Symposium EP8 : Resonant Optics---Fundamentals and Applications

In this talk, we consider fundamental optical phenomena at the interface of nonlinear and singular optics in artificial media, including theoretical and experimental studies of linear and nonlinear light-matter interactions of vector and singular optical beams in metamaterials. We show that unique optical properties of metamaterials open unlimited prospects to “engineer” light itself. Thanks to their ability to manipulate both electric and magnetic field components, metamaterials open new degrees of freedom for tailoring complex polarization states and orbital angular momentum (OAM) of light. We will discuss several approaches to structured light manipulation on the nanoscale using metal-dielectric, all-dielectric and hyperbolic metamaterials. These new functionalities, including polarization and OAM conversion, beam magnification and de-magnification, and sub-wavelength imaging using novel non-resonant hyperlens are likely to enable a new generation of on-chip or all-fiber structured light applications.
The emergence of metamaterials also has a strong potential to enable a plethora of novel nonlinear light-matter interactions and even new nonlinear materials. In particular, nonlinear focusing and defocusing effects are of paramount importance for manipulation of the minimum focusing spot size of structured light beams necessary for nanoscale trapping, manipulation, and fundamental spectroscopic studies. Colloidal suspensions offer as a promising platform for engineering polarizibilities and realization of large and tunable nonlinearities. We will present our recent studies of the phenomenon of spatial modulational instability leading to laser beam filamentation in an engineered soft-matter nonlinear medium.
Finally, we introduce so-called virtual hyperbolic metamaterials formed by an array of plasma channels in air as a result of self-focusing of an intense laser pulse, and show that such structure can be used to manipulate microwave beams in a free space.

Realization of dynamic metasurfaces composed of arbitrarily reconfigurable phased arrays of antennas is a currently a widely-embraced objective for nanophotonics research, and has numerous applications including beam-steering and shaping, holography, and optical correlation. To achieve this, a fast active optical component for spatial control of the phase and amplitude of light in the mid-IR is needed. Graphene is an ideal material for use in such a device as it has a highly tunable dielectric constant in the mid-IR which varies with charge carrier density and therefore can be tuned using an electrostatic gate^[1]. Previous studies have demonstrated the potential use of graphene/gold antenna structures for tuning absorption and phase, showing a theoretical phase tunability of up to 240°.^[2,3]

We demonstrate mid-infrared beam steering by metasurfaces composed of electrically controlled reconfigurable graphene-gold antenna arrays operated in reflection mode at a wavelength of 8.3 um. By independently gating antenna elements, real-time beam steering is achieved. To experimentally realize this device, we pattern a periodic array of gold antennas of 1.2μm length spaced by 50nm on graphene on a 500nm thick SiN_x membrane with a gold back reflector. This introduces a gap plasmon mode, which exhibits a 360° phase shift upon reflection at zero bias and a strongly confined electric field at the graphene surface. By electrostatically gating the graphene, we can then modulate this resonance and actively change the reflection phase by 215° at the operation wavelength. By electrically isolating each element of the metasurface and addressing it with a distinct voltage to achieve the desired phase, we will experimentally show that a reflected beam can be steered by 5° in the far field.

1. Brar, Jang, Sherrott, Lopez, Atwater NanoLetters 13, 2541 (2013)
2. Y. Yao et al, Nano Lett. 2014 Nov 12;14(11):6526-32
3. Z. Li and N. Yu, Appl. Phys. Lett. 102, 131108 (2013)

Many exciting fundamentals and applications have been demonstrated in linear optical phenomena arising from two-dimensional metamaterials, usually called metasurfaces. Very recently, interesting and sometimes surprising results have been obtained when optical metasurfaces are studied in the nonlinear regime. In this talk we will present some of these results mostly for dielectric metasurfaces.

