Nanotechnology has enabled the realization of hybrid devices and circuits constructed from highly-engineered nanoscale metal and semiconductor building blocks. In electronics, it is well known how the distinct material-dependent properties of metals and semiconductors can be combined to realize important functionalities, including transistors, memory, and logic. In this presentation, I will describe several optoelectronic devices for which the geometrical properties of the constituent semiconductor and metallic nanostructures are tuned in conjunction with the materials properties to realize multiple functions in the same physical space. These devices capitalize on the notion that nanostructures possess a limited number of resonant, geometrically-tunable optical modes whose hybridization and intermodal interference interaction can be tailored in a myriad of useful ways.

The ability to design and to control nanoscale light-matter interactions using metal-dielectric planar optical chips defines new exciting challenges in the rapidly growing fields of nanoplasmonics and nano-optics. Efficient schemes for nanoscale electromagnetic field enhancement, concentration and control over predefined spatial/spectral bandwidths can be enabled by the manipulation of both propagating and non-propagating electromagnetic fields in resonant scattering nanostructures with multiple length scales. In particular, recent advancements in the design and fabrication of multi-particle arrays of metal-dielectric nanostructures have demonstrated unique opportunities for the engineering of novel functionalities and planar optical devices that leverage photonic-plasmonic coupled resonances, such as broadband optical nano-antennas, near-field concentrators and extractors, laser nanocavities, circular light scatterers and on-chip nonlinear optical elements.
In this talk, I will discuss some of our recent results on the design, nanofabrication and optical characterization of periodic and aperiodic photonic-plasmonic coupled nanostructures for a number of optical engineering device applications. Specifically, I will focus on retardation and multiple scattering effects in nanoplasmonics as a strategy to boost the intensity of optical near-fields at the nanoscale. I will then introduce our work on circular multiple scattering in photonic-plasmonic spiral structures and demonstrate their potential for the engineering of broadband light emission enhancement, solar cell energy harvesting, and the generation and control of Orbital Angular Momentum (OAM) of light using both metallic and dielectric arrays. The generation of well-defined numerical sequences of OAM values by light scattering from engineered particle arrays is relevant to a number of device applications in singular optics, secure communication, classical and quantum cryptography. Finally and time permitting, I will present our recent work on the design and engineering of novel types of Si-compatible light emitting nano-devices, namely the coaxial dielectric slot and the Metal-Insulator-Metal (MIM) coaxial cavity, both supporting nanoscale localized modes for broadband light emission enhancement or Erbium radiation at 1.54µm and the manipulation of spontaneous emission rates at multiple frequencies on planar optical chips.

Nanoscale semiconductor structures such as nanowires and nanoparticles with high refractive index can support optical resonances that give rise to strong light matter interaction and optical response. These resonances lead to use of semiconductor nanostructures for compact and efficient photonic, optoelectronic and photovoltaic devices. One unique aspect of semiconductor optical resonators, comparing to plasmonic resonant structures made of metals, is that one could gain access to both far-field (scattering) and near-field (absorption) properties of the resonators at the same time, allowing us to comprehensively study and engineer resonant optical phenomena. To show case this interesting notion, we will demonstrate theoretical analysis and experimental observation of optical Fano resonance in a single Si nanostructure, which is often considered challenging comparing to plasmonic structures. Furthermore, due to the advantage of employing semiconductor nanostructure, we can for the first time characterize both scattering and absorption near an optical Fano resonance and identify their very different spectral features. Finally, we would further explore magnetic response from resonant Si nanostructures and its implication for engineering dielectric metamaterials and metasurfaces.

Dielectric structures can provide strong, tunable optical resonances that are similar to those in plasmonic structures. However, much less has been known about the dielectric resonance. Here we demonstrate the general correlation between the optical resonance with the leaky optical mode in dielectric objects. This correlation is evidenced by a linear dependence of the resonance on the physical features of the structure, and can be excellently interpreted with a simple Fabry-Perot model. Similar correlation can be generally found in dielectric structures with different dimensionality (1D nanowires and 0D nanoparticles) or different morphology (rectangular or triangular cross sections). This fundamental understanding can serve as a powerful predictive tool to guide the engineering of optical resonances.

Recent progress on optical antenna has led to many exciting new devices. However, one of the key issues of optical antenna is the increasing metal loss at shorter wavelength. While this is not a significant limitation for some applications, reducing the optical antenna loss is a major priority for many important applications such solar energy harvesting.
I will present some of our new findings based on metallo-dielectric optical nano-antenna that show excellent properties over a wide spectral range. Most importantly, they can simultaneously produce a high antenna gain and directivity.

Although it is often assumed that all light-matter interactions at optical frequencies are mediated by electric dipole transitions, we regularly observe higher-order transitions. For example, we see magnetic dipole emission every day from the many lanthanide ions (such as trivalent europium and erbium) that help to illuminate everything from fluorescent lighting to telecom fiber amplifiers. Magnetic dipole and electric quadrupole transitions also play an important part in the light emission from transition metal ions and semiconductor quantum dots. In this presentation, we will experimentally characterize the "forbidden" transitions in a range of solid-state emitters, and we will illustrate how these higher-order processes can provide new tools for active electronic and photonic devices.

Over the past decade, significant effort has been focused on the field of plasmonics and its applications in enhanced spectroscopy, light emitters, waveguides, and absorbers for photodetectors. The enhanced capabilities are made possible by exciting plasmons at the interfaces of metals, and within metallic nanostructures. While plasmonic nanopillars have attracted attention for their ability to achieve extreme sub-wavelength photon confinement, large concentrations of optical energy, and enhancements to various optical processes, they are severely limited by the inherent and detrimental losses due to the high carrier densities within metals. Therefore, it is highly desirable to identify new materials capable of supporting such sub-diffraction limited electromagnetic modes without the disadvantages that arise from such excessive losses.
We report on sharp surface-phonon resonances in 1um tall nanopillars fabricated from semi-insulating silicon carbide, with diameters ranging from 100-250 nm, as exemplifying one potential system for meeting these goals in the mid-wave IR, SiC in particular should prove useful militarily and commercially important 8-12 um atmospheric window. In a polar semiconductor, surface electromagnetic modes can be supported by optical phonons, where the charge separation between the atoms is akin to charge carriers in the plasma. The coupling of electromagnetic waves to optical phonon modes in polar semiconductors such as SiC, AlN or GaAs drives the real part of the dielectric constant ε_r negative at infrared frequencies, as is necessary for confined subwavelength optical modes, in perfect analogy to conventional plasmons. These “bound-charge” surface phonon polaritons are the electromagnetic equivalent of the free-electron surface plasmons. While electronic plasmas are damped by scattering on a time scale of tens to hundreds of fs, optical phonon modes decay via much weaker anharmonic phonon-phonon interactions on a time scale of a few ps. For nanoscale surface modes, this improves the attainable Q factors by an order of magnitude (from several tens to several hundreds). We report on Q-factors from these initial structures in excess of 40 with calculations showing that values up to 500 being possible. Furthermore, the Q factor was found to be only weakly dependent upon the size of the nanostructure. These large quality factors were achieved with field confinements of the incident mid-IR wavelengths on the order of lambda;/40-lambda;/75. We observe two peaks that can be tentatively assigned as transverse and longitudinal modes, however electromagnetic simulations indicate that these surface modes demonstrate a much more complex field profile than would be anticipated from an spherical subwavelength particle. We also observe surface-enhanced IR absorption from a deposition of anthracene, illustrating the promise of these systems for applications such as molecular spectroscopy.

We experimentally demonstrate the directional emission of polarized light from single semiconductor nanowires. The directionality of this emission has been directly determined with Fourier micro-photoluminescence measurements of vertically oriented InP nanowires. Nanowires behave as efficient optical nanoantennas, with emission characteristics that are not only given by the material but also by their geometry and dimensions. By means of finite element simulations, we show that the radiated power can be enhanced for frequencies and diameters at which leaky modes in the structure are present. These leaky modes can be associated to Mie resonances in the cylindrical structure. The radiated power can be also inhibited at other frequencies or when the coupling of the emission to the resonances is not favored. We anticipate the relevance of these results for the development of nanowire photon sources with optimized efficiency and controlled emission by the geometry.^[1]
[1] G.Grzela, R. Paniagua-Domínguez, T. Barten, Y. Fontana, J. A. Sánchez-Gil, J. Goacute;mez Rivas, Article ASAP, Nano Letters (2012), DOI: 10.1021/nl301907f

One winding of a metal helix can be seen as a planar split-ring resonator, one end of which is pulled into the third dimension. For several windings, the interplay of the helix resonance together with the Bragg resonance gives rise to a one-octave broad circular-polarization stop band (Science 325, 1513 (2009)). Arrays of such helices can be used as a compact broadband circular polarizer. To further improve the performance of the device in terms of bandwidth and extinction, we have previously explored tapered-helix metamaterials (Appl. Phys. Lett. 100, 101109 (2012)), which do increase the bandwidth but still suffer from some finite circular-polarization conversion, which limits the extinction and the purity of the emerging polarization.
In this contribution, we focus on metamaterials composed of metallic quadruple-helices arranged into a square array. By recovering a four-fold rotational axis, circular polarization conversion can strictly be eliminated. However, metamaterial circular polarizers based on quadrupel helices, unlike single helices, inherently require absorption of the constituent metal. Otherwise, the combination of a four-fold rotational axis and time-inversion symmetry strictly forbids circular-polarizer action. Our symmetry analysis is confirmed by extensive numerical calculations comparing results for perfect electric conductors with those for a free-electron Drude metal with finite damping (Opt. Express, in press (2012)).
We also present our experimental status regarding fabricating such complex structures. We combine three-dimensional (3D) stimulated-emission-depletion (STED) direct-laser-writing (DLW) optical laser lithography using negative-tone photoresists and gold electroplating.

We report here results of experiments that indicate modified optical absorption in plasmonic nanoparticles excited by narrowband radiation relative to white light excitation. The plasmonic resonance frequency and spectral shape for metallic nanoparticles is dependent on the charge density of the particle [1]. We have recently proposed theoretical models for the ‘plasmoelectric effect&’ in which optical excitation of metallic nanostructures can result in changes of charge density, producing electrochemical potentials (i.e. plasmoelectric potentials) [2]. When a plasmonic nanostructure is illuminated at frequencies higher than the plasmon frequency, a plasmoelectric response will increase charge density and blue-shift the plasmon resonance to produce increased absorption. Increased heat from absorption thermodynamically drives this response. At frequencies lower than the plasmon frequency, a plasmoelectric potential will red-shift the plasmon resonance by decreasing charge density.
We have performed optical experiments to excite and probe plasmoelectric potentials. Optical absorption experiments can manifest a plasmoelectric response because the induced change in charge density is indicated by a change of the resonance frequency. Monochromatic radiation at frequencies on either side of the neutral particle plasmonic resonance will induce a plasmoelectric potential, altering charge density in the structure and increasing absorption at that frequency. However, radiation at the resonance frequency will not induce a plasmoelectric response because no change in charge density will produce increased absorption at this frequency. Thus, in a plasmoelectric system, extinction spectra measured by scanning monochromatic illumination will show apparent broadening of the resonance compared with spectra measured under white light illumination.
To monitor the plasmoelectrically induced change in the optical spectrum we use a lock-in amplifier technique to measure the extinction of monodisperse (8% coeff. of variation) 60nm diameter Ag and Au colloids in solution (OD=1.2). For white light illumination extinction measurements, the sample was placed in the beam path of a white light source before a monochromator, while for monochromatic illumination extinction measurements, the sample was placed in the beam path after a monochromator. Strikingly, we found spectra collected with monochromatic illumination at a power density of 0.2W/cm^2 showed up to a 3% increase in extinction on either side of the extinction peak when compared to spectra collected under white light illumination. Further, increases in extinction to either side of the resonance scale with incident intensity and are of similar magnitude to theoretical predictions. These results are strong evidence for an optically induced plasmoelectric potential in noble metal nanoparticles.
[1] S. K. Dondapa, et al., Nano Letters 2012, 12 (3)
[2] M.T. Sheldon, et al., 2012 in submission, arXiv:1202.0301

One-dimensional photonic crystals (1D PCs), also known as Bragg stacks (BSs), represent a promising class of “smart” environmentally responsive nanostructures featuring optically encoded stimuli detection. Aptly, these periodic nanostructures are functional systems with an inherent response to physical stimuli such as temperature and humidity changes, and are capable of translating the chemical fingerprint of chemical and biological analytes into a visibly perceptible optical read-out. A variety of fabrication methods and numerous combinations of organic and inorganic materials together with surface functionalization and morphology tuning opens up new avenues to the design of simple, yet versatile sensing devices. Optical sensing is realized via tracking the behavior of the photonic stop band - a characteristic spectral region allowing for the modulation of the transmission/reflection properties of PCs. By virtue of the tunability of the stop band position, the intensity of light propagating through the Bragg stack is modulated upon varying the environmental conditions. Herein, we present a detection principle based on utilizing 1D PCs as responsive optical filters for intensity tuning of narrow-band light sources. The tunable range for the stop band modulation lies in the visible region and can thus be detected in a straightforward and inexpensive fashion by a visible-light photodetector.[1] We demonstrate a route towards the bottom-up assembly of a fully functional, integrated miniature sensing platform and show temperature, humidity and chemical analyte detection as a proof of the proposed sensing concept.
[1] A. T. Exner, I. Pavlichenko, B. V. Lotsch, G. Scarpa, P. Lugli, Towards Low-Cost Thermo-Optic Imaging Sensors: A Detection Principle Based on Tunable 1D Photonic Crystals, 2012, submitted.

As new techniques are employed to obtain smaller and smaller nanoparticles, difficulties rise in characterizing and utilizing these nanoparticles for applications, such as biological sensing. Non-Fluorescent nanoparticles are especially difficult, however, in some cases, resonant absorption introduces an avenue for optical imagining. In this study, we implore the photothermal heterodyne imaging (PHI) technique, which enables studies of individual weakly absorbing nano-objects in various environments. Briefly, PHI microscopy uses a tightly focused time-modulated, resonant heating beam superimposed with a nonresonant probe beam. The heating beam is absorbed by the nanoparticle and a photoinduced change in the refractive index of the environment produces a measurable signal. Taking advantage of the dramatic index of refraction change occurring around a thermotropic liquid-crystalline phase transition, we demonstrate a 40-fold signal-to-noise ratio enhancement for gold nanoparticles imaged in 4-cyano-4prime;-pentylbiphenyl (5CB) liquid crystals over those in a water environment. We studied the photothermal signal as a function of probe laser polarization, heating power, and sample temperature quantifying the optimal enhancement. This study established photothermal microscopy as a valuable technique for inducing and/or detecting local phase transitions at the nanometer scales, which allows studies of phase change dynamics and confinement effects.

Noble metal nanoparticles exhibit optical excitations known as surface plasmons. Plasmonic nanoparticles are in the focus of attention because of their interesting electric and optical properties. These types of materials produce a large enhancement of the local light intensity under external illumination. Plasmons are highly related to the specific particle size and shape. There are various synthetic procedures which allow us to fine tune these parameters in order to adjust their plasmonic response. However, the enhancement of the local light increases particularly when particles are arranged in closely spaced configurations. This is due to the formation of hotspots with high electromagnetic fields. Thus, a critical role in the hot spot generation is the inter-particle gap distance.
Controlled assembly using colloidal chemistry is an emerging and promising field for high yield production of metal nanoparticle clusters with small inter-particle gaps. However, most of the reported methods rely on the use of nucleic acids or other organic molecules as linking elements, which yield long separation distances and thus small plasmon coupling. Additionally, only simple clusters such as dimmers and trimmers have been efficiently synthesized. In this work, we report the controlled assembly of gold nanospheres into well-defined nanoparticle clusters with large coordination numbers (up to 7) and high symmetry. We further demonstrate ultrasensitive direct and indirect surface-enhanced Raman scattering (SERS) sensing, thus corroborating the outstanding optical performance of these clusters with robust enhancement factors over 3 orders of magnitude higher than those of single particles.
Ref:
N. Pazos-Perez et al., Angew. Chem. Int. Ed., (2012-DOI: 10.1002/anie.20120701)

Metallic rugate structures are theoretically investigated for achieving near-perfect absorption in the visible and near-infrared regions. Our model builds on nanoporous metal films whose porosity (volume fraction of voids) follows a sine-wave along the film thickness. By setting the initial phase of porosity at the top surface as 0, near-perfect absorption is obtained. The impacts of various structural parameters on the characteristic absorption behaviors are studied. Furthermore, multiple peaks or bands with high-absorption can be achieved by integrating several periodicities in one structure. The rugate absorbers show near-perfect absorption for TE and TM polarizations and large incident angles.

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Over the last two decades, metallic nanomaterials such as gold, silver and copper have attracted much more attention due to their unique electronic and optical properties compared to the "bulk" system [1]. These properties are strongly dependent on the characteristics of the metallic nanomaterials like the size, shape, environment and nature of the materials [2]. Noble metal nanomaterials present a strong absorption in the visible region coined as the collective oscillation of the conduction electrons and named surface plasmon polariton. Such resonance occurs when the incident photon frequency matches with the collective oscillations of the conduction electrons of the metallic nanoparticles. Due to the discrete size of the materials, the possibility to confine the charge density oscillations (Surface plasmons) and thus, the local electric field at the vicinity of the nanomaterials opens new avenues in biology, photonics and catalysis [3]. While the linear optical processes in small metallic nanomaterials are well understood, few investigations have been carried out to understand the nonlinear optical processes involved in metallic nanomaterials [4]. In nonlinear optics, two photons of frequency omega; are converted into one photon of frequency 2omega; and arises from the quadratic susceptibility of the material. This process is consequently forbidden, under the dielectric dipole approximation in media with inversion symmetry and makes the nonlinear optical techniques, good candidates for probing interfaces and surfaces where the centrosymmetry is broken [5].
In this study, we investigated the second harmonic emission of gold nanomaterials with different size, shape and state of aggregation using a second harmonic generation microscope. Angular resolved measurements were carried out to quantify and qualify the second harmonic emission modes in spherical nanomaterials, aggregates of nanomaterials and in gold nanowires. The study led to the identification of nonlocal dipole and local quadrupole emission in spherical nanomaterials depending on the size, the shape of the nanomaterial. By changing the polarization orientation in anisotropic metallic nanomaterials like nanowires or aggregates, it has been possible to select and control the mode of emission (dipole or quadrupole modes). Finally, we demonstrate the possibility to fashion the cartography of the local electric fields by selecting the excitation wavelength and the polarization state of the excitation source.
References
[1] P. C. Ray, Chem.Rev, 110(9), 5332-5365, 2010.
[2] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters(1995), Springer, Berlin.
[3] RW Murray, Chem. Rev, 108, 2688, 2008.
[4] J. Nappa, I. Russier-Antoine, G. Benichou, B.C Jonin, P.F Brevet, J. chem.Phys, 125,184712,2006.
[5] J.P Abid, P-F. Brevet, H.H Girault, J.Phys.Chem C. 111(25), 8849, 2007.

Metal nanoparticles are being used in nanosensing and optical devices due to their surface plasmon resonances (SPR). The SPR peak positions and intensities are critical parameters for their applications and they have been shown to depend on the particle size and shape [ 1,2 ]. There are reports that SPR can also be modified by the dielectric surroundings, such as substrates; but no systematic experimental study with high spatial resolution is available. Electron energy-loss spectroscopy (EELS) data collected by scanning transmission electron microscope (STEM) with a monochromator significantly facilitates the ability to detect dark modes and weak dipoles for localized SPRs with high accuracy, and high spatial and energy resolution [2]. We have utilized this technique to measure the effect of the nature of substrates with different refractive index (n) values on the localized SPRs of gold nanoparticles. The STEM-EELS data were obtained with a beam size on the order of 1 nm and 0.14 eV energy resolution from individual gold nanoparticles on various substrates (i.e., carbon, SiO2, TiO2, and HfO2) with n values ranging from 1 to 2.6. A comparative study including nanoparticles in vacuum and gaseous atmosphere will also be presented.
[1.] J. A. Scholl et al. Nature 2012 483, 421-428
[2.] V. Myroshnychenko et al. Nano Letters 2012 12 (8), 4172-4180

The recently introduced concept of coherent perfect absorption and its interpretation in terms of time reversed lasing has attracted significant attention as a mean to achieve perfect absorption in weakly absorbing media [1]. In this framework, we demonstrate that the weak optical absorption in nanometric layers of CdSe/CdS quantum dot-in-rods [2] or of Ce doped yttrium aluminum garnet [3] can be overcome by exploiting coherent absorption, i.e., by the interference and dissipation of the light scattered at the different interfaces of the sample. The excitation of eigenmodes of the structure is intrinsically related to the interference pattern that traps the incident radiation in the absorbing layer. We make use of this enhanced coherent absorption to increase the fluorescence emission from nanometric layers of light emitting materials.
[1] W. Wan et al., Science, 331, 889 (2011).
[2] G. Pirruccio et al., Phys. Rev. B 85, 165455 (2012).
[3] S. Murai et al., Opt. Mat. Expr. 2, 1111 (2012).

In this study, we develop a highly-sensitive optical fiber surface-enhanced Raman scattering (SERS) sensor by using interference lithography. While one facet of the optical fiber is patterned with silver-coated nanopillar array as a SERS-active substrate, the other end of the probe is used, in a remote end detection, to couple the excitation laser into the optical fiber and send the SERS signal back to the Raman spectrometer. SERS performance of the probe is characterized using trans-1,2-bis(4-pyridyl)-ethylene (BPE) monolayer and an enhancement factor of 1.2×10^7 can be obtained by focusing the laser light directly onto the nanopillar array substrate (front end detection). Furthermore, we demonstrate that this probe can be used for the in-situ remote sensing of toluene and 2,4-dinitrotoluene vapor by the remote end detection. Such a fiber SERS probe is very promising for molecular detection in various sensing applications.

Advances in the field of metamaterials have enabled unprecedented control of light-matter interactions. Metamaterial constituents support high-frequency electric and magnetic dipoles, which can used a building blocks for new materials capable of negative refraction, electromagnetic cloaking, and enhancing magnetic or chiral transitions in ions and molecules. While all metamaterials to date have existed in the solid-state, considerable interest has emerged in designing a colloidal metamaterial or ‘metafluid&’. Such metafluids would combine the advantages of solution-based processing with facile integration into conventional optical components.

Here we demonstrate the colloidal synthesis of an isotropic metafluid that exhibits a strong magnetic response at visible frequencies. Protein-antibody interactions are used to direct the solution-phase self-assembly of discrete metamolecules comprised of silver nanoparticles (36 nm diameter) tightly packed around a single dielectric core (90 nm diameter). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are used to characterize the structure of the assembled metamolecules. TEM tomography is also used to confirm the three-dimensional structure.

The electromagnetic properties of the metamolecules are simulated with a generalized multi-particle Mie theory, a semi-analytical solution of Maxwell&’s equations for a system of spheres. Near and far field calculations are performed to elucidate the nature and spectral position of the electric and magnetic modes. The magnetic dipole mode, spectrally in the 600 nm to 700 nm range, is confirmed in the simulations through its strongly enhanced magnetic field and circulating displacement currents.

The electric and magnetic modes of individual metamolecules and the bulk metamaterial solution are directly probed with optical scattering and spectroscopy. Single metamolecules are characterized by polarization filtered darkfield scattering spectroscopy in an optical microscope. Using crossed polarizers to filter out the electric dipole scattering, we directly observe a strong magnetic mode in single metamolecule measurements. A home-built angle and polarization resolved ensemble light scattering system was used to directly measure the strong magnetic dipole scattering from the bulk metafluid solution. Using a 633 nm HeNe excitation source, we observe magnetic dipole scattering from the metafluid that is approximately 12% of the strength of the electric dipole scattering and more than 10 times greater than the control sample of unassembled silver and dielectric nanoparticles. This colloidal metamaterial can be synthesized in large-quantity and high-quality, and may accelerate development of advanced nanophotonic and metamaterial devices.