All-dielectric metasurfaces provide a platform to engineer magnetic and electric resonant modes in nanoscale resonators with very low loss. Fabricating such dielectric metasurfaces from different types of semiconductors can be used to enhance their second and third order nonlinearities significantly. We will present nonlinear optical measurements for metasurfaces fabricated from Silicon and III-V semiconductors. For example, we measure second and third harmonic generation with efficiencies orders of magnitude higher than what is obtained from their bulk constituents, for excitation wavelengths below their bandgaps. The particular geometry of the dielectric nanoresonators also imparts polarization selection rules that are different from the bulk constituents and often require invoking surface harmonic generation due to the high surface to volume ratio in top-down fabricated nanoscale resonators.

Metamaterials have recently received a lot of attention in the context of non-linear optics, since they allow dramatically boosting light-matter interaction and enhancing the strength of non-linear processes. Different non-linear phenomena, such as second-harmonic generation, difference-frequency generation and phase conjugation, have been shown to exist in sub-wavelength metamaterial structures with exceptionally large efficiencies [1]. Furthermore, the flexibility offered by metamaterials over the control of light flow allowed the realization of non-linear structures with unprecedented functionalities, such as non-linear gradient metasurfaces with full control over the non-linear emission directions [2].
Here, we present another exciting opportunity offered by non-linear metamaterials, namely the realization of non-reciprocal devices without any type of external biasing. Such structures are based on materials with third-order non-linear effects, i.e., materials with intensity-dependent refractive index or saturable absorption. The proposed devices are designed to provide different local field intensities when excited from different directions, so that the non-linear variation of the refractive or absorption indices, and subsequently the transmission through the structure, are different for different propagation directions. A common problem of such structures is the existence of a trade-off between transmission asymmetry (isolation) and maximum transmission. Here we theoretically prove that this problem is the result of time-reversal symmetry and show how it can be overcome by using multi-stable non-linear responses. Furthermore, in the case without multi-stability, we derive fundamental bounds about the maximum transmission that can be achieved for a given transmission contrast. Following this general theory, we design non-linear isolators based on multi-quantum-well metamaterials, exploiting their giant non-linear response.
References
J. Lee, M. Tymchenko, C. Argyropoulos, P. Y. Chen, F. Lu, F. Demmerle, G. Boehm, M. C. Amann, A. Alù, and M. A. Belkin, “Giant Nonlinear Response from Plasmonic Metasurfaces Coupled to Intersubband Transitions,” Nature, Vol. 511, No. 7507, pp. 65-69, July 2, 2014.
M. Tymchenko, J. S. Gomez-Diaz, J. Lee, M. A. Belkin, and A. Alù, “Gradient Nonlinear Metasurfaces,” Phys. Rev. Lett., in press, 2015.

Photonic crystals (PhCs) are resonant optical materials that can be used to efficiently modulate the propagation direction of light and redistribute the photon energies throughout the electromagnetic spectrum. Emerging micro- and nanotechnology have pushed the fabrication of solid structures down to nanoscale. However, most PhCs have limited tunability and structural reconfigurability, attributed to their solid nature and the relatively static optical constants of solid materials. Here, we evaluated, for the first time, an active 3D photonic crystal assembled from reconfigurable intersecting microplasma column arrays, with electron densities varying from 1 x 10¹³ cm^-3 to 1 x 10¹⁶cm^-3, in all the three dimensions. A significant photonic stopband with strength up to 52% gap-midgap ratio is observed under intermediate electron density level (>1×10¹⁵ cm^-3) when the permittivity contrast between plasma and the background material becomes sufficiently large through finite-difference time-domain (FDTD) interpretation. With such a large n_e tunable range, the corresponding electromagnetic responses can potentially cover the extremely high frequency (EHF), e.g. 30GHz to 300GHz, which has been broadly used in radio astronomy and remote sensing. It is observed that strong surface plasmon resonances co-exist under all electron densities and shift to higher frequency with the increase of plasma frequency. The overlapping of stopband resonance (strong reflectance) and surface plasmon resonance is observed under certain range of the electron densities. The capability to temporal and spatial modulate microplama arrays in three dimensions, in combination of the complex three dimensional geometry, provides abilities to control isotropic optical responses including but not limited to surface plasmon resonance and photonic band gap. We therefore anticipate the resonant electromagnetic system using microplasma as the tunable medium would be a valuable system to study and would potentially lead to a number of novel and important applications in both gigahertz and higher frequency regime.

Modifying spontaneous emission decay rate of quantum emitters by coupling them to nanostructured environments has been an active area of research during last decade due to number of interesting potential applications in sensing, light-emitting diodes (including single-photon sources) and solar thermophotovoltaics.
We report spontaneous emission decay rate modification of InP quantum dots coupled to plasmonic cavity that incorporates degenerately doped semiconductors, namely, transparent conducting oxides such as indium-tin-oxide (ITO) and transition metal nitrides such as titanium nitride (TiN). We use rf magnetron sputtering to fabricate ITO and TiN films. By changing sputtering parameters, such as the nitrogen flow rate for TiN, Ar/O₂ flow rates for ITO and growth temperature, we control the carrier concentration, and, hence, complex refractive index in ITO and TiN films. In case of TiN the experimentally accessible carrier concentrations range from 2.8 x10²² cm^-3 to 5.6 x10²² cm^-3 while in case of ITO they range from 5 x10¹⁹ cm^-3 to 10²¹ cm^-3.
First, we fabricate multiple plasmonic cavities that have identical geometry but different carrier concentrations of the incorporated semiconductors. The studied plasmonic cavities consist of a silver back reflector, SiO₂ (or HfO₂) spacer, TiN or ITO film and GaN nanorod. We place InP quantum dots in the middle of the metal-oxide-semiconductor structure and perform time-resolved photoluminescence (PL) measurements to define spontaneous emission decay rate of InP quantum dots coupled to the fabricated plasmonic cavities.
Next we apply electrical bias between TiN (or ITO) film and silver back reflector. Ag/SiO₂/TiN stack forms a metal-oxide-semiconductor (MOS) capacitor: applying electrical bias forms electron accumulation in the semiconductor at the semiconductor/ SiO₂ interface thus modifying local density of optical states (LDOS) in the vicinity of the considered structures that modifies the decay rate of the InP quantum dots coupled to the structure as well as their far field radiation pattern. Using our time-resolved PL setup we measure the modification of the emission decay rate as a function of applied bias. We show two-fold decrease of the spontaneous emission decay rate under applied electrical bias. This work may have important applications in development of on-chip optical elements.

Metasurfaces composed of planar arrays of sub-wavelength artificial structures show promise for light manipulation, and have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces and orbital angular momentum manipulation and detection over a broad range of electromagnetic spectrum [1]. However the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after fabrication, thus limiting their use in a wide range of applications.

We experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. The metasurface we study consists of a gold back plane, an ITO layer (17 nm) followed by an aluminum oxide layer (5 nm) on which we pattern a gold nanoantenna array. The identical antennas are connected either to right or left external gold electrodes via connections to create electrical gates. Unlike previous optical frequency metasurfaces which used differences in antenna geometry or orientation to impose different phase shifts for each antenna element, the metasurface we study here is periodic, and different phase shifts by metasurface elements are achieved by application of different bias voltages to adjacent antenna electrodes. Each metasurface antenna element is effectively an MOS capacitor with the Au antenna serving as a gate and ITO functioning as a field effect channel [2]. When applying an electrical bias between the antenna gate and the underlying ground plane, the carrier concentration at the Al₂O₃/ITO interface increases or decreases by forming a charge accumulation or depletion layer [3]. This results in modulation of the complex permittivity of ITO and variation of the reflected phase and amplitude from each antenna element.

By using a Michelson interferometer, we measure a phase shift of π and ~ 30% change in the reflectance at wavelength of ~1550 nm by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz, and electrical switching of +/-1 order diffracted beams, a basic requirement for electrically tunable beam-steering phased array metasurfaces. In addition to the fundamental interest of tunable metasurfaces, these structures have many potential applications for future ultrathin optical components, such as dynamic holograms, tunable ultrathin lens, reconfigurable beam steering devices, nano-projectors, and nanoscale spatial light modulators.