The major hurdles to implementing solar energy on a large scale are cost, manufacturing throughput and elemental abundance. A 50% cost reduction would make solar energy competitive without subsidies in most world markets. Thin-film solar cells (such as CdTe) cost about 50% as much to manufacture as crystalline silicon, but have roughly the same cost per watt of output power due to their lower efficiency (typically 12%). Doubling their efficiency to bring them in-line with crystalline silicon while maintaining low manufacturing cost would cut the cost per watt in half and could have a large impact on the world energy market.
This talk will discuss a novel geometry for low-cost and high performance thin-film solar cells: metal-semiconductor core-shell nanowires. Solid semiconductor nanowires (as have been used for photovoltaic applications) have particularly strong absorption in the TM polarization (electric field parallel to the length of the nanowire) but less in the TE polarization (electric field perpendicular to the length of the nanowire). Because metal nanowires have a strong localized surface plasmon resonance in the TE polarization, the core-shell geometry may therefore provide the best response for unpolarized light (like solar irradiation). In fact, our analytical Mie calculations show that such radially symmetric structures outperform their semiconductor nanowire counterparts by absorbing over 25% more photons. We will discuss the fundamental mechanisms for these extreme absorption enhancements, including plasmonic/photonic mode coupling and splitting, based on Mie theory and FDTD simulations.
Optimized structures absorb more than 95% of above band gap photons with 15-40 times lower film thickness than needed in a thin-film geometry. These optimized structures have large metal core diameters (~100 nm) and thin semiconductor shells (46 nm for CdTe, 20 nm for a-Si), which would also allow for improved fill factors via lower series resistance and shorter diffusion lengths, and ultimately for high efficiencies with low-cost and abundant semiconductors.

GaAs nanowire (NW) arrays represent an approach to enable high-quality, lattice-mismatched growth via radial strain relaxation, and also to reduce material usage and cost of photovoltaic and photoelectrochemical devices. Even at very low areal fill fractions, GaAs NW arrays exhibit strong absorption due to light trapping effects. In NW arrays, light can potentially couple into several types of modes: i) photonic crystal modes, ii) longitudinal modes (Fabry-Perot resonances), and iii) optical waveguide (radial) modes. We report here the very strong light absorption and current collection properties of sparse (<5% fill fraction) GaAs NW arrays grown on both GaAs and Si substrates, supported by experiments, simulations and analytical theory. These results reveal that GaAs NW arrays with 4% fill fraction absorb between 60 and 100% of the incident light, and that the absorption fraction depends strongly on wavelength and incidence angle. These absorption characteristics indicate a 15-25x enhancement in the effective cross section of the NWs.
We assess the mechanism for the observed strong absorption enhancement and find that mechanism iii) coupling to radial waveguide modes makes the dominant contribution.
Optical absorption and electrochemical photocurrent collection was performed for MOCVD-growth NW arrays (L_wire=3um, D_wire=150nm, pitch=600nm) as a function of wavelength (350-900nm) and illumination angle (0-60deg) was measured experimentally and modeled via three-dimensional, full-field electromagnetic simulations. Experiments, electromagnetic simulations and analytical modal analysis showed excellent qualitative and quantitative agreement across the spectrum and range of incident angles. The strong absorption of the wire arrays is explained by coupling into resonant leaky and guided optical waveguide modes, which is enabled by scattering of incident light from neighboring wires. The identification of specific TE and TM wire modes that are responsible for absorption peaks was carried out via a comparison of analytical solutions using fundamental optical waveguide theory and the spatially-resolved electric field profiles of the wire cross sections obtained from electromagnetic simulations. Good agreement was discovered between the calculated resonant wavelengths of a given mode and the electric field profiles observed at the absorption spectral peaks.
Ultimately, the elucidation of the mechanism responsible for the high absorption of low fill fraction GaAs NW arrays will facilitate optimization of light absorption and, thus, improve III-V NW array optoelectronic performance. Thin-film GaAs solar cells presently hold the 1 Sun efficiency record and have demonstrated the promise of single crystal GaAs for future photovoltaic devices. NW arrays represent a means to dramatically reduce the material usage of high-quality GaAs, which is an important element for GaAs to reach its potential as a TW scale solution to the solar energy challenge.

Wing scales of butterflies and moths provide over 175 000 types of 3D hierarchical sub-micrometer structures. In this presentation, we will introduce how these natural textures can be directly replicated in seven common metals (Ag, Au, Co, Cu, Ni, Pd, Pt) through a simple chemical route with potential optical, thermal, magnetic, electric and catalysis applications. Among them, Au butterfly wing scales have exhibited excellent properties as surface-enhanced Raman scattering substrates, in terms of high-sensitivity (10^-13 M, R6G, 10-fold of magnitude higher than commercial counterparts), high-reproducibility (gene-engineered structures), and low cost. As supports, they have also doubled the fluorescence intensity of some fluorescence probes, compared with their present human-designed counterparts. These findings could help bring high-quality and affordable SERS and FEM substrates as consumables for trace-amount chemical detection to ordinary laboratories across the world.
References
1. Yongwen Tan, Jiajun Gu*, Linhua Xu, Xining Zang, Dingxin Liu, Wang Zhang, Qinglei Liu, Shenmin Zhu, Huilan Su, Chuanliang Feng, Genlian Fan, and Di Zhang*,
"High-density hotspots engineered by naturally piled-up subwavelength structures in three-dimensional copper butterfly wing scales for surface-enhanced Raman scattering detection”, Adv. Funct. Mater., 2012, 22, 1578-1585.
2. Yongwen Tan, Jiajun Gu*, Xining Zang, Wei Xu, Kaicheng Shi, Linhua Xu, and Di Zhang*,
"Versatile fabrication of intact three-dimensional metallic butterfly wing scales with hierarchical sub-micrometer structures”, Angew. Chem. Int. Ed., 2011, 50, 8307-8311.
3. Yongwen Tan, Xining Zang, Jiajun Gu*, Dingxin Liu, Shenmin Zhu, Huilan Su, Chuanliang Feng, Qinglei Liu, Woon Ming Lau, Won-Jin Moon, and Di Zhang*,
"Morphological effects on surface-enhanced Raman scattering from silver butterfly wing scales synthesized via photoreduction”, Langmuir, 2011, 27, 11742-11746.

The optics of zinc oxide nanowire arrays for application in silicon rod solar cells was simulated and compared to experimental results. A Finite Difference Time Domain (FDTD) approach was used to simulate the optical wave propagation in zinc oxide nanowire arrays coated with an amorphous silicon layer. A surface growth model was applied to the nanowire array in order to determine realistic rod dimensions. The influence of the wire design on the absorption of the structure is discussed and optimal dimensions of the zinc oxide wire is determined. High absorption in the structure can already be achieved for zinc oxide nanowires with a length of 200 nm. The highest absorption was reached for a nanowire spacing of 400 nm to 600 nm. The optical simulations will be compared to measurements of the absorption and reflection of zinc oxide nanowire arrays with and without an amorphous silicon coating.

The light scattering at subwavelength objects provides an important mechanism for control of light. However, the well-established scattering theory, Mie theory offers little physical insights into the shift of optical phase during the scattering. This has historically prohibited the effort to manipulate the scattering phase for light control. We elucidate the scattering phase shift of dielectric object rooted in leaky modes of the object by analyzing the light scattering with a different theoretical framework, coupled leaky mode theory, and. The new theoretical framework is capable of determining structure parameter window to achieve 2pi phase change for given incident light in a very intuitive simple way. It also further predicts the structure parameter window is insensitive to dielectric material with different refractive indexes due to the leaky mode nature of phase change. The intuitive correlation with leaky modes can enable unprecedented capabilities to manipulate the phase of light by leveraging on scattering at rationally-designed objects with desired modal properties.

Localized surface plasmon resonance (LSPR) is a label-free biosensing technique employing plasmonic nanostructures to detect local refractive index change induced by biomolecules in the vicinity of these nanostructures. In analogy to surface plasmon resonance (SPR) sensor in a cuvette, LSPR is resistant to bulk refractive index fluctuation yet remains comparably sensitive for biosensing purpose. LSPR has the advantage over SPR in that the overall system size is smaller, and not affected by normal temperature fluctuations during measurement. However, mass production of a cheap but effective LSPR substrate remains challenging.
In this paper, a self-assembly gold nanoisland structure was synthesized on transparent glass substrate by a simple two-step deposition-growth process. The first step involved depositing an ultra thin film of gold with nominal thickness of 5 nm by thermal evaporation at 1× 10-7 torr. Then the gold coated substrate was placed into a high temperature oven and annealed at 450°C for 10 hours. By first observation, the annealed substrate turned from pale green to dark pink. Upon scanning with atomic force microscopy, it was revealed that nanoislands of about 100 nm to 150 nm wide with height of 60 nm in average were formed. Optical extinction measurement showed that the absorption peak was about 560 nm with full-width-half-maximum of 100 nm, so dark pink color iwas observed. For the biosensing demonstration, Bovine serum albumin (BSA) and Anti-BSA bio-affinity interaction was measured using the self-assembly gold nanoisland LSPR sensor. Anti-BSA was functionalized onto the sensing site and BSA of known concentrations, i.e. 1 ug/ml was injected. The result showed LSPR spectral intensity change of 650 counts at the resonance dip of 634 nm. With standard deviation of spectral intensity fluctuation at 7 counts, the detection limit of BSA was estimated at about 12 ng/ml which was comparable with that of LSPR systems with more elaborate nanostructures. The LOD of the present system can be further improved by implementing phase measurement and further nanostructure improvement.

One of the most compelling aspects of plasmonics is the ability to confine electromagnetic radiation in subwavelength modes at metal/dielectric interfaces - a promising characteristic for miniaturizing photonic communications technology at the scale and density of electronics. In order to simultaneously achieve low waveguide propagation loss and high mode confinement, we require chip-based hybrid photonic/plasmonic circuits that feature i) low-loss silicon photonic waveguides, ii) high-confinement plasmonic waveguide building blocks, and iii) methods for efficient mode coupling between them. The channel plasmon polariton (CPP) configuration supports and confines slot plasmon polaritons in a highly confined channel. Light can be efficiently coupled from silicon-on-insulator ridge waveguides to channel plasmon polariton waveguides for which the guided mode exhibits an electric field perpendicular to the channel sidewalls (similar to the TM MIM plasmonic mode). By proper control of mode polarization in the silicon-on-insulator waveguide, parasitic excitation of surface plasmon polaritons (SPPs) can be suppressed on the free metal surface at the junction between the silicon waveguide and channel plasmon waveguide.
In this work, we present near field scanning optical measurements demonstrating efficient coupling into the channel plasmon polariton mode from Si ridge waveguides at lambda;0 = 1520nm. Light was coupled in 700nm Si ridge waveguide via grating couplers. By varying the incident angle at the grating coupler, light can be coupled into either the TE (E-field parallel to the chip surface) or TM (E-field perpendicular to the chip surface) polarized modes in the silicon ridge waveguide. When these modes are coupled from the silicon waveguide into the 200×900nm Au channel waveguides, we can excite either the CPP mode (from Si guide with TE polarization) or the SPP mode (from Si guide with TM polarization). By measuring the exponential decay in CPP waveguides that are longer than the propagation length, we are able to extract a CPP mode propagation length of L = 10mu;m, and by measuring the interference pattern in waveguides that are shorter than the CPP propagation length, we are able to extract the real part of the mode index of n = 1.05, both of which are in agreement with simulation-based modal analysis.
We further demonstrate optical power splitting at ultracompact 90-degree 4-way splitters by crossing two CPP waveguides and find that the CPP mode splits four-ways, coupling equal power into each of the junction arms. This configuration forms the basis for ultracompact plasmonic interferometers comprised of intersecting four CPP waveguides.

The ability to precisely control the topography, roughness, and chemical properties of metallic nanostructures is crucial for applications in plasmonics, nanofluidics, electronics, and biosensing. Here we demonstrate a simple and facile method to produce embedded nanoplasmonic devices that can generate tunable plasmonic fields on ultra-flat surfaces. With template stripping, isolated metallic nanoparticles and wires are embedded in optical epoxy, which is capped with a thin silica overlayer using atomic layer deposition. The top silica surface is topographically flat and laterally homogeneous, providing a uniform, high-quality biological substrate, while the nanoplasmonic architecture hidden underneath creates a tunable plasmonic landscape for optical sensing, imaging, and manipulation. We use the localized surface plasmon resonance of gold nanodisks embedded underneath flat silica films for real-time kinetic sensing of the formation of a supported lipid bilayer and subsequent receptor-ligand binding. We also demonstrate that an optical epoxy layer with large area embedded nanoparticles can be easily peeled off to produce flexible nanoplasmonic devices.

The strong optical field confinement in optical micro-/nano-cavities leads to many fundamental studies and exciting applications in nanophotonics. The sizes of these conventional dielectric cavities cannot be smaller than wavelength scale for efficient photon confinement, which is limited by the low refractive indices of nature materials. We have investigated theoretically indefinite medium with hyperbolic dispersion can be used to miniaturize optical cavities due to the supported unbounded large wave vectors. Here, by incorporating multilayer indefinite metamaterials, we experimentally demonstrate deep-subwavelength optical cavities with sizes down to ~Îraquo;/12. The metamaterial structure consists of alternating thin layers of silver (Ag) and germanium (Ge). These cavities are highly anisotropic, resulting in a hyperbolic iso-frequency contour (IFC). The hyperbolic dispersion permits the access to wave vectors much larger than that in air and the tremendous momentum mismatch between the metamaterial and the air causes the total internal reflection at the interface. By cutting the metamaterial into a nanoscale cube, 3D optical Fabry-PAtilde;copy;rot cavity can be formed with a high effective refractive index equals to k/k_0. Since arbitrary large wave vectors can be reached along the unbound IFC hyperbolic curve, the cavity size can be arbitrarily small. Multilayered metamaterial with 20nm Ag and 30nm Ge is fabricated in experiments Cavities with different sizes are aligned at the identical resonant frequency of 191 THz for the (1, 1, 1) modes, which exemplifies one of the unique features of indefinite cavities. For the same cavity, the (1, 1, 2) mode has a lower frequency compared to the (1, 1, 1) mode, showing the anomalous mode dispersion. Also, the transmission depth is different for each cavity mode, indicating the coupling between the excitation plane wave and the cavity mode strongly depends on the cavity size and the mode order, which is related to the resonating wave vector supported inside the cavity. The retrieved radiation quality factor extracted from the measured transmission spectra, based on the coupled mode theory, shows vertical and total radiation quality factor is proportional to the fourth power of effective index. Also, the indefinite metamaterial cavities allows unnaturally high effective refractive index and a maximum value of 17.4 is demonstrated for the (1, 1, 2) modes. The demonstrated nanoscale metamaterial optical cavities significantly differ from the conventional dielectric cavities, showing anomalous mode dispersion and unique universal fourth power law between the radiation quality factor and the resonating wave vector. This new type of optical cavities will greatly extend the ability to manipulate light at deep-subwavelength scale for potential applications including light-matter interactions.
References:
[1] Yang, X.,* Yao, J.,* Rho, J.,* Yin, X. and Zhang, X. Nature Photonics, 6, 450-454 (2012)

Since the introduction of the extraordinary optical transmission through (EOT) a subwavelength apertures in a metal film by Ebbesen et al. in Ref 1, there has been a significant interest in utilizing such structures for applications from filtering [2] to hyperspectral imaging [3]. Due to the surface plasmon (SP) coupling, a metallic aperture is an efficient antenna for localizing light and enhancing the electric field, however, the minimal lateral dimension of such structures is limited by the fabrication techniques used. While coaxial apertures in a silver film made by a Ga focused ion beam (FIB) milling have been demonstrated to be strong nano-resonators with an ultrasmall mode volume [4], their Q-factor was measured to be only 5% of the maximum attainable theoretical value [5].
Recent advances in the Helium ion microscopy (HIM) have made fabrication with a resolution below 5 nm a reality [6]. In this work, we present an experimental demonstration of maximum theoretical performance of HIM-made coaxial apertures in a thin gold film on a glass substrate. Due to the high resolution of the HIM, the radius of curvature of the corners is much less than the plasmon skin depth rendering the structure as a perfect, defect-free nano-resonator. The measured EOT signal in the visible spectrum has the highest Q-factor possible that is only limited by the choice of the substrate and not the fabrication process. The result is a realization of field localization on the single digit-sub 10 nm-scale that can only be further improved by a different choice of the substrate material.
[1] T. W. Ebbesen et al., Nature 391, 667-669 (1998)
[2] S. Yokogawa et al., Nano Lett. 12 (8), 4349-4354 (2012)
[3] A. Weber-Bargioni et al., Nano Lett. 11 (3), 1201-1207 (2011)
[4] R. de Waele et al., Nano Lett. 9 (8), 2832-2837 (2009)
[5] F. Wang et al., Phys. Rev. Lett. 97, 206806 (2006)
[6] L. Scipioni et al., J. Vac. Sci. Technol. B 28, C6P18 (2010)

We recently reported that brush block copolymers can rapidly self-assemble to form photonic crystals, with tunable reflection spanning the entire visible spectrum. Due to the highly branched nature of this class of macromolecules, the energetic barrier due to chain entanglement is drastically lowered. In this recent work, we have achieved paintable photonic crystals that are chemically robust, and have not shown any signs of degradation for over 18 months. We have developed two unique systems that both achieve reflection peaks in the near infrared, through thermal annealing and direct solvent casting under ambient conditions. We have also demonstrated the ability to blend these copolymers to tune the wavelength of reflection, enabling an economically attractive approach to application-tailored photonic crystals. Our ongoing development of new polymer architectures and self-assembly methods for high fidelity photonic crystals will be discussed.
Specifically, we have demonstrated the “grafting-through” synthesis of brush block copolymers containing polystyrene, polylactide and polyisocyanate brushes. In our initial study, polystyrene/polylactide brush block copolymers were assembled by solvent casting and thermal annealing to yield alternating lamellar morphologies with distinct visible reflections. For a given self-assembly method, the peak wavelength of reflection was found to be a linear function of molecular weight, unlike the analogous linear copolymer. Direct thermal annealing of the polymer melt yielded reflection from the ultraviolet (~300 nm) to the near infrared (~1300 nm), corresponding to lamellar periods of ~70 to 430 nm. Transfer matrix simulations were employed to justify the proposed mechanism of the observed reflection spectra. Good agreement with experiment was observed across the entire weight range (1.08 × 10^6 to 6.64 × 10^6 g/mol) of the polymers, and for each self-assembly method. The refractive indices of each block were measured by ellipsometry, which ranged from 1.45 - 1.65.
We next demonstrated the synthesis of isocyanate-based brush block copolymers, and their ability to form large lamellar nanostructures by simple solvent casting under ambient conditions (room temperature, from dichloromethane). We also showed that by blending two of these copolymers with different molecular weights, we could tune the peak of reflection across the entire visible spectrum, from 375 - 775 nm. The peak wavelength of reflection increased monotonically with the weight fraction of the blend, with an R^2 = 0.989. This work represents a significant step towards truly paintable photonic crystals.
1. Sveinbjörnsson, B. R.; Weitekamp, R. A.; Miyake, G. M.; Xia, Y.; Atwater, H. A.; Grubbs, R. H. PNAS 2012, 109, 14332-14336.
2. Miyake, G. M.; Weitekamp, R. A.; Piunova, V. A.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 14249-14254.
3. Miyake, G. M.; Piunova, V. A.; Weitekamp, R. A.; Grubbs, R. H. Angew. Chem. Int. Ed. 2012, 51, 11246-11248.

Silicon nanocylinders support geometric (Mie) resonances in the visible spectral range, making them promising candidates for many applications, such as sub-wavelength photodetectors, anti-reflection coatings on solar cells, and building block for metamaterials due to their strong magnetic character. To design optimum particle arrays for such applications, it is essential to fully understand the resonant properties of single nanocylinders. However, the small particle size makes it difficult to fully study individual Mie scatterers with standard optical scattering experiments.
Here, we study the resonant optical properties of individual Si nanocylinders using cathodoluminescence (CL) imaging spectroscopy, a novel technique that allows for optical measurements with a spatial resolution far below the diffraction limit. We demonstrate that the 30 keV electron beam of a scanning electron microscope can be used to efficiently excite the optical eigenmodes of the Si nanocylinders. By scanning the electron beam over the particle and collecting the emitted optical radiation (400-700 nm) using a parabolic mirror, the “photonic wavefunctions” of the Si nanocavities can be imaged with 10 nm resolution.
Single silicon cylinders (70-320 nm diameter, 100 nm high) were fabricated on a silicon-on-insulator wafer using e-beam lithography and reactive ion etching. Clear cavity resonances (quality factor Q=10-20) are observed in the CL emission spectra, that red-shift for larger particle diameter, as expected. We make two-dimensional maps of the modal field distribution for each resonance at deep sub-wavelength resolution. The photonic wavefunctions show characteristic radial nodes and anti-nodes, corresponding to different azimuthal and radial mode numbers (m=0-3, n= 1-3) for the different resonances. The lowest order resonance (m=0, n=1) of Si Mie scatterers corresponds to an out-of-plane point dipole, as confirmed by measurements of the angular distribution of the radiated emission, which shows an azimuthally symmetric toroidal distribution. The higher-order modes show an angular radiation pattern that is pointed upward. The results show good agreement with finite-difference time-domain simulations and an analytical 2D dielectric disc model that is used to describe the geometric eigenmodes of the particles.

Predictions of unique and potentially useful properties, such as a negative refractive index, have triggered a quest to implement left-handed (LH) media [1], under the form of artificially engineered structures known as metamaterials [2]. Here we report the first experimental implementation of a bulk metamaterial with a LH electromagnetic response in the ultraviolet (UV). The metamaterial, based on stacked, coupled unit cells of planar plasmonic waveguides incorporating Ag and TiO2 layers [3], was designed using the transfer-matrix method and experimentally demonstrated to exhibit, for TM-polarized light, an all-angle refractive index close to -1 at lambda;=364 nm, for both power flow and phase. Using this metamaterial, we fabricate a Veselago flat lens [1], which we use to demonstrate the first imaging, beyond the near field, of arbitrarily-shaped two-dimensional objects. Moreover, using a pump beam tuned above the bandgap of the constituent TiO2 semiconductor, we demonstrate all-optical active control of UV images transferred by metamaterial flat lens, with an intensity modulation up to 50%. The planar multilayer architecture of the metamaterial is ideally suited for large-area, lithography-free implementation of LH optical elements and coatings able to provide an unprecedented degree of control of light beyond the visible. We will discuss novel optomechanical and imaging applications of this all-angle metamaterial. In particular, the far-field imaging performance of a LH flat lens will be analyzed in detail, highlighting how lens-source interaction conspires to limit resolution over supra-wavelength distances, even in the “perfect lens” [4] scenario of a lossless material with ε = mu; = -1.
(1) V.G. Veselago, Sov. Phys. Usp. 10, 509 (1968). (2) C.M. Soukoulis, & M. Wegener, Nat. Photonics 5, 523 (2011). (3) E. Verhagen et al., Phys. Rev. Lett. 105, 223901 (2010). (4) J.B. Pendry, Phys. Rev. Lett. 85, 3966 (2000).

One-way transmission of light, meaning that light incident from left and right directions show very different transmission coefficients, has attracted enormous attentions and exhibited many applications including optical isolators and optical diodes. Such an intriguing effect normally replies on the presence of a static magnetization or nonlinearity of the medium, which breaks the reciprocity of light propagation. The rapidly growing field of metamaterials provides a novel platform to manipulate light propagation in extraordinary ways, allowing for negative refraction and invisibility cloak, for instance. Recently, researchers have been actively exploring polarization-dependent asymmetric transmission through metamaterials by engineering the interaction of light and sophisticated chiral metamaterial structures.
Here, we demonstrate broadband one-way transmission for linearly polarized light only through a double-layered chiral metamaterial structure in the near-infrared region. Different from previous work that focused on the asymmetric transmission of circularly polarized light, asymmetric transmission of linear polarization is achieved by designing proper in-plane symmetry as well the symmetry along the propagation direction of the chiral metamaterial structure. Interestingly, the transmission for circularly polarized light through the designed metamaterial is symmetric. We fabricate the double-layered metamaterial, which consist of a half-sauwastika and a half-gammadion shaped metallic pattern in each unit cell. The experimental measurements show asymmetric transmission around 150 THz with 50 THz bandwidth, and the the maximum transmission contrast is 1800% between the forward and backward propagation directions. Our results could be highly valuable for developing novel photonic devices such as ultra-thin optical isolators.

Achieving one-way wave transmission, i.e. isolation of forward and backward waves, is one of the key problems of modern electromagnetics [1]. Recent advances with magneto-optical (MO) materials and their integration with photonic crystals [2,3] demonstrate a certain potential for optical isolation. At the same time such structures may encounter complicated designs, fabrication difficulties, and scalability with operating frequency. In this work we propose a concept for a design of nonreciprocal electromagnetic systems, including optical isolators, using extreme-parameter metamaterials. In particular, we show that by merging the principles of extreme-parameter metamaterial design with MO materials it is possible to develop a novel class of optical systems, namely magneto-active nonreciprocal metamaterials. We study the case of epsilon-near-zero (ENZ) metamaterials, infiltrated with MO materials, and show theoretically in the context of effective medium approach that in such metastructures optical isolation for circularly polarized waves is possible, i.e. such media are transparent for forward circularly polarized waves of a given handedness and opaque for backward propagating waves of the same handedness. We numerically study two potential implementations of such optical isolators. In particular, we analyze wave propagation in MO-material-filled rectangular waveguide near its cut-off frequency for its TE10 mode, and a metal-MO stack. We demonstrate, using numerical simulations, that in the presence of magneto-optical activity the optical isolation is achieved in such geometries.
[1] R.J. Potton, “Reciprocity in optics,” Rep. Prog. Phys. 67 717754 (2004).
[2] A. B. Khanikaev, S. H. Mousavi, G. Shvets, and Y. S. Kivshar, “One-Way Extraordinary Optical Transmission and Nonreciprocal Spoof Plasmons,” Phys. Rev. Lett. 105, 126804 (2010).
[3] K. Fang, Z. Yu, V. Liu, S. Fan, “Ultracompact non-reciprocal optical isolator based on guided resonance in a magneto-optical photonic crystal slab”, Opt. Lett. 36, 4254-4256 (2011).

Taming and manipulating electromagnetic waves and fields by tailoring the parameters of metamaterials such as permittivity and permeability provide novel approaches for designing devices and components. One category of metamaterials is epsilon-near-zero (ENZ) materials and structures, in which for any given frequency, the apparent wavelength is effectively increased as the relative permittivity nears zero. As a result the distance (d) between two points, e.g., between the two sources and/or scatterers, although physically sizable, becomes electrically small when compared with the effective wavelength inside the structure. Here, we explore the degree of incoherence caused by randomness in the location of electromagnetic sources and scatterers and how this can be reduced and controlled by embedding these sources and scatterers in an ENZ environment. First, this is done analytically for a plane wave of random phase. Second, the concept is explored numerically for a collection of dipoles with randomized positions. Furthermore, we investigate this phenomenon in emission of quantum dots in ENZ metastructures such as a waveguide near its cut-off frequency. ENZ materials can open up fascinating possibilities for extending the coherence length of emitters and scatterers, leading to unprecedented light-matter interaction with long spatial coherence length.

Ohmic loss in metal based metamaterials continues to be one of the primary impediments to their application at infrared and visible frequencies. Dielectric metamaterials offer one potential solution to this issue by eliminating ohmic loss as well as avoiding saturation in the magnetic response at high frequencies. In this work, we will discuss our recent efforts to develop three-dimensional purely dielectric metamaterials at optical frequencies based on structured silicon. These resonant dielectric metamaterials are employed for achieving near-zero refractive index at optical frequencies. Formed from stacked silicon rod unit cells exhibiting both electric and magnetic dipole Mie resonances, these metamaterials exhibit impedance matched near-zero refractive index at optical frequencies allowing near unity transmission. We present experimental evidence of a nearly isotropic low-index response including angular selectivity of transmission and directive emission from quantum dots placed within the metamaterial. These isotropic low-loss metamaterials can be applied for a variety of applications including directional emitters, filters, and compact lens systems.

The need for reducing or even compensating of the losses is a key challenge for metamaterial technologies. One promising way of overcoming the losses is based on incorporating a gain material into the metamaterial structure. Therefore, it is of vital importance to understand the mechanism of the coupling between metamaterials and the gain medium. In addition, these ideas can be used in plasmonics to incorporate gain to obtain new nanoplasmonic lasers. We will present our FDTD numerical new results with gain in metamaterials. In this talk, we address the question of what is a good conductor for use in metamaterials and plasmonics.

Optical coatings, which consist of one or more films of dielectric or metallic materials, are widely used in applications ranging from mirrors to eyeglasses and photography lenses. Many conventional dielectric coatings rely on Fabry-Perot-type interference, involving multiple optical passes through transparent layers with thickness on the order of the wavelength to achieve effects such as anti-reflection, high-reflection, and dichroism. Highly absorbing dielectrics are typically not used because it is widely accepted that light propagation through such media destroys interference effects. We have shown that under appropriate conditions interference can instead persist in ultra-thin (a few to tens of nm thick), highly absorbing films, and demonstrate a new type of optical coating comprising such a film on a metallic substrate, which selectively absorbs various frequency ranges of the incident light (Ref.1). These coatings have low sensitivity to the angle of incidence and require minimal amounts of absorbing material (e.g. 5-20 nm for visible light). This technology has potential for a variety of applications from ultra-thin solar cells and photodetectors to optical filters, and even the visual arts and jewelry.
In parallel and related work we have shown that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media (Ref. 2). This design is implemented with an ultra-thin (~lambda;/65) vanadium oxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at lambda; = 11.6 µm. The structural simplicity and large tuning range (from ~80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.
1. M. A. Kats et al. Nature Materials PUBLISHED ONLINE: 14 OCTOBER 2012 | DOI: 10.1038/NMAT3443
2. M. A. Kats et al. Appl. Phys. Lett. 26 November 2012. In press

The ubiquitous spin-orbit interaction destroys the rotational symmetry of particles&’ spin degree of freedom and introduces a universal transverse spin current regardless the particle nature of being an electron or photon. The relativistic spin-orbit coupling of electrons results in intrinsic spin precessions and therefore spin-polarization-dependent transverse currents, leading to the observation of spin Hall effect and the rapidly flourishing field of spintronics. Since the coupling between charge&’s spin degree of freedom and its movement is essentially identical to the coupling of the transverse electric and magnetic components of a propagating electromagnetic filed, to conserve total angular momentum, an inhomogeneity of material&’s index of refraction can cause momentum transfer between the orbital and the spin angular momentum of light along its propagation trajectory, resulting in a transverse splitting in polarization. Such a photonic spin Hall effect was recently proposed and a unified theory has been developed to describe the spin-orbit interaction, the geometric phase, and the precession of the polarization of light in weekly inhomogeneous media as well as the abrupt discontinuous interfaces between homogenous media.
The experimental observation of spin Hall effect of light, however, is fundamentally challenging since the amount of momentum that a photon carries is exceedingly small. The study of spin-orbit interaction of light was only possible very recently by accumulating the effect through multiple reflections or utilizing ultra-sensitive quantum weak measurements with pre- and post-selections of spin states. Here we demonstrate experimentally the strong and controllable interactions between the spin and the orbital momentum of light in a resonating thin metasurface - a two-dimensional electromagnetic metamaterial with a designed in-plane phase retardation over the wavelength scale. In such an optically thin material, the resonance-induced anomalous “skew-scattering” of light destroys the axial symmetry and we observed PSHE even at the normal incidence. In stark contrast, for conventional interfaces between two homogeneous media, the spin-orbit coupling apparently vanishes at the normal incidence.

Three dimensional (3D) optical negative index metamaterials (NIMs) are artificially engineered structures that have been demonstrated in recent years for operation both in the near infrared and visible wavelength ranges. These systems exploit multilayer metal-dielectric stacks, with open fishnet geometries that involve unit cells with sub-wavelength dimensions. The fabrication processes typically use focused ion beam (FIB) techniques to machine directly, in a serial fashion, the necessary structures in uniformly deposited multilayer assemblies over small areas (hundreds of µm2), both for near infrared [1] and visible [2] wavelengths. Realistic applications demand, however, the ability to build, at high throughputs, large area 3D NIMs. This talk will describe several soft lithographic printing methods that are capable of fabricating high performance NIMs, in a scalable manner, with the potential for use in low cost manufacturing. We begin by reviewing ananotransfer printing technique for fabricating 3D, fishnet NIMs with sizes of >75cm2 and high figures of merit in the near infrared wavelength range (1.8 µm -2.4 µm). The resulting structures incorporate 2x107 times more unit cells than those previous reports [1] and are formed with throughputs 1x108 times higher than those possible with state-of-art FIB systems. An advanced version of this fabrication sequence exploits subtractive printing to form similar 3D NIMs, but with operation in the visible wavelength range (529 nm - 720 nm) and the telecom band (1.35 µm - 1.6 µm) in a manner that enables otherwise comparable sizes, levels of performance and processing throughput. 3D NIMs of this type with unit cell pitch of 300 nm show peak values of the negative index of -0.73 at 710 nm, which represents the lowest experimentally observed wavelength range for negative index of refraction in this class of NIM [4]. Finally, we describe how subtle, non-ideal features of the structures can be eliminated by careful selection of materials, deposition methods and fabrication techniques [5]. The collective results provide important practical steps toward engineered metamaterials for realistic applications over a broad range of wavelengths.
1. Valentine, J., et al., Three-dimensional optical metamaterial with a negative refractive index. Nature, 2008. 455: p. 376-380.
2. Garcia-Meca, C., et al., Low-Loss Multilayered Metamaterial Exhibiting a Negative Index of Refraction at VisibleWavelengths. Physical Review Letters, 2011. 106.
3. Chanda, D., et al., Large-area Flexible 3D Optical negative Index Metamaterial Formed by Nanotransfer Printing. Nature Nanotechnology, 2011. 6: p. 402-407.
4. Gao, L., et al., Subtractive Printing Based Large Area, Three-dimensional Visible and Telecom Band Negative Index Metamaterials. To be submitted
5. Gao, L., et al., Angular Growth Improvement for Printed Three-dimensional Visible Negative Index Metamaterials. To be submitted

We propose and demonstrate a method for light-assisted self-assembly of nanoparticles using photonic crystal templates. Laser light incident on a photonic-crystal slab excites guided resonance modes of the structure, enhancing the intensity of the electromagnetic field near the slab. Different resonance modes correspond to different periodic, near-field patterns, each of which creates an array of optical trapping sites for nanoparticles. The nanoparticles thus assemble into a 2D crystal pattern. The crystal structure can be reconfigured by changing laser wavelength or polarization.
We present the design, fabrication, and characterization of photonic-crystal slabs that trap polystyrene particles in square and rectangular arrays. We then demonstrate reconfigurable light-assisted self assembly of nanoparticles experimentally.
We expect our method to be useful both for templated nanofabrication of complex nanophotonic materials and for reconfigurable tuning of nanophotonic devices such as optical filters.
Unlike in standard colloidal self-assembly processes, which result in simple triangular, close-packed structures, our method allows the directed assembly of nanoparticles into a variety of lattices and with complex unit cells.

Photonic integrated circuits offer excellent prospects for high speed complex optical applications on a chip scale. During the past decade, silicon-on-insulator (SOI) has emerged as one of the most promising integrated optics platforms due to the high refractive index contrast it provides, which allows for building ultra-compact devices. However, silicon has a narrow indirect bandgap (1.1 eV) and centrosymmetric crystal structure, which not only limits its operation to wavelengths above 1100 nm but also precludes important active functionalities, such as light emission, optical nonlinearity and the linear electro-optic (Pockels) effect.
In this talk, we present our approaches to integrate nonlinear optical materials on silicon substrate for building active photonic circuits. Our choice of materials are III-Nitride. We show that both GaN and AlN thin film can achieve low loss, wide-band waveguiding on an engineered silicon substrates. We further demonstrate the use of III-nitride on silicon material systems for achieving active nonlinear optical functions that are not permitted in crystalline silicon, including x(2) optical nonlinearity and Pockels&’ effect for electro-optic modulation.

The ability to observe, measure and manipulate individual molecular interactions is central to our understanding of complex chemical pathways and biological processes. Fine single molecule analyses of conformational changes, intramolecular distances, and/or adhesion forces are typically carried out by methods such as optical tweezers, Förster energy transfer schemes and other molecular ruler platforms. Since quantitative measurements at the molecular level continue to be at the core of biological and materials R&D, it will be crucial to develop novel sensing architectures that can be inserted directly into systems for in situ diagnostics and/or be engineered to spatially measure nanomechanical forces. In this talk, I will present some of our recent work on subwavelength optical waveguides and show how near-field light-matter interactions can be leveraged to provide feedback on molecular distance changes and forces. Both dielectric-dielectric and dielectric-plasmonic coupling effects will be discussed and how these can be used to design large area or single element force probes. The all-optical design of obtaining feedback makes these systems ideal for new force sensing devices, imaging technologies and high-throughput nanomechanical analysis.

Due to limitations in device speed and performance of silicon-based electronics, silicon optoelectronics has been extensively studied to achieve ultrafast optical-data processing. However, the biggest challenge has been to develop an efficient silicon-based light source since indirect band-gap of silicon gives rise to extremely low emission efficiency. Although light emission in quantum-confined silicon at sub-10 nm lengthscales has been demonstrated, there are difficulties in integrating quantum structures with conventional electronics. It is desirable to develop new concepts to obtain emission from silicon at lengthscales compatible with current electronic devices (30-100 nm), and therefore cannot use quantum-confinement effects. We will demonstrate an entirely new method to achieve bright visible light emission in “bulk-sized” silicon coupled with plasmon nanocavities from non-thermalized carrier recombination via Purcell enhancement. Highly enhanced emission quantum efficiency (>1%) in plasmonic silicon, along with its size compatibility with present silicon electronics, provides new avenues for developing monolithically integrated light-sources on conventional microchips.

We report several methods to transfer silicon photonic devices, including waveguide circuits and high-Q cavities, from the traditional silicon-on-insulator substrates to a variety of new substrate materials, including polymeric films and chalcogenide glasses. The optical performances of the devices are unaffected by the transfer processes. The novel silicon-on-anything photonic systems will enable new applications in flexible and tunable photonic devices and mid-infrared silicon photonics for chemical sensing and optical communication.

Photonic resonant structures have been used to improve modal properties of broad area semiconductor lasers and obtain integrated beam combining of these lasers in recent years. For a single emitter, it has been demonstrated that the width of single-mode semiconductor laser can be increased by at least two orders of magnitude using the transverse Bragg reflection of two dimensional periodic nanostructures. While for a laser array, the Bragg diffraction can be also used to obtain integrated coherent beam combining. In this talk, we will first describe a photonic crystal structure to obtain the single mode operation of large-area, edge-emitting semiconductor lasers. Pulsed and CW operation of electrically pumped, single-mode photonic crystal broad area lasers with single-lobe, diffraction-limited far-fields are experimentally demonstrated at room temperature. A wavelength tuning sensitivity 80 times smaller than a conventional DFB laser is also achieved for the photonic crystal Bragg laser. We then discuss a novel integrated laser architecture in which Bragg diffraction is used to realize simultaneous modal control and coherent combining of broad-area diode lasers. Our experimental results show that two 100mu;m wide, 1.3mm long InP broad-area lasers provide near-diffraction-limited output beam and are coherently combined at the same time without any external optical components. Furthermore, our design can be expanded to a coherently combined broad-area laser array that turns a laser bar into a coherent single mode laser with diffraction-limited beam quality.

Plasmonic and metamaterials have introduced tremendous research interest within last decade and has become an increasingly important field in nanophotonics. The extraordinary optical properties of plasmonic structures and metamaterials have opened up a variety of novel applications such as super resolution imaging, deep-subwavelength waveguides, ultrasensitive sensors and etc. In this talk I will review some of our recent work on active and passive control of light by using plasmonics and metamaterials. Plasmonic structure illumination microscopy, combining structured illumination technique and surface plasmon interference, represents a unique approach for high speed super resolution optical imaging. Our most recent experimental results will be presented with resolution improvement factor ~3 compared to that of conventional microscope. Metalens, comprising metal-insulator-metal waveguides, are designed to perform super resolution focusing and Fourier transform. Such lens can also act as “Janus lens” to break the forward backward imaging symmetry and the detailed imaging characteristics will be discussed. An integrated plasmonic OLED structure will also be introduced for super contrast biomedical imaging. Other topics such as plasmonic enhanced high efficiency high speed LEDs, dispersion engineering in compound lenses, and super resolution optical lithography will also be briefly discussed.

Organic solar cells (OSCs) have been a highly interesting field in recent years, as they have a strong potential to realize low cost solar cells which are highly portable and deployable due to their flexibility and light weight. Compared with inorganic solar cells, PSCs usually suffers from the insufficient light absorption due to the thin active layer restricted by the short exciton diffusion length and low carrier mobilities. Attempts to optimize both the optical and electrical properties of the photoactive layer in OSCs inevitably result in a demand to develop a device architecture that can enable efficient optical absorption in films thinner than optical absorption length [1,2,3]. Here, we report the use of dual metallic nanostructures to achieve the broad light absorption enhancement, increased short-circuit circuit (Jsc) and improved fill factor (FF) simultaneously based on a small-bandgap polymer donor of poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b&’]dithiophene-2,6-diyl]-alt-[2-(2&’-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) in BHJ cells [4]. The double metallic nanostructure consists of 2D arrays of metallic nanograting electrode as a back reflector and the metallic nanoparticles (NPs) embedded into the active layer. Apart from the waveguide modes and diffractions, we simultaneously introduce hybridized surface plasmonic resonances (from Ag nanograting) and localized plasmonic resonances (from Au NPs) to successfully achieve a broadband absorption enhancement. The detail understanding has been described with our theoretically studies. Consequently, we improve PCE to reach 8.79%[5] by improving both optical properties and electrical properties of OSCs through introducing dual plasmonic nanostructures which contribute to the practical application of OSCs for photovoltaics.
[1] H. A. Atwater, A. Polman, Nat. Mater. 2010, 9, 205.
[2] J. You, X. Li, F.X. Xie, W.E.I. Sha, J.H.W. Kwong, G. Li, W.C.H. Choy, and Y. Yang, Adv. Energy Mat., DOI:10.1002-aenm.201200108.
[3] X.H. Li, W. E.I. Sha, W.C.H. Choy, D.D.S. Fung, and F. X. Xie, J. of Phys. Chem. C, 10, 1021, 2012.
[4] L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J. Hou, Angew. Chem. Int. Ed. 2011, 50, 9697.
[5] X. Li, W.C.H. Choy, L. Huo, F. Xie, W.E.I. Sha, B. Ding, X. Guo, Y. Li, J. Hou, J. You, and Y. Yang, Adv. Mater., 2012. 24, 3046.

The magneto-optical activity that results from the interaction of polarized light with magnetized media gives way to an induced rotation and ellipticity of the light polarization. These phenomena afford the basis for potentially novel devices in data control of optical communications/data storage and sensing. This has, in turn, spurred the research on new materials exhibiting large magneto-optical responses at the operating wavelengths. The strategies towards magneto-optical enhancement are essentially based on the dramatic intensification of the light-matter interaction when media are intentionally nanostructured to couple resonantly with photons of certain wavelengths.
In previous works, we have demonstrated the efficiency of coupling magneto-optics to photonic band-edge effects in magnetophotonic crystals [1, 2], where at frequencies close to the band edges the group velocity of light is dramatically slowed down and, therefore, photons of those wavelengths couple very intensively with the medium. Exploiting this mechanism, we have achieved enhanced magneto-optical responses at near-band edge wavelengths in three-dimensional magneto-photonic crystals (3D-MPCs) [1, 2]. Here we envisage an alternative strategy to boost magneto-optic signals by coupling to plasmonic resonances. Thus, the incorporation of magnetic nanoparticles into plasmonic structures provides an alternative pathway to modulate the magneto-optical spectra and enhance the response at specific wavelengths. In this case, we have exploited the huge increase of the electromagnetic energy density associated with plasmons that are excited in extremely confined regions around metal/dielectric interfaces. With this in mind, we have coated corrugated gold/dielectric interfaces with magnetic (nickel and iron oxide) nanoparticles [3]. We have found that the magneto-optical spectra at visible wavelengths are strongly modified when the magnetic nanoparticles are incorporated into plasmonic structures formed either by Au voids or Au nanodisk arrays. In particular, we find that the magneto-optical activity is enhanced by up to around one order of magnitude for wavelengths that are correlated to the excitation of either propagating or localized surface plasmons. In addition, we demonstrate that this strong magneto-optical activity is not merely the result of the reflectance modification associated to diagonal terms of the permittivity tensor, but to an intrinsic enhancement of the optical activity related to the off-diagonal permittivity coefficients. Our results demonstrate the potential of exploiting light polarization in plasmonic and photonic structures as a powerful strategy to customize the magneto-optical spectral response of magnetic materials and to obtain optimized materials for applications such as sensing or optical communications.
[1] J.M. Caicedo et al., ACS Nano, 5 (2011) 2957.
[2] O. Pascu et al., Nanoscale, 3 (2011) 4811.
[3] M. Rubio-Roy et al., Langmuir, 28 (2012) 9010.

We study surface plasmon mediated oxidation of hydrazine for ambient temperature growth of nitride semiconductor films. Conventional methods for growing nitride films require high temperatures (greater than 700 K) in order to dissociate high-energy nitrogen bonds in precursor molecules such as ammonia and hydrazine. Alternatively, localization of ultraviolet radiation via surface plasmons can provide high concentration of optical energy for resonant dissociation of nitrogen bonds in these precursor molecules. Therefore, UV surface plasmon induced photochemistry can provide a new method for deposition of nitride materials at ambient temperature.
We have performed numerical simulations and experimental characterization of aluminum nanostructures optimized for hydrazine decomposition at 248 nm. Al is studied because it exhibits a surface plasmon in the deep UV, between 100 nm and 300nm, and is about eight times less lossy at 200 nm compared with Au, for example. Full wave simulations (FDTD method) were used to numerically study properties of the surface plasmon in Al nanostructures. Simulations vary the height and pitch of Al gratings for a plasmonic resonance at 248nm. We observe a near-field enhancement factor up to 25x in optimized designs. UV reflection and transmission characterization of nanofabricated Al devices show absorption maximum consistent with simulations.
We also study the efficiency of hydrazine decomposition via excitation of the UV surface plasmon with the test structures under vacuum. A pulsed 10 W m^(-2) laser at 248nm was used to couple light into surface plasmon of the Al test structures after a layer of hydrazine was cryogenically adsorbed to the surface. Gas phase analysis with mass spectrometry show increases of the decomposition products of hydrazine from plasmonic structures after UV excitation compared with control substrates. Our results indicate that surface plasmon mediated oxidization of hydrazine is a promising route to enable ambient temperature growth of nitride semiconductor films.