References:
1. N. Yu et al., “Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction,” Science 334, 333 (2011)
2. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-Order Index Change in Transparent Conducting Oxides at Visible Frequencies,” Nano Lett. 10, 2111-2116 (2010).
3. H. W. Lee et al., “Nanoscale Conducting Oxide PlasMOStor,” Nano Lett. 14, 6463 (2014).

Metasurfaces have yielded very interesting insights into fundamental physics like the photon spin hall effect [1] and unique applications [2] such as ultrathin optical elements and holography. However most of the previous research has encompassed static structures, whose properties are determined by nanoscale geometry and are fixed once fabricated. More recently, actively controlled metasurfaces have emerged with active media/mechanisms that include phase change materials [3], mechanical strain [4] and liquid crystal dynamic polarization5. Of particular interest is field-effect tunable permittivity modulation using transparent conducting oxides (TCO) [6], based on carrier-induced changes in the complex refractive index in a thin TCO active layer in proximity to a nanoscale resonant structure.

We report and experimentally demonstrate an actively controlled gate-tunable Fano resonant plasmonic metasurface operating in the visible region, where –strikingly - the operating voltages for reflectance and complex index modulation are
We explain this behaviour via field-effect dynamics and major field confinement in the ITO active layer, particularly at the corners of resonantly excited structures, and have corroborated this idea with full wave electromagnetic simulations. A carrier concentration change in the ITO from 5E20 cm-3 to 8E20 cm-3 within an active layer of 2 nm is used to model the effect of millivolt-scale switching of ITO permittivity.
Switching the accumulation layer to a depletion layer by reversing the bias, leads to interesting complementary effects such as increased absorption in the same structures. The reflectance modulation is also polarization sensitive, as an increase/decrease in reflection/transmission/absorption is polarization dependent, also seen in the light of mode interactions in such Fano resonant systems. Further investigation into this unique response will be presented.

[1]. X. Yin, Z. Ye, J. Rho, Y. Wang and X. Zhang, Science 339 (6126), 2013 [2]. N. Yu and F. Capasso, Nature Materials 13, 139-150, 2014 [3]. M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S.
Walavalkar, J. Ma and H. A. Atwater, Opt.
Express 17(20), 2009
[4]. I. M. Pryce, K. Aydin, Y. A. Kelaita, R. M. Briggs and H. A. Atwater, Phil. Trans. R. Soc. A 369, 3447-3455, 2011 [5]. O. Buchnev, J. Y. Ou, M. Kaczmarek, N. I. Zheludev and V. A. Fedotov, Opt. Express 21(2), 1633-1638, 2013 [6]. H. W. Lee, G. Papadakis, S. P. Burgos, K. Chander, A. Kriesch, R.
Pala, U. Peschel, and H. A. Atwater, Nano Lett., 14(11), 2014

Thermal emission impacts a wide variety of applications, including thermophotovoltaics, photovoltaics, photon-enhanced thermionic emission, selective solar absorption, incandescent lighting, and spectroscopy. Ordinary structures generally emit a broad range of wavelengths, angles, and polarizations. However, highly-selective thermal emission has potential to greatly improve performance in many of these applications. While prior work has explored a wide range of structures to provide some degree of control of one or more of these attributes, there is an ongoing challenge in combining readily-fabricated, simple structures made of appropriate materials with the desired functionality. Here, we will focus on using metasurfaces in conjunction with refractory materials as a platform for achieving selective control of emission. These structures are built from sub-wavelength elements that support localization of surface plasmon polaritons with appropriate attributes. Modeling is performed using rigorous coupled wave analysis (RCWA) of absorption, plus Kirchhoff’s law of thermal radiation, and is further validated using finite-difference time domain (FDTD) as well as a direct thermal emission simulation in at least one case. This strategy is first applied to samarium-doped glass to achieve highly wavelength- and polarization-sensitive emission suitable for applications such as thermophotovoltaics. It is then also applied to various shallow periodic structures to achieve both symmetric and asymmetric angular selectivity with near-blackbody emissivity. We have studied the effects of changing the period, depth and shape of the periodic unit cell on the direction angle, angular spread, and magnitude of coupled radiation mode. Such structures can be considered arbitrarily directional sources that can be carefully patterned on metallic surfaces to yield a thermal lens with a designed focal length and/or concentration ratio; the benefit of this approach is that it can enhance the view factor between thermal emitters and receivers, without restricting the area ratio or separation distance.