Several recent reports have demonstrated shifts of the plasmon resonance frequency of metal nanostructures when an external electric field alters free carrier density in the metal. [1] A simple thermodynamic argument indicates that a related phenomenon, termed the ‘plasmoelectric effect&’, will induce changes of free carrier density in metal nanostructures under narrowband excitation at a frequency near their plasmon resonances. This manifests itself as an electrochemical potential, i.e. a plasmoelectric potential, that can drive current through a circuit load. [2] Unlike the more familiar thermoelectric or photovoltaic effects, the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and the incident radiation. Radiation at higher frequencies induces an increase of electron density in the nanostructure that blue-shifts the plasmon resonance. This response is entropically favored due to the increased heat that accompanies the increased absorption. Similarly, radiation at lower frequencies decreases electron density in the nanostructure to induce a red-shift of the absorption maximum.
We report the results of experimental tests of these predictions from the electrical and optical response of colloids of monodisperse Au or Ag nanoparticles spin-cast on ITO films. Scanning Kelvin probe force microscopy (KPFM) provides nanoscale resolution of the surface potential of device structures while varying the frequency of incident radiation near the plasmon resonance. Under 1 W cm^-2 single-frequency illumination, we measure induced potentials of ± 15 mV from 60 nm Au particles, with a characteristic sign change in the electronic charge as the illumination frequency is scanned from the blue side to the red side of the particle absorption maximum, near 520 nm. In accord with theory, the magnitude of induced potentials scales with particle surface density and illumination intensity. Additionally, power-dependent and frequency-dependent increases of optical absorption from samples under monochromatic illumination indicate shifts of the plasmon resonance when compared with the spectral response of samples under white light illumination. We report optically induced absorption increases of up to 3% under 2 mW cm^-2 monochromatic excitation. We observe clear evidence for the theoretically predicted trends in the particle size-dependence and frequency-dependence of the electrochemical potential and plasmonic absorption. Our results provide deeper insight into the underlying mechanism of the plasmoelectric effect and highlight potential applications for this new optoelectronic phenomenon.
[1] Dondapati, et al. (2012). Voltage-Induced Adsorbate Damping of Single Gold Nanorod Plasmons in Aqueous Solution. Nano Letters, 12(3), 1247-1252.
[2] Sheldon et al. (2012) The Plasmoelectric Effect: Optically Induced Electrochemical Potentials in Resonant Metallic Structures. arXiv:1202.0301

In recent years, engineered light-matter interactions at the nanoscale have enhanced the sensitivity of spectroscopic techniques such as Surface Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Infra-Red Absorption spectroscopy (SEIRA). Nanostructured surfaces and nanoparticles supporting strong optical-frequency electric resonances have allowed SERS and SEIRA to reach single molecule and attomolar sensitivity, respectively. In this presentation, we draw on insights from SERS and SEIRA to investigate the chiral counter-part of this problem: enhancing Circular Dichroism (CD) spectroscopy using optical nanoantennas.

We begin our study by tackling the problem analytically in the dipolar limit. Our model predicts that isotropic nanostructures supporting optical-frequency electric or magnetic dipoles are sufficient to enhance locally the excitation of a molecule's chiral polarizability and thus its circular dichroism spectrum. However, simultaneous electric and magnetic dipoles are necessary to achieve a net, spatially averaged enhancement. In order to quantify our analytical predictions, we use the Boundary Element Method (BEM) to rigorously solve for the CD enhancement factors on the surroundings on different optical antennas. We first study the response of an electrically resonant 10 nm radius silver sphere. As predicted by the analytical model, the plasmonic response of the particle permits enhancement of the CD signal of a chiral molecule by a factor of 6 at the top and the bottom of the particle. However, averaging the CD enhancement factor on a surface surrounding the sphere yields no net enhancement. This situation is modified when considering an isotropic antenna supporting both electric and magnetic resonances. As a case study, we investigate the ability of a 75nm Si sphere to enhance chiral light matter interactions in its surroundings. Our simulations indicate that this particular system can enhance CD spectroscopy both locally and on average by nearly one order of magnitude.

Our results indicate that neither plasmonic resonances nor structural chirality are required to enhance CD spectroscopy in the presence of nanoscale objects. Our contribution provides a theoretical framework to understand chiral light-matter interactions at the nanoscale and sets the basic design rules to achieve field enhanced circular dichroism spectroscopy in the presence of nanoantennas. These results may trigger the development of new field-enhanced technologies such as surface enhanced CD or VCD spectroscopy. Such techniques would permit the sensing of chiral molecules with higher sensitivity and enable studies of protein-folding dynamics on lower, more biologically-relevant concentrations.

Electric fields are generally thought to dominate light-matter interactions. According to the semi-classical Bohr model, electric dipole transitions are approximately ten thousand times stronger than magnetic dipole transitions, and thus the interaction of the magnetic field of light with most natural materials can be neglected. Recently, however, progress in the fields of plasmonics and metamaterials has illuminated the importance of magnetic light-matter interactions. Conducting nanostructures have been engineered to support optical frequency current loops, but these magnetic resonances are often at near-infrared and lower energies.
In this presentation, we theoretically and experimentally explore visible frequency magnetic fields in a plasmonic nanoparticle. The plasmonic structure under study is the three-dimensional analogue of the split-ring-resonator: the metal-dielectric nanocrescent. This asymmetric core-shell structure supports multiple resonances, each consisting of degenerate electric and magnetic character. We demonstrate that these resonances can be tuned across visible and near-infrared frequencies through variations in the size and shape of the nanocrescent. We theoretically predict electric and magnetic field enhancements as large as 100 throughout the core of the particle, and calculate enhancement of the spontaneous emission of electric and magnetic dipoles as high as 500 in the visible spectral regime.
Through an angled and rotating electron-beam evaporation procedure, we fabricate nanocrescents with dielectric cores and noble metal shells with diameters ranging from 50 to 300 nm. Using polarization-sensitive and angle-resolved cathodoluminescence spectroscopy, we map the local density of optical states of the crescent and identify multiple electric and magnetic resonances throughout the visible spectrum, in agreement with the theoretical predictions. Next, we probe the magnetic near-fields of nanocrescents with time-resolved fluorescence of embedded magnetic dipole emitters. To do so, we use solution-based techniques to synthesize Eu3+-doped cubic-phase NaYF4 spherical nanoparticles for the dielectric cores of nanocrescents. These particles exhibit the purely magnetic 5D0 to 7F1 transition of Eu3+ at 590 nm, along with the purely electric 5D0 to 7F2 transition at 610 nm. By observing the tuned emission of Eu3+ inside the nanocrescent core with time-resolved fluorescence spectroscopy, we explore the effect of the nanocrescent resonances on the rate of spontaneous emission of the magnetic dipole. Our results demonstrate the use of a magnetic dipole emitter as a probe of the magnetic local density of optical states in a plasmonic nanostructure, and, more generally, illustrate the importance of the magnetic component of light-matter interactions.

In a conventional flat plate solar cell under direct sunlight, light is received from the solar disk, but is re-emitted isotropically. This isotropic emission corresponds to a significant entropy increase in the solar cell, with a corresponding drop in efficiency. Using a detailed balance approach with wave-optical models for ultrathin solar cells, we show that limiting the emission angle of a high quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38% with a single junction. The highest efficiencies are predicted for a thin, light trapping cell with an ideal back reflector. However the scheme is robust to a non-ideal back reflector, with efficiencies above 35% achievable. Furthermore, due to enhanced light trapping with this approach, full absorption can be achieved in a GaAs cell that is only 50nm thick. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells, but also for higher overall efficiencies. [1]
As an initial proof of principle, we have designed a wavelength-scale dielectric layered structure to limit the emission angle for wavelengths where radiative emission occurs. While this structure provides limited light trapping enhancements, it is suitable for experimental demonstration of enhanced voltage due to photon recycling in a conventional, flat plate GaAs cell with high radiative efficiency and limited emission angle. This structure, with total thickness of about 2 microns, consists of alternating layers of silicon dioxide and titanium dioxide along with an anti-reflective coating of alumina and magnesium fluoride. The sputtered dielectric films are characterized via x-ray reflectometry and spectroscopic ellipsometry. While based on a traditional Bragg stack, the layer thicknesses have been optimized to enhance transmission, such that it is not purely periodic. Transfer matrix method calculations indicate >90% transmission from 700-900nm at normal incidence with angle restriction to 16 degrees at maximum photoluminescence intensity in our initial design. Furthermore, based on the measured open circuit voltage of the conventional, flat plate GaAs solar cells, we have found an external radiative efficiency of 35% which we use in a realistic detailed balance model that also accounts for proprietary details of the solar cell and the measured photoluminescence spectrum. Using this model with transfer matrix method results for our initial optical design, we predict an open circuit voltage increase of 5.6mV with no change in current, allowing for the first experimental demonstration of increased voltage due to photon recycling with limited emission angle.
[1] Kosten et. al. Light: Science & Applications. (in press)

Controlling phase pickup of light with optical resonators with deep subwavelength dimensions could play a key role in shrinking the dimensions of optical devices. Various nanolenses and metasurfaces based on plasmonic structures have been demonstrated with the ability to manipulate the phase front of incident light within a thin layer, while the possibility of employing dielectric optical antennas to control the phase of light have remained relatively unexplored. Dielectric optical antennas based on silicon (Si) nanostructures can support leaky mode resonances, which can confine light within subwavelength, high-refractive-index nanostructures. Here, we show that the phase of light scattering from Si nanostructures varies rapidly across their optical resonances, and the phase pickup can be engineered by varying the size and shape of Si nanostructures. Also we demonstrate a nanophotonic planar structure composed of a pair of Si nanoparticles of asymmetric sizes that pick up different phase of scattering, and the interference effects give rise to directional scattering in a wavelength-dependant fashion and therefore color splitting. The dielectric optical antennas can avoid the intrinsic losses in metals, and the planar Si nanostructures can be easily fabricated by standard nanopatterning techniques. The ability to engineer the phase front of light at the subwavelength scale through a thin planar Si structure provides novel approach for light steering, planar lenses, polarizer and light trapping.

Metamaterials are artificial composite materials whose extraordinary electromagnetic (EM) properties are engineered by their subwavelength structures, geometries and materials. Recently, self-assembly techniques have opened new routes for the fabrication of optical metamaterials due to the extraordinary capacities of the chemical synthesis of nanoparticles. However, the imperfect control of the size of synthetic nano-objects and the structural symmetry by self-assembly remains great challenging for the desired metamaterial properties. Here, we report a self-selective assembly method to control the structural symmetries and metamaterial properties. Structural symmetries are controlled by interfacial self-assembly of coupled gold nanorods. Metamaterial responses exhibiting optical isotropic electric and magnetic modes are achieved by selectively disassembly process using plasmonic responses as symmetry-based feed-back controls. Our method introduces a paradigm shift for scalable on-demand manufacturing of functional metamaterials and can be generalized to other functional 3D nanostructures.

In the emerging field of thermoplasmonics, Joule heating associated with optically resonant plasmonic structures is exploited to generate nanoscale thermal hotspots. The ability to control and probe thermal processes at the nanoscale has opened the door for several promising applications in medicine, chemical catalysis, and data storage. In the present study, new methods for designing and thermally probing thermoplasmonic structures are reported. A general design rationale, based on Babinet&’s principle, is developed for understanding how the complementary version of ideal electromagnetic antennas can yield efficient nanoscale heat sources with maximized current density. Using this methodology, we show that diabolo antennas are much more suitable for heat generation compared with their more well-known complimentary structure, the bow-tie antenna. We also develop a new thermal microscopy method based on the temperature dependent photoluminescence lifetime of thin-film thermographic phosphors to experimentally characterize the thermal response of various antenna designs. Data from FDTD simulations and the experimental temperature measurements are used to confirm the validity of the design rationale. It is also shown that the thermal microscopy technique, with its robust sensing method, could overcome some of the drawbacks of current micro/nanoscale temperature measurement schemes.

Photonic microcavities gained much attention due to their rather high quality factors (Q factors) and their small mode volume [1]. Recently, also plasmonic resonators came into focus due to their potential for, e.g., sensing applications or photovoltaic devices [2,3,4]. Surface-plasmon-resonators usually suffer from relatively small Q factors below 100, however, using a microdisc resonator, Min et al. demonstrated experimentally that the Q factors of whispering-gallery surface plasmon modes are larger than 1000 [5]. Furthermore, Xiao et al. theoretically investigated a toroidal whispering-gallery microcavity of silica coated with silver which supports, assisted by surface plasmons, a strongly localized mode on the outer surface with high Q factors of up to 1000 [6].
In this work we experimentally demonstrate that a silver-coated dielectric bottle resonator exhibits hybrid photon-plasmon modes with high Q factors of up to 1000. The mode spectrum is excited by Rhodamine 800 dye molecules embedded in the bottle resonator and detected in a micro-photoluminescence setup. Our measurements, supported by sophisticated three-dimensional finite-difference time-domain simulations, prove the existence of hybrid modes which feature a dielectric whispering-gallery-mode-like field distribution inside the resonator and a strongly localized plasmon-mode-like field distribution on the outer silver surface. Our structures combine the high Q factors of dielectric resonators and the strong field enhancement by surface plasmons at the resonator surface, which makes them particularly interesting for sensing applications.
We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the Graduiertenkolleg 1286.
[1] K. J. Vahala., Nature 424, 839 (2003).
[2] A. Polman, Science 322, 868 (2008).
[3] A. Rottler et al., Opt. Lett. 36, 1240 (2011).
[4] H. A. Atwater and A. Polman, Nat. Mater. 9, 205 (2010).
[5] B. Min et al., Nature 457, 455 (2009).
[6] Y.-F. Xiao et al., Phys. Rev. Lett. 105, 153902 (2010).

As Surface plasmons (SPP) can be confined to small nanostructure sized areas - much smaller than the wavelength of the exciting free space photons - they are thought to combine the benefits of photonics and microelectronics, i.e. high speed and small dimensions, in future integrated devices. To take advantage of this feature it is necessary to control the propagation of SPPs. A recent approach is tailoring the SPPs effective refractive index using metamaterials or nanostructured dielectrics placed at the surface of the film carrying the SPPs [1, 2]. First results based on this technique were presented in [3] for a plasmonic Lüneburg lens, i.e. a lens with a focal spot at its outer perimeter, using Fluorescence imaging and leakage radiation microscopy.
In this work we investigate the interaction of SPPs with a plasmonic Lüneburg lens in the near field using near field scanning optical microscopy. Gray-scale electron beam lithography is used to prepare a dome-shaped resist structure with a diameter of 19 µm and a center height of 300 nm on-top of a gold film to obtain the effective index profile of a Lüneburg lens for SPPs propagating at the film surface at 1.71 eV. Next to the Lüneburg lens (3 µm from the outer perimeter) a grating coupler (a = 400 nm) for the excitation of SPPs is milled into the gold film with focused ion beams. The SPPs are launched to propagate through the lens and the near field pattern is scanned. We clearly identify a focal spot in the near field signal at the outer perimeter of the lens. In addition, we observe a beating pattern arising from further plasmon waves excited by higher orders of the grating coupler. The measured near field pattern could very well be modeled with finite element methods as well as with an analytical model using the effective refractive index approach.
We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the Graduiertenkolleg 1286.
[1] P. A. Huidobro et al., Nano Letters, vol. 10, issue 6, pp. 1985-1990 (2010)
[2] Y. Liu et al., Nano Letters, vol. 10, issue 6, pp. 1991-1997 (2010).
[3] T. Zentgraf et al., Nature Nanotechnology, vol. 6, issue 3, pp. 151-155 (2011).

Herein, we demonstrate how surface plasmon polariton optics or plasmonics can improve the performance of highly efficient dyes to be likely employed in solid-state lighting (SSL). We make use of large area arrays of aluminum nanoantennas fabricated by an imprint lithography technology that sustain collective plasmonic resonances. This enables to shape the angular pattern of the emission, beaming most of the light into a very narrow angular range in a defined direction. The enhancement for unpolarized emission reaches a factor of 60 at certain frequency in the forward direction and a factor of 14 when integrated over all the emission range of the dye. This behaviour is the result of the emission of the dye into collective plasmonic resonances known as surface lattice resonances (SLRs) that arise from the coupling of localized surface plasmon polaritons to diffracted orders in the array [1,2]. SLRs have a large spatial extension [3] and can couple very efficiently to free space radiation due to their hybrid photonic-plasmonic character. These features lead to the highly directional emission in defined directions [4].The possibility to tune the dispersion of SLRs by varying the shape and dimensions of the nanoparticles, the lattice structure and the period of the array, opens the possibility to fully control the emission of different light emitters integrated in plasmonic-based LEDs. In addition, we have investigated the combination of these structures with high power standard blue LED sources, showing that the plasmonic structure acts as an integrated optical component to shape the emission pattern of the dye layer. Plasmonics provides a reliable platform for state-of-the-art lighting applications. These results open a new path for fundamental and applied research in SSL, wherein plasmonic nanostructures can mould the spectral and angular distribution of the emission with unprecedented precision.
References
[1] G. Vecchi et al., “Shaping the fluorescence emission by lattice resonances in plasmonic crystals of nanoantennas” Phys. Rev. Lett. 102, 146807, 2009.
[2] V. Giannini et al., “Lighting up multipolar surface plasmon polaritons by collective resonances in arrays of nanoantennas” Phys. Rev. Lett. 105, 266801, 2010.
[3] G. Vecchi et al., “Surface modes in plasmonic crystals induced by diffractive coupling of nanoantennas”, Phys. Rev. B 80, 201401, 2009.
[4] S. R. K. Rodriguez et al., “Quantum rod emission coupled to plasmonic lattice resonances: A collective directional source of polarized light”, Appl. Phys. Lett. 100, 111103, 2012.

Metal nanowire networks have emerged as promising alternatives for transparent conducting oxides used in flexible electronics. However, these composite electrode materials tend to suffer from large surface resistance due to the narrow contact area at the nanowire-nanowire junctions in the network. We present a direct-write laser process which takes advantage of plasmonic resonance in silver nanowires to weld junctions and increase the contact area. This resonant laser process efficiently transfers optical energy to plasmonic modes in the silver nanowires, allowing the localization of energy to the junctions while preserving the organic and other temperature sensitive substrates. We investigate the plasmonic effects of this laser processing through FDTD simulations, and verify these findings with SEM and TEM microscopy of silver nanowires exposed to varying amounts of laser radiation. We also apply the FDTD simulations to better understand the role that the silver nanostructures have in the bulk optical transparency, exploring the effect of plasmonic mediated optical transmission in the film. The laser processing of silver nanowire networks results in lower macroscopic sheet resistance of the electrode by an order of magnitude while preserving the optical transparency. Finally, we verify the process by depositing the silver nanowires on an otherwise complete hybrid organic photovoltaic cell. We are able to successfully laser process the nanowire electrode on top of the organic layer and perform I-V characterization of the device which shows enhanced fill factor and efficiency compared to a standard grid electrode.

We present two different vantage points to explain very effective light absorption in a thin semiconductor film on the top of a judiciously chosen metal substrate with a finite conductivity. First, we consider Fabry-Perot resonator model for a layered air/semiconductor/metal system with light incident from the air region. Incident light is firstly transmitted from air to the semiconductor layer. After propagating to the bottom of the semiconductor layer, it is bounced back at the semiconductor/metal interface. There is a reflection phase pickup. If the metal has an infinite conductivity, this reflection phase pickup is π. In the optical regime, however, metals exhibit a finite conductivity and the reflection phase pickup becomes smaller than π. At the semiconductor/air interface, the reflection phase is near zero. The minimum reflection takes place when the total round-tip phase is an integer multiple of 2π.
To verify the suppressed reflection, the full-field distribution is calculated by using the transfer matrix method. In addition to the reflection coefficient, this full-field simulation provides us with the intensity of the electromagnetic field distribution in each layer, allowing for the rigorous analysis for the absorption in the semiconductor layer and the metal substrate separately. It turns out that the semiconductor thickness for the enhanced absorption is in good agreement with that expected from the reflection minimum in the resonance condition. This result demonstrates that the enhanced absorption indeed comes from the coupling of incident light into the resonance mode formed in the semiconductor film.
Next we analyze the absorption behavior in terms of allowed leaky modes supported by this system. Their properties can be examined by solving the characteristic equation of the guided mode in the air/semiconductor/metal layer. The mode analysis shows that the resonance frequency coincides well with that from the reflection phase analysis and the transfer matrix method above. It can hence be inferred that the energy of incident light is coupled to the leaky mode in the multilayer waveguide and undergoes absorption in the semiconductor layer.
In conclusion, the significantly enhanced absorption in the thin semiconductor layer on the top of the metal substrate is presented and understood in two distinct pictures that provide valuable insights into the nature of the absorption enhancement. The full-field simulation and the mode analysis reveal that the most of the energy is actually absorbed in the semiconductor layer by the resonant coupling of incident light into the leaky mode. We believe that the proposed method can open a new way to implement very efficient and ultrathin photodetectors.

Over the last several decades, innovative light-harvesting devices have evolved to achieve high efficiency in solar energy transfer. Research on the mechanisms for plasmon resonance is very desirable to overcome the conventional efficiency limits of photovoltaics. The influence of localized surface plasmon resonance on hot electron flow at a metal-semiconductor interface was observed with a Schottky diode composed of a thin silver layer on TiO2. The photocurrent is generated by absorption of photons when electrons have enough energy to travel over the Schottky barrier and into the titanium oxide conduction band. The correlation between the hot electrons and the surface plasmon is confirmed by matching the range of peaks between the incident photons to current conversion efficiency (IPCE, flux of collected electrons per flux of incident photons) and UV-Vis spectra. The photocurrent measured on Ag/TiO2 exhibited surface plasmon peaks; whereas, in contrast to the Au/TiO2, a continuous Au thin film doesn&’t exhibit surface plasmon peaks. We modified the thickness and morphology of a continuous Ag layer by electron beam evaporation deposition and heating under gas conditions and found that the morphological change and thickness of the Ag film are key factors in controlling the peak position of light absorption.

Sculpted electromagnetic beams can serve as optical tweezers, allowing small objects to be accelerated, manipulated, or trapped with light alone. In the subwavelength regime, plasmonic optical traps provide the key to overcome the limitations of conventional optical traps that arise from the diffraction limit and the high power levels required for trapping in this regime.
In this work, we theoretically and experimentally investigate plasmonic coaxial apertures as low-power optical traps for nano-sized specimens. When a particle interacts with the near field of a coaxial aperture illuminated with a linearly polarized plane wave, it experiences an optical trapping force. Unlike many prior plasmonic traps, the proposed coaxial structure traps particles at the surface of the aperture rather than inside it. Consequently, further manipulation and processing can be performed on the trapped particle. Theoretically, we systematically examine this induced optical force using Maxwell stress tensor formalism and finite-difference time-domain simulations. The coaxial aperture we consider consists of a semi-infinite, 150-nm-thick silver slab with an embedded 25nm-thick silica ring to form a coaxial aperture with inner and outer radii of 60 nm and 85 nm respectively. Illuminating this aperture with plane wave from the backside produces two hot spots at the output side of the aperture where the targeted particle is trapped. We show that the induced trapping potential at these hot spots can stably trap dielectric particles smaller than 10 nm in diameter while keeping the trapping power level below 20 mW. By tapering the thickness of the coaxial dielectric channel, trapping can be extended to sub-2 nm particles. Our results also indicate that the trapped particle experiences a pulling force of the order of tens of pico-newton which can be utilized to apply localized forces on micro and nanoscale objects with nanometer precision. Experimentally, we investigate arrays of coaxial aperture traps in 100 nm Au films, fabricated using focused ion beam milling. The coaxial apertures were integrated in a flow cell where fluorescent nanoparticles flow over the aperture and are selectively trapped at the resonant wavelengths of the aperture array around 680 nm. Additional 488 nm laser source is used to illuminate the fluorescent particles. The typical sizes of the dye-doped polystyrene particles used in the trapping experiment are 40 nm and 20 nm. Our results demonstrate the potential of coaxial plasmonic apertures in enabling optical trapping and manipulation of dielectric particles with sizes previously inaccessible.

Metamaterials are artificial composites consisting of array of periodic sub-wavelength resonator structures. They can modify the electromagnetic waves in unique ways not achieved by naturally occurring materials. Until now most of the metamaterials functionality is demonstrated within a narrow spectral frequency range. However, in many cases it would be desirable to adapt the metamaterial to its environment by tuning its frequency. In this work, the electromagnetic resonance of the metamaterials is controlled by substrate deformation. Further, the resonance Q factor of metamaterials is improved by integrating an interdigitated structure to the design.
The metamaterials fabricated on a flexible substrate modifies the inter-cell capacitance when mechanically strained, and hence shifts the resonance frequency of resonators. Electromagnetic device design and flexible electronics fabrication techniques are combined to demonstrate such mechanically tunable metamaterials operating at terahertz frequencies. Two different I-shaped resonator design, with and without interdigitated structure is studied. The I-1 design (without interdigitated structure) and I-2 design (with interdigitated structure) are comprised of a planar array of resonators micro-fabricated on a highly elastic polydimethylsiloxane (PDMS) substrate. These resonator designs were simulated and experimentally verified for uniaxially stretching.
When the resonators where mechanically strained, the I-1 design resonator shows a resonance frequency shift of 30 GHz, whereas the I-2 design with interdigitated structure shows a further improvement of resonance frequency shift by 10% (upto 40 GHz). The I-2 design exhibits higher sensitivity to applied mechanical strain compared to the I-1 design. The robustness of the fabricated metmaterials was verified by repeated stretching and relaxing cycles. In our experiment greater than 8% of the tuning range was achieved with good repeatability. A large continuous tunabality of the frequency can be achieved by utilizing resonant structures on elastomeric substrates. This study promises applications of elastomeric metamaterials in remote strain sensing and other mechanically controllable metamaterial-based devices.

Controlling the dispersion and directionality of the emission of nanosources is one of the major goals of nanophotonics research. One of the ways to achieve this control is to couple the emission to Bloch modes of periodic structures. Recently, it has been shown that arrays of bottom-up grown semiconductor nanowires can form a 2D photonic crystal.^[1] The light emission of a source localized in each nanowire, governed by the Bloch modes of the photonic crystal, was recorded by scanning each direction of interest. Here, we present the first measurements of light emission from nanowire photonic crystals by using Fourier microscopy. With this technique we efficiently collect the directional emission of nanowires within the numerical aperture of a microscope objective. We also demonstrate that the directionality of the emission can be easily controlled by infiltrating the photonic crystal with a high refractive index liquid.^[2] This work opens new possibilities for the control of the emission of nanosources that could lead to development of novel light emitting devices.
[1] S. L. Diedenhofen, O. T. A. Janssen, M. Hocevar, A. Pierret, E. P. A. M. Bakkers, H. P. Urbach, J. Goacute;mez Rivas, ACS Nano 5, 5830 (2011)
[2] Y. Fontana, G.Grzela, E. P. A. M. Bakkers, J. Goacute;mez Rivas, submitted (2012)

Advanced photon management using nanostructures provides exciting opportunities for enhancing optoelectronic device performance. In this talk, I will present two examples of photon management at the nanoscale: nanocone solar cells and metal nanowire transparent conducting electrodes. Nanocone solar cells are simple structures combing an efficient antireflection and light trapping across a broad band of spectra and over a wide range of incident angles while enhancing the charge carrier collection and maintaining low dark current. Using amorphous Si, we demonstrate high power efficiency for both substrate and superstrate configurations. We also extend this concept to ultrathin single-crystal Si solar cells. In the second example, I will present novel metal nanowire networks as transparent conducting electrodes to replace the existing indium tin oxides. Metal nanowires with diameters smaller than and with separations larger than the wavelength of the light can allow the sunlight pass through without significant reflection or scattering back. We show that these metal nanowire networks provide high optical transmittance at very low sheet resistance.

We report on a design and fabrication of thin film silicon solar cells with polarization-independent, angle-insensitive, broadband spectral response by direct coupling of incoming light to the resonant modes of subwavelength-scale Mie nanoresonators incorporated into the thin film crystalline silicon active layer. Our prototype structures consist of a lithographically-patterned two-dimensional periodic array of 150 nm thick Si nanoresonators on silica substrates. A crossed trapezoid shape[1] of rectangular cross section absorbers is used to excite broadband Mie resonances across the visible spectra to achieve broadband and polarization-independent light absorption.
Full-field electromagnetic simulations were used to design parameters and maximize broadband absorption, with a 420% overall enhancement relative to planar 220 nm thick Si films. This design featured trapezoidal Si resonators with 200 nm and 300 nm long bases, 150 nm height and periodicity of 600 nm. (18.5 mA/cm2 for 220 nm thick Si film)
We have experimentally tested our predictions by optical absorption spectroscopy and photovoltaic spectral response measurements in planar and Mie nanoresonator-patterned 220 nm thick Si-on-insulator (SOI) film silicon solar cells. Nanoresonator patterned silicon thin film devices were fabricated on SOI wafers using electron beam lithography and reactive ion etching techniques after removal of the Si substrate. Angular-resolved reflection-transmission measurements were performed using an integrating sphere set-up. Photovoltaic spectral response measurements were made using lateral Schottky and p-i-n photodiodes fabricated using photolithography techniques. Optical absorption and spectral response measurements reveal that crossed trapezoidal Mie resonant structures give angle-insensitive broadband increase compared to planar and crossed rectangular structures, as predicted by full-field simulations.
1. Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Nat Commun 2, 517.

Light trapping in thin films solar cells using periodic metallic backreflectors has been extensively studied. It is now well established that a properly designed periodic or random pattern of metallic scatterers, integrated with the metallic back contact of the solar cell, can serve to efficiently scatter light to in-plane waveguide modes of the solar cell. This light trapping strongly enhances the red response of the cell and at the same time enables the fabrication of thinner solar cells without compromising on efficiency.
A major drawback of the metallic structures is that they also absorb a significant fraction of the incident light due to Ohmic dissipation, which reduces the light trapping effect. Here, we demonstrate a novel backscattering geometry that is entirely based on dielectric scatterers. The structures suffer no losses and can provide better light trapping than structures with metallic back scatterers.
Our work is based on thin-film amorphous Si solar cells on glass, made by sequential deposition of a Ag back contact, a ZnO buffer layer, the a-Si:H n-i-p active layer (350 nm thick) and an ITO transparent conducting top layer. In the new light trapping architecture the solar cell is built up from an unstructured planar metallic backreflector, on which a structured dielectric ZnO layer is made. The ZnO surface is structured into an array of dielectric nanohemispheres with diameters in the range 300-400 nm and a height of 150 nm. The active a-Si:H layer, grown by PE-CVD, and the ITO layer are conformally deposited over the patterned ZnO layer. Substrate-conformal imprint lithography in combination with ZnO deposition is used to make the dielectric nanostructures.

Numerical simulations show the ZnO nanoparticles act as resonant dielectric Mie scatterers in which light is trapped in a geometrical resonance and subsequently scattered into waveguide modes of the a-Si:H layer. As the resonant mode resides inside a dielectric structure it incurs no absorption losses, and all light trapped inside the cavity is scattered very efficiently into the a-Si:H layer. Simulations show the ZnO cavity has a scattering cross section that exceeds the geometrical cross section of the void over the complete 350-800 nm wavelength range, making this a broadband light trapping geometry.
In experiments we systematically compare devices in which only the ZnO is structured to devices in which only the Ag is structured or both the Ag and the ZnO are structured. Both flat and Asahi-type rough glass substrates are used as substrates, and chemical-mechanical polishing is used to flatten ZnO on some of the rough structures that are conformally grown onto Asahi glass. In this way a systematic comparison is made between different planar and patterned geometries. We present photocurrent measurements on all cell geometries to demonstrate the advantage of this novel back scattering light trapping concept.

Transparent conducting oxides (TCOs) such as indium tin oxide or zinc oxide play an important role in a variety of technologies for which large area electronic and optical access is required. In many applications, TCO films serve a dual role as a transparent coating and an electrical contact capable of extracting photocurrent. Despite their success, there is a growing interest in replacing these materials to reduce the cost of electrodes and to increase their mechanical flexibility and optical transmissivity. In this work, we show the feasibility of employing a nanostructured thin aluminum film as a transparent electrode on a silicon Schottky detector. We demonstrate improved photocurrents through polarization and spectrally resolved measurements for electrodes with different grating, grid, and fractal geometries. The optimized nanostructured electrodes can be readily fabricated via large-area, low-cost deposition techniques, such as metal sputtering and nano-imprint lithography.

Surface plasmon resonance is a promising route to spectrally extending and enhancing the photocatalytic activity of semiconductors. Most commonly used photocatalysts, such as TiO₂, absorb less than 5% of the solar spectrum due to large band gap energies. If photocatalysts are to be used for environmental remediation and water splitting the spectral range of photocatalytic activity must be extended into the visible and near infrared. Localized surface plasmon resonance (LSPR) describes the excitation of collective electron oscillations by incident light in smaller than wavelength metal nanostructures. The LSPR wavelength depends on the composition of the nanostructure as well as its environment, such that the LSPR peak can be tuned across the entire solar spectrum. The energy stored in the LSPR can be transferred to the semiconductor through the local electromagnetic field or through the direct transfer of hot electrons, allowing LSPR to act as a tunable photosensitizer.
In this presentation we have synthesized Au/Cu2O, Au/SiO₂/ Cu₂O, and Ag/ Cu₂O composite nanostructures to investigate the plasmonic enhancement of photocatalysis. The core shell structure allows for separation of possible metal self-catalysis effects from plasmon enhanced photocatalysis. Action spectrum measurements show that the metal/oxide photocatalytic activity follows the extinction spectrum of the LSPR. The Cu₂O core shell thickness is varied to demonstrate the spectral tuning of the plasmon enhanced photocatalysis. The plasmon enhanced charge separation mechanisms are investigated by transient absorption spectroscopy. The insulating SiO₂ interlayer and metal core compositions allow for the near field and charge transfer mechanisms to be separated and determined.

We show that the light trapping enhancement of solar cells, and thermal emission properties of nanophotonic structures, can be understood by analyzing the aggregate properties of optical resonances.

Traditionally, absorption in thin films has been considered constrained by the ray optics (or ergodic) limit first proposed and demonstrated by Yoblanovich in 1981. This limit fails to account for non-homogeneous structures in which the local density of states (LDOS) can vary greatly from that of a uniform absorber. Careful consideration of these effects leads to novel photovoltaic structures with enhanced absorption and minimized thickness. Calculations have shown that the LDOS of thin films on high index dielectric slabs can be enhanced by factors of 10 or more, and when plasmonic structures are used, 100 or 1000 fold LDOS enhancements are expected.
Here we present measurements that explicitly demonstrate clear deviation from the ergodic limit through absorption engineering by manipulation of the LDOS. Thin films of ITO (and separately, P3HT:PCBM) were deposited on various substrates including glass and GaP with scattering Lambertian reflectors. When the film thickness is large enough to support a continuum of modes, the ray optics limit was observed. However, the absorption in thinner films began to deviate from this limit. This is to be expected as the modes of the film become discrete and a larger fraction of the film&’s volume is in close proximity to the substrate. The observed absorption can be explained by a careful calculation of the LDOS and analysis of the light path. This approach utilized absorption measurements made using an integrating sphere and a monochromatic light source. The expected absorption was then calculated using a combination of ray optics and FDTD simulations. We believe that these results will lead to the realization of advanced photovoltaic structures and new approaches to antireflection coatings.

We present a new theoretical analysis on the fundamental limit of solar absorption in semiconductor materials. In stark contrast with the existing studies, which study the fundamental limit of solar absorption by using idealized material system with extremely weak and wavelength-independent absorption, we study the fundamental limit in real semiconductor materials of practical solar cells, including silicon, amorphous silicon, CdTe, and CIGS. Our study builds upon a theoretical model we have recently developed, coupled leaky mode theory (CLMT). The CLMT demonstrates that the light absorption of semiconductor structures is governed by the coupling of incident light with leaky modes of the structure, and that the absorption contributed by one single leaky mode is determined by the radiative loss of the leaky mode. Our study indicates that the maxima solar absorption depends on the number of intrinsic leaky modes in the structure. To approach this maximum fundamental limit, the key issue is to maximize the number of the leaky modes with radiative loss in a proper range.

Traditional light trapping strategies for bulk solar cells have relied on random, roughened surfaces to scatter the incident light into the internal modes of the device. More recent work on thin film solar cells has examined wavelength-scale periodic texturing. This can greatly enhance the local optical density of states (LDOS) and increase light matter interaction in the device, leading to enhanced absorption. However, periodic texturing limits the population of internal modes by momentum conservation, allowing only a small subset of modes to be populated at a given angle. To achieve maximum population of internal modes at all incident angles, some disorder or randomness must be incorporated into the design of such absorber layers. We report on a study of disordered two-dimensional thin-film photonic crystal (PhC) slabs as a prototypical light trapping design that embodies the compromise between traditional periodic structures and random texture.
We use a combination of finite-difference time domain (FDTD) simulations and rigorous coupled wave analysis (RCWA) to simulate optical absorption in disordered photonic crystals slabs. We first optimize absorption in simple periodic lattices and identify general trends and design rules for designing periodic photonic crystal absorber layers. For a 200nm Si photonic crystal slab, we can enhance absorption by as much as 200% compared to a planar slab with a 2 layer planar dielectric antireflective coating.
We show that introducing defects into otherwise perfect photonic crystal lattices can further increase absorption in two ways: first, by increasing the local density of optical states (LDOS) within the absorber region; and second by increasing the incoupling efficiency to the photonic crystal slab. For a test case of a 200nm Si slab with air holes having a diameter of 232 nm and a square lattice constant of 290 nm, we find that an optimized sublattice of defects can enhance absorbed photocurrent by ~3 mA/cm2. We also investigate the angular response of this structure and find that a consistent photocurrent enhancement is achieved for angles up to 80 degrees from normal incidence. We have developed optimal designs for a variety of other periodic designs, including optimized square and hexagonal lattices and always find that in each case there is a defect mode that increases absorption over that of the periodic photonic crystal. Additionally, we further investigated other designs such as a roughened low index scattering layer above the PhC that serves as a separate incoupler to more fully populate the modes of the absorbing layer.

Resonant nanostructures integrated with thin-film solar cells can significantly enhance the photovoltaic conversion efficiency. Here, we present a novel resonant light trapping architecture composed of an array of TiO₂ nanocylinders printed on thin crystalline silicon slabs, and demonstrate large enhancements in both the light incoupling and light trapping in thin film silicon solar cells.
Bulk Si wafers and thin Si slabs with thicknesses in the range 1-20 micron were used in the experiments. Full-wafer scale arrays of TiO₂ cylinders were printed on the surface using substrate conformal imprint lithography in combination with evaporation and liftoff. The TiO₂ cylinders act as Mie nano-resonators with resonances in the 400-800 nm spectral range and have scattering cross sections that are 5-8 times the geometrical area. Due to the preferential forward scattering of light stored in these cavities, the nanoparticle-covered wafers show extremely low reflectivity. [1] We combine the imprint technique with Al₂O₃ atomic-layer deposition for surface passivation to realize surface recombination velocities as low as 1 cm/s.
First, we demonstrate that reflectivities as low as 2.3% (averaged over the AM1.5 solar spectrum) are achieved for a passivated Si wafer with an optimized square arrays of TiO₂ Mie scatterers (particle diameter 300 nm, height 150 nm, pitch 450 nm). This is much lower than the reflectivity due to a standard antireflection coating. Moreover, the reflectivity is low over a very broad angular range (+/- 60 degrees).
Next, we use numerical simulations to study the effect of the nanoparticle arrays on light trapping in thin Si slabs. Compared to planar slabs we find that the angular redistribution and mode coupling of light due to scattering from the Mie resonators leads to a strong enhancement in photocurrent. Our calculations show that it is possible to realize a 10 micron-thick Si solar cell with an efficiency as high as 20% by employing an optimized Mie resonator architecture (250 nm diameter, 450 nm pitch). Surface recombination velocities of less than 10 cm/s and carrier bulk lifetimes > 1 ms are essential in order to achieve such high efficiency. The imprint technique, in combination with the use of a passivated flat surface is ideal to achieve this.
Finally, we demonstrate the use of TiO₂ Mie resonator arrays on top of back-contact hetero-junction crystalline Si solar cells with efficiencies above 17%. A full optical and electrical characterization of the cells with an optimized TiO₂ Mie light trapping coating will be presented.
[1] P. Spinelli, M. Verschuuren, and A. Polman, Nature Comm. 3, 692 (2012)

The use of Aluminum for plasmonic nanostructures opens new possibilities, such as access to short-wavelength regions of the spectrum, CMOS-compatibility, and the possibility of low-cost, sustainable, mass-producible plasmonic materials. We examine the properties of individual Al nanorod antennas with cathodoluminescence. This approach allows us to image the local density of optical states of Al nanorod antennas with a spatial resolution greater than 20 nm, and to identify the radiative modes of these nanostructures across the visible and into the UV spectral range. Graphene has emerged as an outstanding material for optoelectronic applications due to its high electronic mobility and unique doping capabilities. Here we demonstrate electrical tenability and hybridization of plasmons in graphene nanodisks and nanorings down to 3.7 microns in the mid-IR. By electrically doping patterned graphene arrays with an applied gate voltage, we observe radical changes in the plasmon energy and strength. Plasmon hybridization and electrical doping in nanorings of suitably chosen dimensions are key elements for bringing the optical response of graphene closer to the near infrared, where it can provide a robust, integrable platform for light modulation, switching, and sensing.

The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed towards controlling their size, shape, and composition, most nanowire lasers currently suffer from emitting at multiple frequencies simultaneously, arising from the longitudinal modes native to simple Fabry-Pérot cavities. Cleaved-coupled cavities, two Fabry-Pérot cavities that are axially coupled through an air gap, are a promising architecture to produce single-frequency emission. The miniaturization of this concept, however, imposes a restriction on the dimensions of the inter-cavity gaps because severe optical losses are incurred when the cross-sectional dimensions of cavities become comparable to the lasing wavelength. Here we theoretically investigate and experimentally demonstrate spectral manipulation of lasing modes by creating cleaved-coupled cavities in GaN nanowires. Lasing operation at a single ultraviolet wavelength at room temperature was achieved using nanoscale gaps to create the smallest cleaved-coupled cavities to date. Besides the reduced number of lasing modes, the cleaved-coupled nanowires also operate with a lower threshold gain than that of the individual component nanowires. Good agreement was found between the measured lasing spectra and the predicted spectral modes obtained by simulating optical coupling properties. This agreement between theory and experiment presents design principles to rationally control the lasing modes in cleaved-coupled nanowire lasers.

Parity-time (PT)-symmetric media, a class of photonic synthetic media, are enabling optical devices capable of asymmetric reflection and unidirectional power propagation. As the name implies, such systems are symmetric under joint operations of parity (P) and time (T) operators. In optics, these conditions can be satisfied if the optical potential (i.e. the refractive index) satisfies the symmetry of n(r)=n^* (-r). By tuning the loss or gain balance, the eigenvalues of PT-symmetric systems can undergo a phase-transition from being purely-real-valued to complex conjugate pairs. This exceptional point defines the PT-device transition from symmetric, reciprocal light propagation to asymmetric and unidirectional optical transport.
In this presentation, we investigate PT-symmetric plasmonic materials and metamaterials. Particular attention is given to two-dimensional planar waveguides composed of alternating layers of silver and a dielectric with refractive index of n=3.2±ik, where +/- refers to loss and gain, respectively. We begin by investigating a 5-layer PT-symmetric plasmonic waveguide with k tuned from 0 up through 0.5. As k is increased, branch points appear in the dispersion relation of the modes. The symmetric, positive index plasmon modes - lying below the surface plasmon resonance frequency - evolve into loss or gain modes with highly asymmetric spatial profiles. Moreover, the asymmetric, negative index plasmon modes - lying above the surface plasmon frequency - evolve into negative index modes with gain and loss. Then, we utilize this 5-layer waveguide as the unit cell of an isotropic metamaterial exhibiting negative index at visible frequencies. As the loss-gain balance is tuned, we show how this metamaterial morphs from an isotropic material to a hyperbolic material with highly asymmetric reflection coefficients. For example, at a wavelength of 500nm, the circular equi-frequency surface (EFS) of this metamaterial becomes elliptical as k is increased to 0.5. This modification is accompanied by a merging of the bands of the fully-periodic metamaterial. The skewed, non-orthogonal Bloch modes of the PT-symmetric plasmonic metamaterial leads to strong birefringence upon illumination. Our results highlight the applications of PT-symmetry for tunable isotropic and hyperbolic metamaterials, and may enable new asymmetric nanophotonic media capable of unidirectional invisibility or coherent spaser absorption.

Plasmonics has achieved significant progress in shrinking the size of optical components and optoelectronic devices below the free-space diffraction limit for visible light. Many important scientific and technological applications require the use of mid-infrared light, but some of the advantages of plasmonic devices are lost in this wavelength range using traditional materials. The difficulty arises due to the relatively weak surface plasmon mode confinement in noble metals in this lower frequency regime. In order to concentrate and guide mid-infrared light at lengths scales smaller than the diffraction limit, new plasmonic materials systems are required. Here we investigate highly-doped InAs as a prospective material for mid-infrared plasmonics. We first directly excite surface plasmons in InAs using a modified Otto configuration and measure surface plasmon propagation lengths and confinement factors, confirming its viability as a mid-infrared plasmonic material. Then we demonstrate tuning of InAs surface plasmons across the mid-infrared both passively through doping and actively through electrostatic gating. InAs is a promising material for mid-infrared plasmonic devices due to its unique combination of desirable optical properties, tunability and compatibility with well established III-V materials systems.

Metamaterials based on metal/dielectric multilayers and superlattices are promising candidates for achieving sub-wavelength imaging, sub-wavelength light concentration, negative refraction, and engineered absorption and emission from quantum emitters. Such metamaterial devices are currently suffering from low performance due to undesirable properties of their metallic building-blocks. Recently there has been a major effort in finding better substitute metallic materials that could lead to the realization of high-performance devices and new designs.
TiN/(AlxSc1-x)N superlattices (with 0.6The optical properties extracted from ellipsometry measurements suggest that the superlattice shows a strong uniaxial anisotropy with positive and negative real permittivity values in the in-plane and out-of plane directions in the green to red part of the visible spectrum. This gives rise to hyperbolic dispersion in this metamaterial that could lead to many interesting applications. The spectral region where hyperbolic dispersion occurs is varied by changing the metal-to-semiconductor thickness ratio. The effects of surface and interface roughness on the metamaterial properties are also presented.

Plasmon energies can be tuned across the spectrum by simply changing the geometrical shape of a nanostructure. Plasmons can efficiently capture incident light and focus it to nanometer sized hotspots which can enhance electronic and vibrational excitations in nearby structures.[1] Another important but still relatively unexplored property of plasmons, is that they can be efficient sources of hot energetic electrons which can transfer into nearby structures and induce a variety of processes. This process is a quantum mechanical effect: the decay of plasmon quanta into electron-hole pairs. I will discuss how plasmon induced hot electrons can be used in various applications: such as to induce chemical reactions in molecules physisorbed on a nanoparticle surface;[2] to inject electrons directly into the conduction band of a nearby substrate;[3] and to induce local doping of a nearby graphene sheet.[4]
References
[1] N.J. Halas et al., Adv. Mat. 24(2012)4842
[2] R. Huschka et al., JACS 133(2011)12247; S. Mukherjee et al., TBP 2012
[3] M. W. Knight et al., Science 332(2011)702, Z.Y. Fang et al., NL 12(2012)3808
[4] Z.Y. Fang et al., ACS Nano 6(2012)10.1021/nn304028b

We discuss the light interaction, use and applications of optical nanoantennas in both their linear and nonlinear operation. These engineered nanoparticles may be used to enhance and tailor the linear and nonlinear response of optical materials and they may realize novel optical devices for optical communications, computing, energy harvesting, sensing and polarization control. By translating some of the familiar radio-frequency concepts to optical antennas, we have proposed in recent years a variety of exciting possibilities for optical antennas in order to realize nanodevices with linear and nonlinear properties not available in conventional optical materials and systems. In our talk, we review our recent theoretical, numerical and experimental results involving individual nanoantennas and collections of them, showing that these plasmonic nanoparticles may realize the true bridge between unconventional nanoscale optical processing and far-field propagation and radiation. We will describe compact polarizers, switches, memories and filters based on arrays of nanoantennas, and discuss how it may be possible to realize complete nanophotonic systems involving wireless optical links and other optical components to feed, tune and control optical radiation and wave propagation. We will also show how periodic or aperiodic arrangements of nanoantennas in specific configurations may realize bulk metamaterial properties fundamentally different from the ones of their constituents or of any available optical material, and show their potential application to manipulate light propagation in exotic ways.