An efficiency of generation of energetic plasmonic carriers in metal nanostructures depends strongly on the optical design and material composition. In our studies, we demonstrate the ability to generate large numbers of hot plasmonic carriers in specially-designed hybrid nanostructures [1,2,3]. Optical generation of hot electrons becomes especially efficient in plasmonic nanostructures with electromagnetic hot spots [1,4]. Theoretically, the problem of generation of energetic plasmonic electrons is treated using the quantum mechanical approaches based on the kinetic DFT and the equation of motion of the density matrix [3,4,5]. A size of nanocrystal is a crucial parameter determining a shape of the energy distribution of optically-excited plasmonic carriers. In large nanocrystals, most excited carriers have very small excitation energies and the electron distribution resembles the case of the plasmon wave in bulk. For metal nanocrystal with smaller sizes (less than 10nm) or in nanostructures with hot spots, the optically-excited state incudes a large number of carriers with high energies [1,3-5]. Nanostructures with plasmonic hot spots or with strong electromagnetic enhancement generate unusually large numbers of energetic electrons, which can be observed using the ultrafast spectroscopy [1,4]. Overall, our investigations have shown that the resonant optical geometry of a metastructure is crucial for the hot-electron generation and related photo-catalysis [6]. The results obtained in this study can be used to design plasmonic hot-electron nanodevices for photo-catalysis, photo-detectors and solar-energy applications.

[1] H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L. Khosravi Khorashad, L. V. Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015).
[2] W. Li, Z. J. Coppens, L. V. Besteiro, W. Wang, A. O. Govorov, J. Valentine, Nature Communications 6, 8379 (2015).
[3] A.O. Govorov, H. Zhang, V. Demi, and Y. K. Gun’ko, Nano Today, 9, 85 (2014).
[4] H. Zhang and A. O. Govorov, J. Phys. Chem. C 118, 7606 (2014).
[5] A.O. Govorov and H. Zhang, J. Phys. Chem. C, 119, 6181 (2015).
[6] L. Weng, H. Zhang, A. O. Govorov, and M. Ouyang, Nature Communications 5, 4792 (2014).

Perfect absorbers and emitters enjoy widespread interest owing to their potential application in energy harvesting and sensing devices, including photovoltaics, thermal emission control, as well as bolometer and photodetector design. Here we report on design and fabrication of broadband, polarization- and angle-insensitive perfect absorbers and emitters based on sparse III-V compound semiconductor nanowire (NW) arrays, and demonstrate a versatile large-area low cost nanowire array fabrication scheme using a method based on mechanical peel-off from bulk substrates. Specifically, we address design and fabrication for the UV to near-IR and near-IR to mid-IR wavelength ranges using InP (E_g=1.34eV) and InSb (E_g=0.17eV), respectively, as active absorbers.
Herein, we present results on InP and InSb NW array broadband absorbers, supported by experiment, simulation and analytic theory. Electromagnetic simulations indicate that tapered and multi-radii NW arrays with 5% fill fraction (ff) can achieve greater than 95% broadband absorption (λ_InP=400-900nm, λ_InSb=1.5-5.5µm) and exhibit ~25% absorption enhancements over uniform arrays of equal material usage. Experimentally, InP broadband absorbers were fabricated via electron beam or nanoimprint lithography and reactive ion etching techniques. Optical characterization of 5% ff InP NW arrays (r=50-90nm, h=2µm) embedded in PDMS and peeled off from the substrate exhibited >80% broadband absorption (unpolarized light, λ=400-900 nm, incidence angle = 0-30°) and close agreement with simulation. In the near- and mid-IR, a 5% ff InSb tapered NW array (h=20 μm, r_upper=100 nm, r_lower=550 nm) achieves an average absorption of 95% from 1.5 to 5.5 μm. Additionally, we report designs for switchable absorption in which a thin, planar VO₂ layer above the array enables active thermal tunability, effecting a 50% modulation, from 87% (insulating VO₂) to 43% (metallic VO₂) average absorption.
The absorption mechanism in both material systems is identical – strong coupling of incident light into resonant waveguide modes of the NWs, most notably the HE₁₁ mode. The high refractive indices and large absorption coefficients, arising from direct interband transitions, of InP (UV-Vis-NIR) and InSb (NIR-SWIR-MWIR) make them ideal materials for dielectric waveguides with high confinement and high loss (i.e. absorption), respectively. Due to the spectral dependence of the resonant HE₁₁ mode on wire radius, NW array optical response can be passively tuned via geometric modifications, and a broadband resonant response can be achieved via (1) a multi-wire unit cell – an array containing sub-lattices of NWs with different radii, and (2) NW taper. The former method increases the number of resonant wavelengths and the latter creates a continuous spectrum of resonant wavelengths. Additionally, the symmetry of square-packed arrays and the minimal dispersion of the HE₁₁ waveguide mode result in polarization- and angle-insensitivity, respectively.

Resonant nanostructures have the ability to trap, redirect and manipulate light. We investigate the optical properties of silicon nanowires on top of a silicon substrate, with a particular emphasis on their ability to suppress reflection and enhance light transmission into the silicon substrate. After exploring the properties of a single resonator, we consider array of nanowires, starting from a weakly-coupled regime where the optical response of the array resembles the single resonator to a strongly-coupled, metamaterial regime. Metamaterials are artificially engineered materials offering the unique opportunity to tailor their optical properties and functionalities. Effective medium approximation is often used to describe the optical permittivity of metamaterials. We demonstrate that non-resonant nanostructures can be described with a first order approximation, while second order terms in the effective medium modeling are needed to account for resonant metamaterials. In this talk, we propose the use of resonant structures and metamaterials as antireflection coatings for silicon-based solar cells. We demonstrate that a quarter wavelength-thick uniform silicon nitride layer can be replaced by a linear array of silicon nanowires having appropriate filling fraction, size and thickness. We provide a design guideline to achieve low reflectivity and similar spectral features for different sizes of the wires. Dependence on the angle of incidence is also investigated. Taking advantage of the properties of single resonators, we design and fabricate bimodal arrays of 2D and 3D nanostructures in order to extend the spectral bandwidth and increase light transmission over a large wavelength range, covering both the solar spectrum peak and the silicon absorption peak. As a result, we measure average reflectivity in the 400 nm - 900 nm range lower than 5% corresponding to a 45% increase of the maximum achievable short circuit current compared to flat silicon interface. The optical response of silicon resonators and metamaterials have been simulated through Transfer Matrix Method and full-field Finite-Difference Time-Domain calculations performed with commercial software package (Lumerical). Nanostructures were fabricated using Focused Ion Beam on silicon wafer. Reflectivity has been measured using a confocal microscope.

We will report on our development of flexible metamaterials (fabrics) that can provide passive radiative cooling by utilizing optical resonant effects in polymer microfibers. In particular, we will discuss new types of fabrics, which can help people feel cooler by simply allowing thermal emission from the skin to pass through the clothes rather than being trapped inside [1].

The new wearable technology is based on exploiting different mechanisms of electromagnetic waves scattering on obstacles that are either smaller or larger than the wavelength of the propagating field [1,2]. The designed fabrics are opaque for the visible light yet transparent for the infrared thermal radiation from the human body, while conventional fabrics mostly block thermal radiation emitted by the skin. The new fabrics have been designed using a combination of optimal material composition and structural photonic engineering. Synthetic polymers that support few vibrational modes were identified as candidate materials to reduce intrinsic material absorption in the IR wavelength range. Individual fibers were designed to be comparable in size to visible wavelengths in order to minimize reflection in the IR by virtue of weak Rayleigh scattering while remaining optically opaque in the visible wavelength range due to strong Mie scattering. We modeled photons interaction with single fibers, fiber bundles, and bundle arrays, which effectively act as flexible polymer optical-thermal metamaterials.