Optical metamaterials with a hyperbolic light dispersion paved the way to novel devices, e.g., for light wave guiding [1], perfect imaging [2, 3] or spontaneous emission enhancement [4, 5]. Interestingly, spin waves travelling in thin ferromagnetic films - under certain conditions - exhibit such a hyperbolic dispersion, which in addition can be easily tailored by an external magnetic field and offers the possibility to build the aforementioned metamaterial devices also for spin waves. Recently, we demonstrated by means of time-resolved Scanning Kerr Microscopy perfect spin-wave imaging of a grating structure realized by rectangular holes in a Permalloy film [6].
Here, we utilize this perfect imaging ability of spin waves for the realization of a novel type of spin-wave filter based on a series of two grating structures in a thin Permalloy film, where high throughput is observed only if the spin-wave image of the first grating coincides with the second grating. Frequency filtering is obtained with this device since the distance between the first grating and its spin-wave image is determined by the slope of the hyperbolic dispersion, which strongly depends on the spin-wave frequency and the external magnetic field. We show that the transmission through the device characteristically oscillates with frequency and that the frequencies of high transmission can be tuned by the external magnetic field.
We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via SFB668 and the City of Hamburg via LExI .
[1] B. Wood, J. B. Pendry, and D. P. Tsai, Directed subwavelength imaging using a layered metal-dielectric system, Physical Review B 74, 115116 (2006)
[2] Z. Liu, H. Lee, Y. Xiong, C. Sun, and X. Zhang, Far-Field Optical Hyperlens Magnifying Sub-Diffraction-Limited Objects, Science 315, 1686 (2007).
[3] S. Schwaiger, M. Bröll, A. Krohn, A. Stemmann, C. Heyn, Y. Stark, D. Stickler, D. Heitmann, and S. Mendach, Rolled-Up Three-Dimensional Metamaterials with a Tunable Plasma Frequency in the Visible Regime, Physical Review Letters 102, 163903 (2009)
[4] T. Tumkur, G. Zhu, P. Black, Y. A. Barnakov, C. E. Bonner, and M. A. Noginov, Control of spontaneous emission in a volume of functionalized hyperbolic metamaterial, Applied Physics Letters 99, 151115 (2011).
[5] H. N. S. Krishnamoorthy, Z. Jacob, E. Narimanov, I. Kretzschmar, and V. M. Menonet, Topological Transitions in Metamaterials, Science 336, 6078 (2012).
[6] S. Mansfeld, J. Topp, K. Martens, J. N. Toedt, W. Hansen, D. Heitmann, and S. Mendach, Spin Wave Diffraction and Perfect Imaging of a Grating, Physical Review Letters 108, 047204 (2012).

We demonstrate strong light emission enhancements at frequencies and angles of induced transparency in plasmonic systems [1]. This surprising behavior, mediated by dark polaritons, is observed for the first time and opposes “normal” emission enhancements in plasmonic systems occurring at resonant frequencies identified from optical extinction maxima. From the strong coupling of localized surface plasmon polaritons in a nanoantenna array to guided modes in a light-emitting slab emerge waveguide-plasmon polaritons. Dark polaritons are dressed states in this coupled system that exhibit far-field transparency but strong near-field interactions with light-emitters. A general framework for the existence of dark polaritons will be presented, and other mechanisms (e.g. diffraction) to achieve similar effects will be discussed. These results hold great promise for solid-state lighting-emitting devices, which may profit from enhanced and directional sources with negligible absorption losses.
[1] S. R. K. Rodriguez, S. Murai, M. Verschuuren, and J. Gomez Rivas, Phys. Rev. Lett. 109, 166803 (2012).

Recently considerable attention has turned towards the relationship between plasmon excitation and the electrochemical potential associated with the electron gas in a conductor. In this paper, we explore this relationship in three contexts: 1) the plasmoelectric effect, a new physical phenomenon that relates resonant excitations in conductors to their electrochemical potentials and 2) field effect tuning of the electrochemical potential and plasmon resonances of graphene nanoresonators and 3) transport of plasmon-excited hot carriers across metal-semiconductor interfaces.
The plasmoelectric potential is an electrochemical potential induced by resonant optical absorption in plasmonic nanostructures. This electrochemical potential results from the dependence of the plasmon resonance frequency on electron density. Electrically connecting two metallic nanostructures with resonant absorption maxima at distinct frequencies, and irradiating both structures with an intermediate frequency, induces electron transport from the high frequency plasmonic resonator to the low frequency resonator. This process is entropically driven by an increase of the absorbed incident radiation, which results from the shifted plasmon resonances induced by the new charge density configuration. We demonstrate experimentally the relationship between plasmoelectric potential and resonant optical absorption via three independent experimental methods.

We report the gate-tunable resonant absorption in lithographically fabricated arrays of graphene ribbon plasmonic nanoresonators with cavity lengths in the 10-100 nm range. Resonant mid-infrared absorption features due to transverse and longitudinal plasmonic cavity resonances are observable, as are plasmonic features that couple to localized phonon modes in the underlying silicon dioxide substrate. The plasmonic dispersion relations for these resonators can be developed by variation of resonant energy with cavity length as a function of gate voltage. The relationship between cavity edge roughness and resonance linewidth will be discussed.
Recently considerable attention has turned towards finding a silver lining in the cloud of plasmonics by extracting energy from the inevitable optical losses resulting from plasmon decay. We assess the prospects energy conversion via hot electron injection from localized plasmon decay across a rectifying metal-semiconductor Schottky barrier junction. Of particular interest is the relationship between the injection current across the Schottky barrier and the polarization of the electric field used to excite the plasmon. We analyze the current injection in Au/Si Schottky barrier nanoantennas excited with fields polarized transverse and parallel to the heterojunction interface.

Resonant metallic nanostructures lead to unconventional optical functionalities that are not easy to attain with natural materials. By carefully designing the shape, size and the surrounding dielectric interfaces of metallic nanostructures, one can control and manipulate the fundamental optical processes such as absorption, emission and refraction. The absorption process is of particular importance for energy conversion and conservation applications. In recent years, there has been growing amount of interest in utilization of resonant metallic nanostructures photovoltaic and thermophotovoltaic applications. Here, we are utilizing the optical losses of metals for increasing light absorption inside the metals. This is a completely different approach for engineering the absorption, in which we employ ultra-thin metallo-dielectric films to obtain so called “black” surfaces. Our broad band absorber (BBA) structure consists of a metal-insulator-metal stack with a nanostructured top silver film composed of crossed trapezoidal arrays. By selectively positioning the resonant frequencies of a particular structure on the energy axis, we achieved broad and ultra-narrow absorption with otherwise lossless dielectrics and highly reflective metals. We carried out detailed numerical analysis to optimize the absorption and identify the resonant mode characteristics in the structures. The BBA structure absorbs on average 80% of the incident radiation through the visible spectrum (400 - 700 nm) within a 180 nm thick thin-film. These experimental results were confirmed by full-field electromagnetic simulations, which agreed well with the experimental results. By altering the parameters such as the trapezoid width and metal and oxide layer thicknesses, once can control and manipulate both localized and delocalized surface plasmon polariton resonances, as well as the waveguide modes. In addition to the broadband absorber, we have recently designed an ultra-narrow band absorber with ~5 nm spectral width using gold nanostructured surfaces. Numerical simulations yielded extremely narrow absorption peaks (less than 5 nm FWHM) with high amplitude near not;perfect absorption. We can control the position of the sharp absorption peak by changing the size of the metallic resonators. We will present our recent efforts on the simulation and experimental realization of narrow-band and broadband absorbers.

Free charge carriers residing in a dielectrically isolated nanoparticle will collectively interact with an applied electromagnetic field, producing surface-bound oscillations known as localized surface plasmon resonances (LSPRs). Plasmonic nanoparticles dramatically modify the electromagnetic field near the particle resulting in remarkable optical absorption and scattering properties applicable to advancements in sensing, spectroscopy, microscopy, and photonics. Recent research explores the potential of heavily doped semiconductor nanocrystals (NCs)^1,2 in an effort to exploit LSPR tunability.³ Free carrier concentrations in semiconductors (10¹⁸-10²¹ cm^-3) support LSPR frequencies in the near- to mid-IR, a region of the spectrum less accessible to metal nanoparticles. Despite silicon&’s importance in electronic and photonic applications, no LSPRs have been reported for doped silicon NCs (SiNCs), in large part due to difficulties with doping semiconductor NCs.⁴ Phase segregation of dopants during synthesis leads to small concentrations of activated free carriers despite large atomic concentrations of dopants. As a result, doping concentrations considered acceptable for bulk Si are insufficient to create free carrier concentrations for LSPRs in SiNCs. Here we describe a process for forming degenerately doped SiNCs which exhibit tunable LSPRs in the range of 0.07-0.3 eV. Using a nonthermal plasma synthesis approach developed by Mangolini et. al.⁵ and successfully advanced for low level doping by Pi et. al.,⁶ we show that intense saturation of an argon silane plasma with a dopant precursor produces SiNCs exhibiting LSPR. Fourier-transform infrared spectroscopy illustrates that SiNC LSPR frequency can be shifted by controlling the dopant concentration delivered to the plasma. Using high resolution transmission electron microscopy and x-ray diffraction, we show that the SiNCs retain their crystalline structure even at the highest doping concentrations. Lastly, we discuss the potential for increasing the frequency range of SiNCs LSPRs further into the near-IR through low temperature annealing and specific plasma synthesis conditions.
¹ Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Nat Mater2011, 10, 361-6.
² Manthiram, K.; Alivisatos, A. P. J Am Chem Soc2012, 134, 3995-8.
³ Garcia, G.; Buonsanti, R.; Runnerstrom, E. L.; Mendelsberg, R. J.; Llordes, A.; Anders, A.; Richardson, T. J.; Milliron, D. J. Nano Lett2011, 11, 4415-20.
⁴ Norris, D. J.; Efros, A. L.; Erwin, S. C. Science2008, 319, 1776-9.
⁵ Mangolini, L.; Thimsen, E.; Kortshagen, U. R. Nano Lett2005, 5, 655-9.
⁶ Pi, X. D.; Gresback, R.; Liptak, R. W.; Campbell, S. A.; Kortshagen, U. R. Appl Phys Lett2008, 92, 123102.
This work was supported primarily by the MRSEC program of the National Science Foundation under Award Number DMR-0819885. Partial support was also provided by the Army Office of Research under MURI grant W911NF-12-1-0407

Plasmon-based resonators have been studied extensitvely due to their small sizes and high degree of photon confinement. Of the many possible designs, metal-insulator-metal (MIM) structure confines much of the light in the dielectric region between metals, thereby reducing the metal-induced losses while still providing high degree of photon confinement. Ideally, the dielectric region should be empty so that the high field intensity can be accessed. However, in many cases, creating such an empty slot requires e-beam lithography or focused-ion beam etching that are not only time-consuming, but also can lead to rough edges that can significantly increase the propagation losses. In this presentation, we demonstrate fabrication of a resonator structure that consists of a gold disk stacked on top of a gold ring using selective etching and directional deposition. The disk and the ring are self-aligned, and are separated by an 80nm air gap, but require only a single photolithography and metal depositions step. Simulations indicate presence of plasmonic whispering gallery mode, with strong suppression of dielectric modes. Transmission results confirm the presence of plasmonic whispering gallery modes, with resonance wavelengths at 1280, 1400, 1530 nm and Q-factors of about 20. As much as 90 % of the mode energy is calculated to be within the gap, and 11 % within 5 nm of metal surfaces. These results suggest that the resonator structure may be a good candidate for many applications such as sensing, and examples of such applications will be presented.

Metallic nanostructures can exhibit different optical properties compared to bulk materials mainly depending on their shape, size, and separation. We present the results of an optical modeling study on ordered arrays of aluminum (Al) nanorods with a hexagonal periodic geometry placed on an Al thin film. We used a finite-difference time-domain (FDTD) to solve the Maxwell's equations and predict the reflectance of the nanorod arrays. The thickness of the base Al film was set to 100 nm, and diameter and height of the nanorods were varied in the range 300-700 nm. Incident light in the FDTD simulations was an EM-circular polarized plane wave and reflectance profiles were calculated in the wavelength range 200-1800 nm. In addition, we calculated spatial electric field distributions around the nanorods for wavelength 300, 500, and 700 nm. Our results show that reflectance of Al nanorods can drop down to as low as about 70%-60%, which is significantly lower than the about 20% reflectance of conventional Al film. In addition to the overall decrease in reflectance, Al nanorod arrays manifest multiple resonant modes (higher-order modes) indicated by several dips in their reflectance spectrums (i.e. multiple attenuation peaks in their absorption profiles). Positions of these dips in the reflectance spectrum and spatial EM field distribution vary with nanorod height and diameter. Multiple reflectance peaks are explained by surface plasmon and cavity resonator effects. Spatial EM field distribution profiles indicate enhanced light trapping among the nanorods, which can be useful in optoelectronic and solar cell applications.

Hematite (α-Fe2O3) is promising material for photocatalytic water splitting to generate hydrogen in terms of stability, abundance in earth, and relatively low band gap energy for optical absorption. However, due to its poor conductivity with short diffusion length (~10nm), photo excited carriers easily recombine before reaching hematite water interface. Hereby we present optimized nanobeam structure that overcomes the limitation of short diffusion length by nanophotonic resonance. Illuminated light is concentrated near the surface of hematite when the feature size of nanobeam is ~180nm, due to high order Mie resonance, resulting in photo excited carriers generating mostly near the surface and minimizing charge recombination inside the bulk of hematite. However, because of the roughness of hematite nanostructure and leaky mode nature of nanoscale resonance, substantial amount of light is scattered as well. We have also shown that the scattered light is enabled to be resonant by designing optimal pitch size of nanobeam array and ITO thickness. In such a system, scattered light, waveguided at ITO layer, is additionally coupled to the resonant mode of neighboring nanobeam, supporting strong resonance at rough metal oxide nanobeam structure. Strong and broad photocurrent enhancement compared to that of an unoptimized nanobeam structure has been experimentally demonstrated. Additionally, it is shown that the resonance at ~180nm nanobeam is independent of polarization supporting degenerate mode for each polarization and that the spectrum region of resonance can be tuned by alternative system designing.

The template-stripping method can offer smooth patterned films without surface contamination. However, the process is typically limited to noble metals such as silver and gold because other materials cannot be readily stripped from silicon templates due to strong adhesion. Herein, we report an improved template-stripping method that can be extended to a large variety of materials including refractory metals, semiconductors, and oxides. To address the adhesion issue, we introduce a thin noble metal layer between the template and the deposited materials. After peeling off all of the deposited materials from the template, the noble metal layer can be selectively removed via wet etching. Thus, smooth patterned films of any desired material can be obtained. Further, we demonstrate template-stripped multilayer structures which have potential applications for photovoltaics and solar absorbers. In other words, an entire patterned device, e.g. which includes a transparent conductor, semiconductor absorber, and back contact, can be fabricated. Since our approach can simply reproduce the patterned films and multilayer structures with nanometer-scale fidelity via reuse of the template, a low-cost and high-throughput process in micro- and nano-fabrication is provided that is highly favorable for electronics, plasmonics, and nanophotonics.

In media with hyperbolic light dispersion a broadband Purcell effect was predicted [1] and recently experimentally investigated, e.g., for IR140 laser dye molecules embedded in polymethyl methacrylate (PMMA) [2] or for CdSe/ZnS colloidal quantum dots [3].
Here, we investigate the Purcell effect for a GaAs quantum well embedded in rolled-up radial metamaterials, which we prepare using the principle of self-rolling strained semiconductor layers [4]. A strained Ag/GaAs/InGaAs layer is detached from the substrate by selectively removing an AlAs sacrificial layer and rolls up into a microtube with multiple rotations. The walls of such microtubes are three-dimensional metamaterials consisting of metal/semiconductor superlattices [5] with the option of embedding high quality and robust quantum emitters into the semiconductor component [6].
In the experiments presented here, we varied the Ag/GaAs thickness ratio eta; = d_(Ag)/d_(InGaAs) to tune the effective permittivity of the metamaterial at the quantum well emission energy (1.63 eV) and thereby change the effective dispersions&’ iso-frequency surface in k space from a closed spherical or ellipsoidal iso-frequency surface (eta; < 0.53) to an open hyperboloidal iso-frequency surface (eta; > 0.53). All rolled-up metamaterials were prepared from the same semiconductor sample to guarantee identical intrinsic properties of the embedded GaAs quantum well. In time-resolved photoluminescence measurements we find a sharp decrease of the embedded quantum well&’s lifetime tau; at this transition with tau; asymp; 430 ps for metamaterials with eta; < 0.53 and tau; asymp; 250 ps for metamaterials with eta; > 0.53. This well corresponds to an increase of the photon density of states as expected at the transition to the hyperbolic dispersion regime.
We acknowledge fruitful discussions with Detlef Heitmann and financial support by the Deutsche Forschungsgemeinschaft via GK1286.
[1] Z. Jacob et al., Appl. Phys. Lett. 100, 181105 (2012)
[2] T. Tumkur et al., Appl. Phys. Lett. 99, 151115 (2011)
[3] H. N. S. Krishnamoorthy et al., Science 336, 6078 (2012)
[4] V. Ya. Prinz et al., Physica E 6, 828 (2000)
[5] S. Schwaiger et al., Phys. Rev. Lett. 102, 163903 (2009)
[6] S. Schwaiger et al., Phys. Rev. B 84, 155325 (2011)

Resonant Mie scattering is a spectacular outcome of light-matter interaction at the nanoscale. Light can confine deep-subwavelength structures and exhibits leaky mode resonant behavior via this scattering. The core diameter determines the number of the modes resonating within the nanowire, which could be described as of whispering gallery type. This phenomenon previously was demonstrated on high-index nanowires for structural coloring purpose [1,2]. Besides, it can be engineered in order to enhance absorption inside the solar cells [3]. Among the various geometries, nano-spheres possess unique coloration features. Unlike cylindrical symmetry, spherical symmetry supplies two distinct features of angle and polarization independence. In addition, this geometry provides us with observation of any hue in the visible spectrum with linear dependence on the nano-sphere diameter.
In this study, we observed large-area coloration of nano-spheres via Resonant Mie scattering. By using recent top-to-bottom fabrication technique we achieved to produce core-shell nanowires and subsequently nano-spheres with extended length, aligned and with extensive uniformity [4]. Controlled color generation in any part of visible and infrared spectral regions can be achieved by the simple scaling down procedure that followed by heat treatment. The nanosphere array are produced for extended lengths, typically hundreds of meters, and embedded in a flexible polymer fiber. Spectral color generation in mass-produced uniform core-shell nano-sphere arrays paves the way for applications such as spectral authentication at nanoscale, light-scattering ingredients in paints and cosmetics, large-area devices, and infrared shielding.
[1] L. Cao et al., Nano Lett. 10, 2649 (2010).
[2] T. Khudiyev, M. Bayindir, et al., Nano Lett. 11, 4661 (2011).
[3] P. Matheu, E. T. Yu, et al., Appl. Phys. Lett. 93, 113108 (2008).
[4] M. Yaman, M. Bayindir, et al., Nature Materials 10, 494 (2011).

Surface plasmon polaritons (SPPs) are electromagnetic waves propagating on the interface between a metal and dielectric. Such SPPs are usually found in metals when operating at optical frequencies. The SPPs find applications in the field of biomedical sensing where they can be used to investigate an analyte near the interface. In this work we utilise a metamaterial design operating at terahertz frequencies to demonstrate that it can support such SPPs. In the absence of dielectric loss the metamaterials can provide a sharp resonance at terahertz frequencies, however, a more practical configuration requires a substrate. Hence, the effects of inclusion of polymer substrate to support spoof SPPs is also studied.
A complementary split ring resonator (CSRR) metamaterial design has been proposed to support such spoof SPPs. In our investigation, a scaled version of CSRR metamaterial design is simulated as a periodic infinite array to obtain the transmission spectra and dispersion losses when incident by a plane wave operating at terahertz frequencies. These simulations were carried out using ANSYS HFSS full-wave electromagnetic simulator and CST Microwave studio. The metamaterial presented in this work employs simple micro-fabrication techniques to demonstrate SPPs operating at terahertz frequencies.
The CSRRs when simulated as a free standing structure demonstrates sharp resonance and their potential for supporting SPPs. From the simulation results it is observed that by addition of substrate there is a red-shift in the resonance frequency and reduction in the magnitude. This substrate effect is minimized by fabricating the CSRR on a low-loss, low refractive index and isotropic substrate like cyclic-olefin copolymer. The ability to fabricate micro-structure metamaterials on large polymer substrate sheets will allow the realization of SPP for biomedical applications. In this work we also demonstrate the fabrication method employed to fabricate CSRR structures on polymer substrate to experimentally demonstrate SPPs.

Precise surface plasmon tuning of metal-based nanostructures is essential in the view of fundamental aspects as well as photonic applications. Sulfide ions (S^2-) readily react with silver atoms (Ag⁰) at room temperature. Using this reaction, the addition of sulfide ions into the Au@Ag core-shell nanocubes yields stable Au@Ag/Ag₂S core-shell nanocubes, which lead to the continuous shifts of the plasmon extinction maximum to the longer wavelength from 500 to 750 nm, and covers the entire visible range.
Since the localized surface plasmon resonance (LSPR) spectra of Au@Ag nanocubes are highly sensitive to local refractive index change induced by compositional variation from Ag (n_Ag ~ 0.05) to Ag₂S (n_Ag2S ~ 2.9), it is possible to observe the change of LSPR spectra of Au@Ag nanocubes occurred by the generation of Ag₂S layer on the surfaces of nanocubes.
We use dark field microscopy (DFM) to study the change of LSPR spectra of individual Au@Ag nanocube at a single particle level. When sulfide ions are added into Au@Ag nanocubes, the ions are reacted with specific rates. It is possible to quantitatively measure the rate of compositional variation from Ag to Ag₂S on surfaces of particles by calculating plasmon extinction maximum shift of the LSPR peak in times progress.