Compared to conventional personal cooling technologies, these metamaterials can provide fully passive means to cool the human body regardless of the person’s physical activity level. The anticipated effect of the new optical-thermal polymer fabrics (tailored into a short sleeve shirt and pants) will be the increase in the body cooling rate by at least 23 W, which in turn will allow for raising summer HVAC set points by >4°F (to ~79°F) leading to significant energy savings.

We will also report on our on-going research on improving the optical, thermal, and mechanical characteristics of the proposed new types of flexible metamaterials as well as on new applications beyond indoors personalized cooling.

[1] Tong, J. K., Huang, X., Boriskina, S. V., Loomis, J., Xu, Y.., Chen, G., “Infrared-transparent visible-opaque fabrics for wearable personal thermal management,” ACS Photonics 2(6), 769–778, (2015).
[2] Tong, J. K., Hsu, W.-C., Huang, Y., Boriskina, S. V.., Chen, G., “Thin-film ‘Thermal Well’ emitters and absorbers for high-efficiency thermophotovoltaics,” Optics, Sci. Reports 5, 10661 (2015).

The advancement of metamaterial functionalities for device applications has been extended to exploit the unique properties of its unit structures. In this research, the change of capacitance in the microgap of metamaterial unit structures is studied and applied for detecting dielectric constant and density of microorganism. Metamaterial structures consist of metallic arrays of electric split ring resonators with micrometer gap at the centre. The dielectric constant and the density of microorganisms are detected by the shift of resonance frequency. The shift of the resonance frequency is determined by the change of the capacitance in the microgap due to the presence of the microorganism. Simulation and experimental studies were done to understand the relationship between the change of capacitance in the micro gap of electric split ring resonator due to the existence of microorganism and the shift of the resonance frequency.

Experimentally, electric split ring resonators were fabricated by electrohydrodynamic (EHD) jet printing technology which is cheap, environmental friendly, and is able to print in large area in different substrate. One of the advantages of electrohydrodynamic jet printing is its ability to vary the size and the shape of the structures, so that several parametric studies could also be easily performed. In order to fabricate the metamaterial structures, 2 kinds of substrate were used, i.e. silicon wafer and flexible polymer (polyimide. The drop on demand techniques by controlling an electrical pulse was applied to eject silver nanoparticles ink from the glass capillary nozzle and make silver micro patterns on the substrates. Electric split ring resonator with several dimensions could be made. Yeast was used as a microorganism to be detected. Meanwhile, terahertz time domain spectroscopy was used to observe the terahertz response of metamaterial structures without yeast and with yeast. The minimum microgap of electric split ring resonator which could be fabricated by EHD jet printing was 5 μm with the size the unit structure was 50 μm. The THz response of this structure produces the resonance frequency at 0.8 THz with saturated shifting of resonance frequency 120 GHz.

Both simulation and experimental studies showed that the number density of the microorganism could make the resonance frequency shift of terahertz response due to the change of the permittivity in the micro gap of electric split ring resonators. Moreover, detection of extremely small amounts of microorganisms could also be performed because the size of micro gap of electric split ring resonators is quite similar with microorganism sizes. Functionalizing these metamaterials sensors with receptors will be useful to make high-speed-on-site specific sensing of microorganism in both ambient and aqueous environment.