Spectral upconversion - the conversion of lower energy photons to higher energy photons - provides a means to increase solar cell efficiency by harvesting sub-bandgap light. Because an upconverter is electrically isolated from the solar cell, it neither introduces recombination pathways for electron-hole pairs, nor requires current matching. Previous theoretical work has shown that upconversion can significantly enhance cell efficiency, but neglected to consider the impact of narrow-band absorption and non-unity quantum efficiencies. Understanding how these characteristics affect the overall efficiency is crucial to developing better technologies.
Here, we employ a detailed balance approach to model a solar cell with a realistic narrow-band upconverter, and determine cell efficiencies as a function of upconverter absorption bandwidth and quantum efficiency. The maximum cell efficiency observed with the addition of a spectrally broadband upconverter is 43.6%, and is achieved with a 1.7eV bandgap cell. Decreasing the absorption bandwidth reduces the efficiency, but significant improvements over the Shockley-Queisser limit are still obtained. For example, the application of a 0.2eV bandwidth upconverter has maximal impact on a 1.5eV bandgap solar cell and boosts the cell efficiency from 30.0% to 37.9%. Our calculations indicate that the ideal spectral locations for the low-energy transitions in an upconverter are within the infrared, and accessible with many existing upconverting systems. For a 1.5eV bandgap cell, the ideal low-energy absorption lines lie at 0.96eV and 1.28eV, while for a 1.1eV bandgap cell, they lie at 0.73eV and 1.00eV.
Such energy levels are readily achievable with the lanthanides, which upconvert light via a number of electric dipole and magnetic dipole transitions. To date, the optimal lanthanide system (oleylamine-coated β-NaYF4:Yb3+,Er3+ nanoparticles sensitized with the carboxylated cyanine dye IR806) has a quantum efficiency of .1%. Our calculations indicate that cell efficiencies would exhibit negligible improvement with this system, but quantum efficiencies of 20% and 50% would yield significant improvements of 0.9% and 4.6%, respectively.
As a means of enhancing the quantum efficiency, we propose a plasmonic nanostructure consisting of an upconverter-doped dielectric core and tapered metallic shell. This plasmonic nanocrescent is characterized by large absorption cross-section enhancements (exceeding 100 throughout the entire core) and strong optical frequency electric and magnetic dipoles. By tuning the electric and magnetic dipole modes into alignment with the lanthanide energy levels, we show that emission rates can be increased by more than a factor of 100. Our results provide a path towards increasing upconversion quantum efficiencies past 10%, and into the regime of technological utility.

Integrated optical circuits composed of dielectric and plasmonic nanoscale structures offer great promises for a wide range of applications such as switching, multiplexing, and amplification of light. The optical functionality of these circuits is determined by their dispersion, i.e. the relation between optical frequency and wave vector that determines the phase and group velocity of light. In an optical integrated circuit these properties often vary on the micrometer scale. So far, no techniques exist to directly measure dispersion in optical integrated circuits at this small scale.
Here, we present a novel method to measure the spatially-resolved dispersion relation in dielectric waveguides using electron beam excitation combined with optical detection. An electron beam in a scanning electron microscope is used as an excitation source that is raster-scanned over the surface, while the emitted light is collected by a half-parabolic mirror placed between the electron column and the sample. Using a spectrometer and a CCD imaging camera both the emission spectrum and the angular emission patterns are measured with a spatial resolution as small as 10 nm.
In dielectric materials, an electron beam can generate Cherenkov radiation when the electron speed exceeds the phase velocity of light in the medium. The intensity and angle of emission depends on the electron energy and the refractive index of the dielectric. For planar silicon waveguides Cherenkov radiation is emitted for electron energies higher than 6 keV. The radiation is emitted under an angle larger than the critical angle for total internal reflection, effectively trapping the light in the waveguide. The large emission angle allows for a large in-plane momentum of the generated light, which can be tuned by varying the electron energy. By matching the parallel momentum of the excited light to that of the waveguide modes, the Cherenkov radiation can be used to couple to these discrete modes. At a given electron energy, multiple modes can be excited at different characteristic frequencies. By varying the energy one can control which modes are excited at what frequencies. In this way the dispersion relation of the different modes can be mapped. We study this effect in detail for 100-nm-thick, 1-5-micron-wide free-standing silicon waveguides made using electron beam lithography in combination with reactive ion etching of a silicon-on-insulator (SOI) substrate.
We probe the excited modes in two different ways. A tapered optical fibre is coupled to the waveguide and the radiation spectrum is collected through the fibre while the waveguide is irradiated with electrons at different energies. In a second geometry, a grating structure is etched into the waveguide and light is coupled out and both the spectrum and angular emission pattern are collected. In this way the in-plane wave vector can be directly measured at each modal resonance frequency.

The oscillatory behavior of the photocurrent of individual gallium nitride (GaN) nanowires decorated with gold (Au) nanoparticles, with the nanowire radius of 40 nm - 200 nm, has been observed and analyzed. The oscillations are attributed to the excitation of the allowed transverse magnetic (TM) modes of the nanowire, which have maximum intensities in the undepleted axial region of the nanowire. The TM electromagnetic modes are generated by the surface plasmon polaritons (SPP) excited by incident light at the Au-GaN interface. Oscillations of the photocurrent are not observed for nanowires with radii less than the width of the depletion layer formed at the Au-GaN interface as a consequence of insufficient number of carriers. Numerical modeling demonstrates that the photocurrent maxima are assigned to the corresponding lowest azimuthally symmetric TM eigenmodes.

Terahertz (THz) Plasmonics waveguide(WG) devices enable faster information transfer and miniaturization in the THz region of the electromagnetic spectrum finding applications in interconnects, spectroscopic techniques etc.[1] Planar semiconductor insulator semiconductor (SIS) WG structure with one or more cavities perpendicular to the axis of the WG form resonant structures, which are of interesting application in THz spectroscopic techniques. The key challenge is to strongly localize the THz surface plasmon polaritons (SPPs) in the insulator region of the SIS WG with cavities and to achieve maximum transmission at the resonant frequencies. InSb is an III-IV semiconductor with negative permittivity values in the THz region which can localize the SPPs at the SI interface. Hence we propose to use InSb based planar plasmonic WG structures[2] with cavities to implement as resonators in THz circuitry for frequency selective filter applications, similar to integrated optical filters in the optical region.
In this work, we present the results of simulation and theoretical analysis of electromagnetic wave propagation, in the insulator region (air) of SIS WG made of InSb on quartz substrate with cavities perpendicular to the WG axis. Using a Finite Element Method (FEM) based simulation tool, for an incident THz (0.1THz to 2THz) electromagnetic wave, SPPs Transverse magnetic fields were localized at the SI interface. Length of the WG and the cavity determines the resonant nature of the structure. By varying the geometrical parameters of the WG and cavity the propagation properties of the device varied. SPPs in the InSb-air interface enhanced the propagation of THz signal inside the WG. Cavities acts like a resonator with strongly localized SPPs at the SI interface, which enables to achieve resonance due to interference effects inside the cavity structure. The device without cavity showed transmission at integral multiples of resonant frequency. When the cavity is introduced into the structure resonant peak arises at odd multiples of frequency in addition to the even multiples of resonant frequencies without the cavity, resulting in the resonant TE10 mode. Maximum transmission of the THz signal at resonant frequencies showed similar performance as MIM WGs based cavity resonator in the optical region[3]. The simulated resonant peaks of the plasmonic cavities are discussed analytically using equivalent circuit model in microwave engineering[4]. These results agree well with the obtain FEM based simulation method. The quality factor of the structure is also explored for the device to be used as frequency selective resonant filters and band stop filters.
[1] R. Articles, Nature Photonics, Volume 1 2007.[2] Giannini et al Opt. Express 18, 2797-2807, 2010[3]Matsuzaki et al Opt. Express 16, 16314-16325, 2008. [4] Pannipitiya et al Opt. Express 18, 6191-6204, 2010.

We report thin film, plasmonic nanoscale superstructures composed of metal and phase change nanocrystals with optical properties tunable from the visible to the near infrared. For metal nanocrystal-based nanostructures, the superstructures were tailored in size, shape, and spacing by room temperature nanoimprint lithography of Au, Ag or Au-Ag alloy nanocrystal dispersions. Various superstructure geometries of nanopillars, nanorods, and nanoholes composed of nanocrystals were fabricated over large areas of up to a few centimeters in scale with high resolution. The room temperature fabrication allows for demonstration of these plasmonic nanoscale superstructures on rigid substrates as well as, on various flexible polymers. Engineering the coupling between the nanocrystals within the nanostructures allows tailoring of the dielectric function of the superstructures from that of a dielectric to that of a plasmonic metal by replacing the long, insulating ligands used in nanocrystal synthesis with the compact ligand thiocyanate. The optical plasmonic resonances of the superstructures were tuned by the choice of nanocrystal building block (Au, Ag or Au-Ag alloy), the dielectric function of the nanocrystal superstructure, and the nanostructure geometry and periodicity. Superstructures were also nanoimprinted from nanocrystals of the reversible phase-changeable material vanadium oxide (VO2). By tailoring their geometry and structural motif, and thermally annealing to control nanocrystal phase, we realized VO2 nanostructures with a low-loss, plasmonic resonance that is reversibly, thermally on-off switchable with temperature.
This work is supported by the US Office of Naval Research Multidisciplinary University Research Initiative (MURI) program grant number ONR-N00014-10-1-0942.

Over the past few years, significant research efforts have been devoted to the study of surface electromagnetic waves involving the coupling between photons and phonons, named surface phonon polaritons (SPP); due to their potential applications to improve the thermal performance of nanomaterials in electronics. The thermal effect of these waves is expected to be particularly important in amorphous dielectrics, which usually have low bulk thermal conductivities, whose values decrease as the size is scaled down. Despite their importance, the SPP thermal conductivity of these materials is not well understood to date; especially at nanoscale.
In this work, the SPP contribution to the thermal conductivity of both a thin film and a tube with circular cross-section of silicon dioxide is investigated based on the Maxwell equations and Boltzmann transport equation. It is shown that: (1) the dispersion relation for the in-plane wave vector can be obtained analytically when the thickness of the film or the outer radius of the tube is of the order of or smaller than 300 nm. This allows determining explicit expressions for the propagation length and analyzing its dependence on the electrical permittivity. (2) A small difference between the permittivities of the two surrounding media can generate large propagation lengths and therefore enhance remarkably the SPP thermal conductivity. (3) The SPP thermal conductivity increases as the thickness of these nanomaterials decreases, and its values can be significantly larger than its bulk counterpart. The SPP and phonon contributions to the thermal conductivity as a function of the thickness of both structures are compared and analyzed. Furthermore, the effects of the size as well as of each azimuthal mode in the tube are also discussed. It is expected that the obtained results can be useful to improve greatly the thermal performance of nanomaterials.

Nanophotonics provides opportunities for optimal tailoring of the photonic density of states. This ability can in turn be explored to control thermal radiation in the far-field, and also the near-field, enhance fluorescent light emission, as well as optimize laser emission. In order to make these phenomena useful for large macroscopic devices, large-area nano-fabrication techniques have to be successfully implemented. In this talk, I will present some of our recent theoretical and experimental progress in exploring these opportunities.

Metallic nanoparticles are of particular interest for biosensing and photovoltaic cells because of their unique optical properties. Their localized surface plasmon resonances (LSPR) strongly depend on their electron densities. We have demonstrated that the LSPR of a single gold nanostructure can be tuned by electrochemical methods. The LSPR is blue-shifted when the electron density of the gold nanostructure increases and it is red-shifted when the electron density decreases. These observations are in an excellent agreement with the results of numerical simulations. The intensities of surface-enhanced Raman scattering can vary several orders of magnitude under different bias. That&’s mainly due to the shift of LSPR positions. Our results pushed one step forward for application of active plasmonic devices that are controlled by electrochemical methods.

The plasmon resonances of two closely spaced metallic particles have enabled applications including single-molecule sensing and spectroscopy, novel nanoantennas, molecular rulers, and nonlinear optical devices. While dimers with gaps greater than 0.5 nm have been thoroughly characterized, their properties with smaller separation distances have been historically difficult to describe due to limitations in lithographic fabrication and self-assembly techniques. In a classical electrodynamic context, the strength of dimer plasmon resonances should increase monotonically as the particle gap size decreases. In contrast, a quantum mechanical framework predicts that electron tunneling will increase conductance across the junction and strongly diminish the dimer plasmon strength for sub-nanometer-scale separations.
In this study, we directly observe the plasmon resonances of coupled metallic nanoparticles as their gap size is reduced to atomic dimensions. Using the electron beam of a scanning transmission electron microscope (STEM), we dynamically manipulate pairs of 10-nm-diameter spherical silver nanoparticles on a silicon dioxide substrate, controlling their convergence and eventual coalescence into a single nanosphere. We simultaneously employ electron energy-loss spectroscopy (EELS) to observe the dynamic plasmonic properties of these dimers before and after particle contact. As separations are reduced from 7 nm, the dominant dipolar peak exhibits a redshift consistent with classical calculations. However, gaps smaller than 0.5 nm cause this mode to exhibit a reduced intensity consistent with quantum theories that incorporate electron tunneling. As the particles overlap, the bonding dipolar mode disappears and is replaced by a dipolar charge transfer mode. Our dynamic imaging, manipulation, and spectroscopy of nanostructures enables full spectral mapping of dimer plasmon evolution, and may provide new avenues for in-situ nanoassembly and analysis in the quantum regime.

The resonant properties of a one-dimensional plasmonic cavity, such as a strip or wire, can be captured well with a simple Fabry-Pérot model, which assumes a resonance condition will be met when the round-trip phase accumulation of a travelling surface plasmon polariton (SPP) inside the cavity equals an integer multiple of 2π. The input parameters of this model are the SPP dispersion relation, which accounts for phase accumulation during propagation in the cavity, and the complex SPP reflection coefficient at the cavity terminations, which includes information about the phase accumulation on reflection. For low order resonant modes, the phase accumulation on reflection can tune the overall cavity resonance as strongly as the cavity length. We will present both experimental data showing that the complex reflectivity and both the real and imaginary parts of the plasmon dispersion relation in a simple 1D plasmonic waveguide can be measured experimentally with electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) by examining the LDOS variations near a single termination in an effectively semi-infinite structure. These variations arise from self-interference of the electron beam with a single SPP reflected from the termination, and thus contain full information about the phase accumulation during propagation and reflection. These experiments as well as finite difference time domain simulations show the complex reflection coefficient to be a strong function of the termination geometry. Strategies for controlling this important quantity will be presented.

Controlling the flow of surface plasmon polaritons (SPPs) over the two-dimensional metal-dielectric interfaces paves a way for on-chip interconnections in the next generation of photonic circuits. Traditional plasmonic elements based on either structuring metal surfaces or placing discrete dielectric structures on metal surfaces suffer from considerable scattering losses due to the discontinuities of the structures and are extremely difficult to reconfigure. Recently, to circumvent these problems, nondiffracting Airy beams have been introduced to plasmonic systems, offering a novel approach to route SPPs over a metallic interface without any physical waveguide structures. Especially, we have experimentally demonstrated plasmonic Airy beams (PABs) as an efficient approach for dynamically routing SPPs along reconfigurable trajectories in real-time. Due to their unique self-healing and self-bending properties, such PABs can propagate against surface roughness and defects, or even getting over obstacles and therefore are promising for varieties of applications ranging from on-chip signal processing to dynamic manipulations of nano-particles. However, the PABs are inherently subjected to the paraxial limit, which prevents the beam self-bending into large angles along a parabolic trajectory, resulting in a limited range of diffraction-free propagation. Here, we overcome the paraxial limit of PABs by introducing nonparaxial accelerating beams (NABs), which are exact solutions of the Maxwell&’s equation. We show that such NABs can spatially accelerate into large angles along different trajectories well beyond the paraxial limit, but still retain nondiffracting and self-healing capabilities. Specifically, we shall demonstrate two fundamental members of NABs, namely Mathieu accelerating beams (MABs) and Weber accelerating beams (WABs). We show that MABs propagate along either circular or elliptical trajectories depending on the initial beam parameters, while WABs travel along parabolic trajectories including Airy beams as a special case at the paraxial limit. Our MABs and WABs could be directly applied for dynamic nano-particle manipulations. More importantly, our results inspire researchers from different areas, including physics, material sciences, and biology to develop new technologies or tools for a variety of applications. In addition, our method can be directly employed to excite nondiffracting accelerating surface waves in other low-dimensional systems, such as graphenes and topological insulators.

Electrons and photons can coexist as a single entity called a surface plasmon, an elementary excitation found at the interface between a conductor and an insulator. While most plasmonic applications - ranging from plasmon photovoltaics to plasmonic optical tweezers - rely on classical effects, quantum phenomena can also strongly influence the plasmonic properties of nanometer-scale systems. In this presentation, I&’ll describe our efforts to probe plasmon modes spanning both classical and quantum domains. We first explore the plasmon resonances of individual metallic nanoparticles as they transition from a classical to a quantum-influenced regime. Then, using real-time manipulation of plasmonic particles, we investigate plasmonic coupling between pairs of particles separated by nanometer- and Angstrom-scale gaps. For sufficiently small separations, we observe the effects of quantum tunneling between particles on their plasmonic resonances. We extend our analysis to probe quantum effects of multi-particle geometries, including those sustaining a magnetic mode. Using the properties of coupled metallic nanoparticles, we demonstrate the colloidal synthesis of an isotropic metafluid that exhibits a strong magnetic response at visible frequencies. The strength of the metafluid magnetic dipole is nearly 15% that of the electric dipole, and is tunable with interparticle separation. Our analysis provides a framework for controlling electric and magnetic light-matter interactions spanning classical and quantum domains, with applications ranging from molecular electronics to catalysis.

Nanoplasmonic structures may exhibit nonlocal response when shrinking their sizes and critical dimensions to a regime where quantum phenomena are expected to become important. In this talk I address a semi-classical hydrodynamic description that goes beyond the common description based on Ohm's law. Furthermore, I will discuss the consequences in the context of field singularities, field enhancement, and blue-shifting of resonances.

In open quantum systems, a very interesting type of coupling, called the anti-Hermitian coupling, has been widely studied. It is caused by the indirect coupling among the quasi-bound states through common continuum decay channels. With such a coupling, some long-lived states are deprived of the coupling strength to the decay channels, while others with enhanced coupling strength to the decay channels having very short lifetime, which are termed “super-radiant” states by Dicke to describe the coherent spontaneous radiation of a gas confined to a sub-wavelength scale. Spectroscopically, this phenomenon is manifested as sharp resonances superimposed on the broad super-radiant states, as also observed in the decay of compound nuclear states in certain nuclear reactions.
We show that for a plasmonic system consisting array of dipole antennas closely packed to each other, the coupling among them can be dominated by the imaginary part, and therefore it serves as an interesting classical analog to open quantum systems. By varying the distance between antennas, the coupling can be tuned continuously from real to imaginary regime. We experimentally demonstrate that the coupling constants that are dominated by the imaginary part introduce an interesting functionality of light manipulation in the deep subwavelength scale: the antennas, originally with low quality factor and overlapping resonant responses, can now be individually excited within a diffraction-limited focus spot. Through the near-field measurement, we show that the optical energy is well localized around the selected antenna and the coupling-induced suppression ratio can be up to 7dB.
Compared with the previous approaches to manipulate the optical hot spots at the nano-scale, the novel anti-Hermitian coupling approach presents a totally different mechanism without involving complicated temporal-spatial light modulation techniques. The continuum mediated anti-Hermitian coupling in plasmonics brings new perspectives towards understanding and controlling light at deep subwavelength scale, offers a convenient way to address individual nanophotonic elements in a deep subwavelength nanoplasmonic circuits, and may find important applications in nanophotonic devices such as nanoscale demultiplexed photodetectors.

Recent work has shown that nanometer-scale metallic particles or antennas with a surface plasmon resonance can enhance the subbandgap absorption, giving increased efficiencies and spectral selectivity. The ability to capture energy normally lost to heat would open new opportunities in photon sensors and energy conversion devices. Dephasing of the electron gas oscillation leads to hot electron-hole pair generation with energy separation determined by the incident photon energy. The efficiency of converting incident photons to hot charge carriers is enhanced in the presence of a plasmon resonance due to the additional absorption pathway. A detailed quantum mechanical (QM) description of this process is needed to model hot carrier collection efficiency because of the plasmon absorption pathway. Here we develop a full QM model for surface plasmon decay into hot carriers and couple it to optical finite element simulations to predict subbandgap absorption and coupling to polarization effects in these devices.
Working in Coulomb&’s gauge, a quantized representation of the plasmon field and photon field has been derived. The interaction Hamiltonian was then derived and used in Fermi's Golden Rule to determine the transition rate from photonic to plasmonic states, as well as the rate from plasmonic states to electronic states. To compute the states we use a standard normal mode computation and an orthogonalized plane wave model with k values populated based on the geometry of the system. The coupling between photons and plasmons depends in our model on the projection of the photon polarization onto the plasmon momentum. Likewise the coupling between photons and electrons depends on the projection of the photon's polarization onto the electron's momentum. The net effect of this is and explicit polarization dependence could manifest as a preference in k-space among the excited carriers and how it influences the photocurrent seen in specific geometries will be presented. The direction of hot carrier motion after excitation and correlation with photon polarization will be discussed using the model we have developed. We will report the comparison between the theoretical predictions and experimental absorption spectra at different wavelengths and with different polarizations for geometries used in the model.

Optical nanocavities enable very large emitter-field interaction strengths as a result of the strong field localization inside of their sub-cubic wavelength volumes. Namely, vacuum Rabi frequencies achievable with semiconductor quantum dots (QDs) in such cavities can be in the range of tens of GHz - several orders of magnitude larger than in atomic cavity QED. In addition to the study of new regimes of cavity QED, these effects can also be employed to build devices for quantum information processing, such as ultrafast quantum gates, nonclassical light sources, and spin-photon interfaces.
Besides quantum information systems, many classical information processing devices greatly benefit from the enhanced light matter interaction in such structures, including optical switches, modulators, and lasers.
We have employed a platform consisting of a single self-assembled InAs QD embedded in a GaAs photonic crystal (PC) cavity to study quantum optics and cavity QED. By employing this platform, we have demonstrated controlled amplitude and phase modulation between two continuous wave optical beams at the single photon level (power less than one photon per cavity photon lifetime). Recently, we have performed all-optical switching between two pulsed, resonant optical beams at the single photon level, with 40ps pulse-duration (implying possible 25GHz switching speeds). The switching is accomplished via saturation of the strongly coupled QD-cavity system.
We have also studied the effects of the photon blockade and photon induced tunneling which result from the anharmonicity of the ladder of dressed states in a strongly coupled QD-nanocavity system. These effects lead to dramatic changes in the transmitted photon statistics, which can be varied from sub-Poissonian to super-Poissonian, and can be employed to generate nonclassical states of light (such as Fock or NOON states) with high efficiency. Finally, we have demonstrated strong coupling between a single quantum dot and a photonic molecule consisting of two coupled PC cavities, which is useful for nonclassical light generation and potentially even quantum simulation]. The semiconductor platform also enables the implementation of electrical control of quantum emitters; we have demonstrated fast electrical control of a single quantum dot strongly coupled to a nanocavity which can be used as an electro-optical modulator with sub-fJ control energies and at tens of GHz speed.
By employing an ensemble of QDs embedded in a similar cavity with an incorporated lateral, lithographically defined p-i-n junction, we have demonstrated the lowest threshold electrically injected laser, with a threshold current of only 160nA (at cryogenic temperatures). At room temperature, this device operates as a single mode LED that is directly electrically modulated at 10GHz speed, and sub-fJ control energy per bit.

Nonlinear optical interactions are essential for a broad range of photonics applications. Such interactions enable all-optical switching and routing, which play key roles in optical communication technologies. There is also significant interest in achieving optical switching at ultra-low photon numbers for applications in quantum information processing and quantum networking. However, most optical switches rely on weak optical nonlinearities and typically require large optical energy for each switching operation.
One promising method for reducing optical switching energies is to exploit strong atom-light interactions in quantum dot (QD)-photonic crystal cavity systems. These interactions can enable the strong coupling regime where the cavity and QD mix to form new dressed polariton states, resulting in a modification of both the QD emission spectrum and cavity spectrum. In the strong coupling regime, the cavity-QD system can exhibit a large nonlinear optical response at low optical powers. Controlling these nonlinearities on fast time scales can enable all-optical switching at extremely low energies.
In this talk, we will present our recent experimental work demonstrating fast nonlinear optical switching between two laser pulses in an integrated, photonic crystal cavity-waveguide system containing a single, strongly coupled QD. Switching is observed with only 140 photons of pulse energy in the waveguide. The cavity-QD coupling is modified by a detuned pump pulse, resulting in a modulation of the scattered and transmitted amplitude of a time synchronized probe pulse that is resonant with the QD and propagates in the waveguide. The temporal switching response is measured to be as fast as 120 ps, demonstrating the ability to perform optical switching on picosecond timescales. In future studies, the switching energy can be further reduced by improving the evanescent coupling efficiency between the waveguide and the cavity.

Individual color centers in diamond have recently emerged as a promising solid-state platform for quantum communication and quantum information processing systems, as well as sensitive nanoscale magnetometry with optical read-out. Performance of these systems can be significantly improved by engineering optical properties of color centers using nanophotonic approaches. In this work we describe a high-flux, room temperature, source of single photons based on an individual Nitrogen-Vacancy (NV) center embedded in a top-down nanofabricated, single crystal diamond nanowires¹, plasmonic nano-apertures², and all-diamond based optical cavities^3,4. Using the nanowire geometry, for example, an order of magnitude brighter single photon source is realized, compared to an NV center in a bulk diamond1. By embedding diamond nanowires in metals 10-fold enhancement of NV&’s spontaneous emission, due to large Purcell effect provided by metallic nanocavity, was demonstrated². Finally, single-photon emission of NVs inside ring and photonic crystal resonator, fabricated directly in diamond, as well as single photon routing in an on-chip optical network has been achieved⁴. In addition to applications in quantum information processing, and owing to its excellent physical and chemical properties, diamond based optical nanostructures are of great interest for applications ranging from optoelectronics and NEMS/MEMS to life-sciences and sensing.
1. T.M. Babinec, B.M. Hausmann, M. Khan, Y. Zhang, J. Maze, P.R. Hemmer, M. Lon#269;ar, "A bright single photon source based on a diamond nanowire," Nature Nanotechnology, 5, 195 (2010)
2. J.T. Choy, B.M. Hausmann, T.M. Babinec, I. Bulu, and M. Lon#269;ar, "Enhanced Single Photon Emission by Diamond-Plasmon Nanostructures.," Nature Photonics, 5, 738 (2011)
3. T.M. Babinec, J.T. Choy, K.J.M. Smith, M. Khan, and M. Lon#269;ar, “Design and Focused Ion Beam Fabrication of Single Crystal Diamond Nanobeam Cavities,” J. Vac. Sci. Tech. B, 29, 010601 (2011)
4. B.J.M. Hausmann, B. Shields, Q. Quan, P. Maletinsky, M. McCutcheon, J.T. Choy, T.M. Babinec, A. Kubanek, A. Yacoby, M.D. Lukin, and M. Lon#269;ar "Integrated Diamond Networks for Quantum Nanophotonics", Nano Letters, 12, 1578 (2012).

Exciton and plasmon resonances in nanocrystals become strongly coupled via Coulomb and electromagnetic interactions leading to characteristic interference effects in optical spectra [1-6]. An interaction between a discrete state of exciton and a continuum of plasmonic states gives rise to interference effects (Fano-like asymmetric resonances and anti-resonances) [2,4]. These interference effects can strongly enhance a visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticles and molecules. If a system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to alter and enhance the circular dichroism (CD) of chiral components [5-8]. In particular, the exciton-plasmon interaction may create new chiral plasmonic lines in CD spectra of a biomolecule-nanocrystal complex [5,7]. Strong CD signals may also appear in purely plasmonic systems with a chiral geometry and a strong particle-particle interaction [6,8]. Recent experiments on the protein-nanocrystal and multi-nanocrystal complexes showed the appearance of strong plasmonic signals in CD spectra [7,8]. Potential applications of dynamic hybrid nanostructures include sensors and new optical and plasmonic materials. [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters 6, 984 (2006). [2] W. Zhang, G. W. Bryant, A. O. Govorov, Phys. Rev. Lett. 97, 146804 (2006). [3] J. Lee, P. Hernandez, J. Lee, A.O. Govorov, and N. A. Kotov, Nature Materials 6, (2007). [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008). [5] A.O. Govorov, Z. Fan, P. Hernandez, J.M. Slocik, R.R. Naik, Nano Letters 10, 1374 (2010). [6] Z. Fan, A.O. Govorov, Nano Letters 10, 2580 (2010). [7] J.M. Slocik, A.O. Govorov, and R.R. Naik, Nano Letters 11, 701 (2011). [8] A. Kuzyk, R. Schreiber, Z. Fan, G. Pardatscher, E.-M. Roller, A. Högele, F.C. Simmel, A. O. Govorov, T. Liedl, Nature, 483, 311 (2012).

We present our recent publication about experimental and theoretical investigations on tuning of the surface plasmon-exciton coupling by controlling the plasmonic mode damping, which is defined by the plasmonic layer thickness. The results reveal the formation of plasmon-exciton hybrid state characterized by a tunable Rabi splitting with energies ranging from 0 meV to 150 meV. Polarization dependent spectroscopic reflection measurements were employed to probe the dispersion of the coupled system. Transfer matrix method and analytical calculations were used to model the self-assembled J-aggregate/metal multilayer structures in excellent agreement with experimental observations.

Strong exciton-photon coupling in microcavities are currently subjects of intense research towards realization of polariton lasing with lower threshold than photon lasing [1]. Particularly, J-aggregates of dye molecules embedded in cavities have attracted enormous attention due to their unique property of large oscillator strength which facilitates the creation of exciton-polariton with large Rabi splitting [2]. Here we present a study of the light-matter strong coupling at room temperature in a planar metal-insulator-metal microcavity containing thin films of the dye [5-chloro-2-(2-[(5-chloro-3-(3-sulfopropyl)-2(3H)-benzoxazolylidene)methyl]-1- butenyl)-3-(3-sulfopropyl)-benzoxazolium inner salt, sodium salt] (DEDOC) J-aggregates. Layer-by-layer (LBL) assembly method is utilized for sequential deposition of cationic poly(diallyldimethylammonium chloride) (PDAC) and anionic DEDOC J-aggregate thin films with controlled thicknesses to be varied over a large range. We characterize the morphological and optical properties of the LBL-assembled J-aggregate thin films as a function of film thicknesses and process conditions. With the angle-resolved reflectivity measurements, we demonstrate that the high absorption constants and thickness adjustability of the PDAC/DEDOC layers allow exciton-photon coupling strength to be optimized such that a Rabi splitting energy exceeding 500 meV in the visible region can be achieved. Our results suggest that the PDAC/DEDOC layers are promising for the future exploration of strong- and ultrastrong-coupling optoelectronic applications.
[1] A. Das, J. Heo, M. Jankowski, W. Guo, L. Zhang, H. Deng, and P. Bhattacharya, Phys. Rev. Lett. 107, 066405(2011)
[2] D. G. Lidzey, D. D. C. Bradley, T. Virgili, A. Armitage, and M. S. Skolnick, Phys. Rev. Lett. 82, 3316(1999)

Bose-Einstein Condensation (BEC) - the ground state accumulation of bosons (particles with integer spin) above a critical density to temperature ratio - has been reported for atoms, photons, and solid-state quasiparticles such as exciton-polaritons. Distinctly, the collective oscillations of conduction electrons in metals, known as surface plasmon polaritons, have never been shown or predicted to undergo BEC. The lack of a suitable (e.g. harmonic) potential for thermalization, combined with the strong radiative and Ohmic losses in metals, are likely the reasons for this. Here we demonstrate BEC in a plasmonic system for the first time. Due to the strong coupling of surface plasmon polaritons in a periodic array of metallic nanorods to excitons in a room-temperature solid-state dye layer, bosonic quasiparticles known as plexcitons are formed. The density of plexcitons is increased through optical pumping, and their thermalization is driven by re-absorption and re-emission in the dye layer, which acts as a heat bath. Above a critical density plexcitons macroscopically populate the ground state of the system, giving rise to a new light-matter state: the plexciton condensate. The photonic component of plexcitons leaks out of the open system, enabling us to analyze the condensate through the angular spectrum and real space imaging. We find the plexciton condensate to be the warmest (effective critical temperature ~ 2600 K) and least massive (7 orders of magnitude lighter than the electron rest mass) of any condensate yet reported. Plasmonics therefore provides a suitable platform for room-temperature solid-state studies of macroscopic quantum critical phenomena, and we anticipate the emergence of plexciton condensates as coherent light sources.

Plasmon resonance has been used for different applications in photonics including nano-resolution near-field microscopy [1] and field enhancement [2]. In this presentation, I will talk about the use of plasmon resonance for thin-film holography. A hologram, typically seen on a credit card, recorded as the corrugation on thin metal film produces a Raman-Nath diffraction (but not Bragg diffraction for Lippmann color hologram), and hence the color in the reconstructed image changes with the angle of view. The hologram we have invented is based on plasmon resonance, and is free from color change (seen in rainbow holograms) in angle [3]. Only the selected colors are reconstructed with white light illumination. The mechanism is similar to the one of surface-plasmon sensors [4], while the resonance is tuned to the angle but the wavelength. 3D color images appear without blur or ghost on the metallic thin-film hologram mounted on glass substrate. Some results will be shown. Enhancement of the diffraction [5] and suppression of the image blur due to the extent of the source [6] are also discussed.
[1] S. Kawata, Y. Inouye, P. Verma, Nature Photonics, 3, 389, 2009.
[2] S. Kawata and V. Shalaev,ed., "Tip Enhancement," Elsevier, 2007.
[3] M. Ozaki, J. Kato, S. Kawata, Science, 332, 218, 2011.
[4] K. Matsubara, S. Kawata, Appl. Spectrosc. 42, 1375,1988; Appl. Opt. 27, 1162, 1988; Opt. Lett, 15, 75, 1990.
[5] S. Maruo, O. Nakamura, S. Kawata, Appl. Opt. 36, 2343, 1997.
[6] M. Ozaki, J. Kato, S. Kawata, Appl. Phys. Lett. in press.

Plasmonic systems and devices have attracted great interest for their ability to resolve features smaller than the wavelength of the light, beating fundamental physical limitations. Metal Superlens was used to transfer such sub-diffraction limited spatial information over a silver slab, while curved metallo-dielectric Hyperlens was able to transform the evanescent waves associated with such sub-wavelength information into propagating ones, making them detectible by ordinary microscopes. In this talk, I will present a new approach based on optical nonlinearity that aims to improve the performance of such devices and relax their stringent working conditions. I will discuss the use of four-wave-mixing (FWM) for metallic superlens, and self-focusing effects in metallo-dielectric hyperbolic medium.

A superlens is capable of imaging the samples at subwavelength resolution by using a thin slab of a material with negative permittivity. In principle, due to the small effective wavelength of surface modes at the interfaces between the negative-permittivity layer and positive-permittivity host, the SL can be seen as a practical version of perfect lens allowing for non-diffraction-limited imaging and sensing. However, a new emerging limitation in the SL is the narrow bandwidth. Although the surface resonance contributes a lot to the ultrahigh resolution of the SL, it also constrains that the SL can only work in the frequency range close to the resonance, which limits superlenses in spectroscopic applications.
In order to overcome this limitation, we present two possibilities based on layered materials. The first idea is a multifrequency superlens with layered phonon-resonant dielectrics. This proposed scheme is based on the fact that additional degrees of freedom for superlensing at multiple wavelengths can be provided by increasing the number of phonon resonant dielectrics in a mulitlayer system. In other words, a subwavelength image can be achieved at many different wavelengths by only one single lens, which is a large advantage over other recently proposed techniques for superlensing. Considering the abundance of polar dielectrics, the wavelength range of our lens can cover from IR to THz frequencies by choosing suitable materials.
The second idea is to design a frequency-tunable “graphene superlens”. This novel graphene-based device enables the enhancement of evanescent waves for near-field subwavelength imaging. Due to the non-resonant enhancement provided by the graphene sheets, this graphene lens yields new promising properties including broad intrinsic bandwidth and low sensitivity to loss, together with a still good subwavelength resolution of around lambda;/7 for a bilayer case and over lambda;/10 for the multilayered configuration. Most importantly, due to the large frequency-tunability via the dynamical tuning of chemical potentials, our proposed graphene lens can act as an ultrabroadband sub-diffraction-limited imaging device to in principle cover the nearly whole range from mid-IR to THz frequencies.

We have demonstrated and analyzed a highly efficient on-chip 3D metal-insulator-metal (MIM) gap-plasmon nanofocusing structure. In this abstract, we combine our previous fabrication/experimental work with in-depth theoretical analysis/discussion to provide a detailed picture of the highly efficient, on-chip 3D nanofocusing process. From the experimental measurements, we have estimated the intensity enhancement of 400 within a 14-by-80 nm² cross-sectional area and the coupling efficiency of minus;1.3 dB (74% transmittance). The ultimate transmission loss and the intensity enhancement of the demonstrated nanofocusing approach are theoretically predicted to be 2.5 dB and ~3.0 × 10⁴, respectively, for the case where light was compressed from a 200-by-500-nm² area into a 2-by-5-nm² area. We realized the 3D MIM nanofocusing structure on a chip by employing electron-beam-induced deposition (EBID)/ focused-ion beam (FIB) and demonstrated highly localized light confinement using the two-photon photoluminescence techniques. Here we highlight three major distinctive advantages of our MIM gap-plasmon nanofocusing device that we have experimentally observed and theoretically understood. (1) Hotspot size: Because we utilize the fundamental MIM antisymmetric mode with no theoretical cutoff, it is possible to reduce the field profile down to the size of the MIM cross-section even in the deep sub-wavelength spaces. (2) Loss optimization: The precisely engineered, linear 3D tapering geometry of our device effectively overcomes the major loss mechanisms present in the large wavevector region where the most power loss occurs during the nanofocusing process. More specifically, we theoretically understand that the loss originating in this region is more prominent in on-chip devices that inherently include relatively shorter tapers due to space limitations. And, employing an optimized linear taper in the MIM plasmonic waveguide could significantly minimize the major losses that occur in a large wavevector regime. (3) On-chip nanofabrication: Our process involves the use of EBID/FIB and standard IC processes that are highly controllable and compatible with other on-chip device fabrication techniques. At the 2013 MRS spring meeting, we look forward to presenting our engineering approach/choices, detailed simulations, nanofabrication processes including the creation of precise vertical taper, and succinct yet sufficient qualitative analysis and discussion of our experimental results (intensity enhancement and transmission rate, and most importantly loss-overcoming nanofocusing mechanisms). We strongly believe that this highly efficient on-chip 3D plasmonic nanofocusing approach will be useful in a variety of on-chip nanoscale photonic/plasmonic applications.

In contemporary Si-based image sensor technologies such as CCDs and CMOS image sensors, color sensitivity is added to photo detective pixels by equipping them with on-chip color filters, composed of organic dye-based absorption filters. However, organic dye filters are not durable at high temperatures or under long exposure to ultraviolet (UV) radiation, and cannot be made much thinner than a few hundred nanometers due to the low absorption coefficient of the dye material. Furthermore, on-chip color filter implementation using organic dye filters requires carefully aligned lithography steps for each type of color filter over the entire photodiode array, thus making their fabrication costly and highly impractical for multi-color and hyperspectral imaging devices composed of more than the three primary colors.
As an alternative approach, it is well known that plasmonic hole arrays in thin metal films can be engineered as optical band-pass filters owing to the interference of surface plasmon polaritons (SPPs) between adjacent holes. Unlike current on-chip organic color filters, plasmonic filters have the advantage of being highly tunable across the visible spectrum and require only a single perforated metal layer to fabricate many colors.
In this work, we report on the optical properties of ultra compact plasmonic hole arrays operating as color filters designed for state-of-the-art Si CMOS image sensors. The hole arrays are composed of hexagonally packed subwavelength sized holed on a 150nm Al film designed to operate at the primary colors of red, green, and blue. The hole array plasmonic filters show peak transmission in the 40minus;50% range for large (>25mu;m2) array sizes and maintain their color fidelity for pixel sizes as small as 1mu;m2. Hole array filters are found to be robust with respect to spatial crosstalk between adjacent pixels within our detection limit and preserve their filtering function in arrays containing random defects. A quantitative analysis of hole array filter transmittance and crosstalk suggests that plasmon coupling via nearest neighbor holeminus;hole interactions rather than long-range plasmonic Block mode interactions play the dominant role in the transmission properties of plasmonic hole array filters.
Furthermore, we report the first demonstration of CMOS plasmonic color imaging. A commercial black and white 1/2.8 inch CMOS image sensor was integrated with a 360×320 pixel plasmonic color filter array composed of hole-array filters in a Bayer mosaic layout. Full color images are taken with off-the-shelf c-mount lenses with focal lengths ranging from 6-50mm, all showing good color fidelity with Delta E (CIE2000) = 3.9-4.4 after a white balance and color matrix correction is applied to the raw image over the wide range of f-numbers ranging from 1.8-16.

Nanoscale antennas sandwiched between two graphene monolayers yield a photodetector that efficiently converts visible and near-infrared photons into electrons with an 800% enhancement of the photocurrent relative to the antennaless graphene device. The antenna contributes to the photocurrent in two ways: by the transfer of hot electrons generated in the antenna structure upon plasmon decay, as well as by direct plasmon-enhanced excitation of intrinsic graphene electrons due to the antenna near field. This results in a graphene-based photodetector achieving up to 20% internal quantum efficiency in the visible and near-infrared regions of the spectrum. This device can serve as a model for merging the light-harvesting characteristics of optical frequency antennas with the highly attractive transport properties of graphene in new optoelectronic devices.

The ability to manipulate the polarization state of light is one of the basic photonic functionalities. Polarization control is usually achieved using the retardation effect in natural birefringent crystals. Metallic nanostructures allow unprecedented flexibility in light polarisation manipulation, since plasmonic excitations have distinct polarisation properties. The coupling of light to plasmonic excitations, that are collective electronic modes in metallic nanostructures, allows one to confine the electromagnetic field on subwavelength scales and manipulate it with high precision. In this talk we will discuss new approaches to light polarisation control based on plasmonic metamaterials with hyperbolic dispersion and non-Hermitian metamaterials with loss-coupled polarisation states. In different realisations, plasmonic metamaterials allow one to achieve very strong circular birefringence and circular and linear dichroism and can be used for designing subwavelength-thin polarization components. We will also discuss the possibility of all-optical ultrafast control of polarisation state of light by employing intrinsic metal nonlinearities.

Metallic nanodevices based on surface plasmon polaritons provide new routes to enhance light matter interactions in subwavelength volumes [1], with major applications in molecular sensing [2], light-emitting devices [3] and photovoltaics [4]. In this context, special attention was recently devoted to optical antennas, counterparts of radio and microwave antennas in the optical regime [5]. By reversibly converting propagating electromagnetic waves into localized energy and modifying the emission properties of individual quantum emitters, optical antennas appear to be essential devices to enhance fluorescence spectroscopy [6], Raman spectroscopy [7] and infrared absorption spectroscopy [8]. However, because of their dipolar properties, such optical antennas exhibit a narrow-band response, and hence are not suitable for applications related to multispectral sensing of biomolecules, nonlinear vibrational sensing and nonlinear plasmonics. Despite significant progress, developing robust optical nanodevices with a significant bandwidth of operation remains an open challenge in plasmonics.
In this presentation, we will report a significant step towards tackling this challenge via the report of an optical antenna operating in a broad range of frequencies [9]. Our work is based on a new class of multifrequency optical antennas incorporating a log-periodic design providing a high electromagnetic intensity enhancement on a bandwidth of several octaves. Experimental demonstration of biosensing in a spectral window of several microns has been performed using molecules functionalized on multifrequency optical antennas [10], and high nonlinear interactions between light and such nanodevices have been measured in a broad range of frequencies [11]. These results open new routes for performing multispectral sensing on the same substrate, developing efficient tunable nanosources of light and enhancing the sensitivity of nonlinear sensing techniques.
References:
[1] S. A. Maier et al, Plasmonics: Fundamentals and Applications; Springer: New York, 2007.
[2] Y. Fu et al, Laser Photonics Rev. 2009, 3, 221.
[3] J. A. Schuller et al, Nat. Mater. 2010, 9, 193.
[4] H. A. Atwater et al, Nat. Mater. 2010, 9, 205.
[5] L. Novotny et al, Nat. Photon. 2011, 5, 83.
[6] A. Kinkhabwala et al, Nat. Photon. 2009, 3, 654.
[7] B. Yan et al, ACS Nano 2009, 3, 1190.
[8] F. Neubrech et al, Phys. Rev. Lett. 2008, 101, 157403.
[9] M. Navarro-Cia et al, ACS Nano 2012, 6, 3537.
[10] H. Aouani et al, ACS Nano 2012, submitted.
[11] H. Aouani et al, Nano Lett. 2012, 12, 4997.

The propagation and emission of light in nanostructures and metamaterials is determined by a complex interplay of electromagnetic fields that vary on a deep subwavelength scale. Standard optical techniques cannot be used to spatially resolve these variations due to their limited spatial resolution. Here, we use Angle-Resolved Cathodoluminescence (CL) Imaging Spectroscopy (ARCIS), to study the excitation of light at the nanoscale by two complementary plasmonic emitters: Au nanoparticles and nanohole.
In ARCIS a 30 keV electron beam is used as a broadband point excitation source which is used to locally drive plasmonic resonances. The electron beam is raster-scanned over the surface with 10 nm resolution and the emitted radiation is collected by a parabolic mirror after which its spectral and angular distribution is determined. Here, we present novel insights in the details of the coupling of the electron beam to metallic nanoparticles and holes, and obtain deep fundamental understanding about the nature of their resonant modes.
Au nanoparticles were made on a Si substrate, and holes in a Au film on Si, both with diameters ranging from 50 to 180 nm. Electron-beam lithography and focused-ion-beam milling were used to fabricate the structures. We measure both the CL spectra and the angular emission patterns for the two complementary geometries. For small nanoparticles (<70 nm) we find a single resonance peak at a wavelength of ~570 nm and an azimuthally symmetric toroidal angular emission pattern corresponding to an out-of-plane-oriented dipolar localized surface plasmon resonance (LSPP). This behavior is independent of where the particle is excited by the electron beam.
For larger nanoparticles (>70 nm) the electron beam position strongly on the particle strongly determines the angular and spectral CL distribution. Clear beaming of light is observed away from the excitation point when the particle is excited on the edge. This beaming effect becomes stronger for large nanoparticles and depends strongly on wavelength. We attribute this beaming effect to the simultaneous excitation of both in-plane and out-of-plane dipolar radiation contributions effectively leading to a tilted dipole moment.
ARCIS measurements on the complementary geometry, holes in the Au film show, show clear resonant modes with distinct angular radiation profiles. Here too, we observe the simultaneous excitation of in-plane and out-of-plane dipole moments. The data are analyzed using an analytical model in which the coupling of a point dipole (representing the electron beam) to the different dipole moments of particles and holes is described by a polarizability tensor that is calculated using a three-dimensional boundary element method. This work shows that nanoscale spatial control over the excitation of elementary plasmonic excitations leads to ultimate control over the spectrum, polarization and angular distribution of plasmonic emitters.

With unprecedented ability to localize electromagnetic field in time and space, nanometer scale laser promises exceptionally broad scientific and technological innovation. However, as laser cavity becomes subwavelength, the diffraction of light prohibits the directional emission, so called the directionality, one of the fundamental attributes of the laser. Here, we have demonstrated a deep sub-wavelength waveguide embedded (WEB) plasmon laser that directs more than 70% of its radiation into an embedded semiconductor nanobelt waveguide with dramatically enhanced radiation efficiency. The unique configuration of WEB plasmon laser naturally integrates photonic and electronic functionality allowing both efficient electrical modulation and wavelength multiplexing. We have demonstrated a plasmonic circuit integrating five independently modulated multi-colored plasmon laser sources multiplexed onto a single semiconductor nanobelt waveguide, illustrating the potential of plasmon lasers for large scale, ultra-dense photonic integration.