Hybrid structures of graphene and metal nanoparticles (NPs) have been actively investigated as higher quality surface enhanced Raman spectroscopy (SERS) substrates. Compared with SERS substrates which only contain metal NPs, the additional graphene layer provides structural, chemical, and optical advantages. However, the two-dimensional (2D) nature of graphene limits the fabrication of the hybrid structure of graphene and NPs to 2D. Introducing three-dimensionality to the hybrid structure would allow higher detection sensitivity of target analytes by utilizing the three-dimensional (3D) focal volume. In this talk, I will present a mechanical self-assembly strategy to enable a new class of 3D crumpled graphene-gold (Au) NPs hybrid nanoplasmonic structures for SERS applications. We achieve a 3D crumpled graphene-Au NPs hybrid structure by the delamination and buckling of graphene on a thermally activated, shrinking polymer substrate. We also show the precise control and optimization of the size and spacing of integrated Au NPs on crumpled graphene and demonstrate the optimized NPs’ size and spacing for higher SERS enhancement. The 3D crumpled graphene-Au NPs exhibits at least one order of magnitude higher SERS detection sensitivity than that of conventional, flat graphene-Au NPs. The hybrid structure is further adapted to arbitrary curvilinear structures for advanced, in-situ, non-conventional nanoplasmonic sensing applications. We believe that our approach shows a promising material platform for universally adaptable SERS substrate with high sensitivity.

The synergy effect in nature could enable fantastic improvement of functional properties and associated effects. The detection performance of surface-enhanced Raman scattering (SERS) can be highly strengthened under the cooperation with other factors. Here, greatly-enhanced SERS detection is realized based on rolled-up tubular nano-resonators decorated with silver nanoparticles. The synergy effect between whispering-gallery-mode (WGM) and surface plasmon leads to an extra enhancement at the order of 10⁵ compared to non-resonant flat SERS substrates, which can be well tuned by altering the diameter of micron- and nanotubes and the excitation laser wavelengths. Such synchronous and coherent coupling between plasmonics and photonics could lead to new principle and design for various sub-wavelength optical devices, e.g. plasmonic waveguides and hyperbolic metamaterials.

Reference: Sci Rep. 2015, 5, 15012.

Tip-enhanced Raman spectroscopy (TERS) is an optical characterization technique that uses a metallic nanoantenna to enable the acquisition of chemical information with nanometer resolution and single-molecule sensitivity. To be effective, the nanoantenna must possess a determined geometry in order to exhibit a strong localized surface plasmon resonance (LSPR) near the incident wavelength, thus facilitating near-field amplification of both incident and Raman-scattered light. However, because nanoscale metal features determine the probe resolution and sensitivity, reliable TERS probe fabrication remains a major challenge. We have developed a new TERS probe based on the self-assembly of colloidal metal nanoparticles that support strong LSPRs. We observe exceptionally high Raman enhancement factors for commercially available AFM probes decorated with Ag nanocubes. Using these nanoantenna, we demonstrate the identification and surface mapping of molecular surface layers patterned with features below the diffraction limit.

This presentation reviews the performance of plasmonic and photonic crystals fabricated on 200 mm diameter wafers for surface enhanced Raman spectroscopy (SERS), label-free sensing, and microarray-based surface enhanced fluorescence sensing (SEFS) applications. 1-D and 2-D periodic nanostructures were fabricated on glass, fused silica, and silicon wafers using optical lithography and semiconductor processing techniques. Wafer-scale optical metrology results were compared to FDTD and RCWA modeling and will be presented along with application-based performance results, including: (1) label-free 1-D photonic crystal sensing of surface binding kinetics using the guided mode resonance (GMR) effect, (2) SERS sensing with 2-D nano-dome arrays utilizing localized plasmonic hotspots, and (3) microarray-based SEFS using 2-D aluminum nano-hole arrays (NHA).
Four 1-D photonic crystal GMR filter designs, composed of TiO₂ slab waveguide layers deposited on top of SiO₂ gratings, were fabricated with pitch varying from 360 - 410 nm for label-free sensing and SEFS applications. The spectral position of the GMR is shifted upon changes in excitation angle or local refractive index. GMR filter wafer-uniformity maps were prepared and quality factors exceeded 200 in some cases. The structures were designed for use in an air environment and showed a large shift (~2.5 nm) in resonance position after binding a thin self-assembled monolayer (~1.6 nm thick) to the photonic crystal surface as a protein mimic. GMR filters also showed reasonable shift (~0.3 nm) during in-situ label-free sensing in an aqueous environment. Subsequently, an improved GMR filter design optimized for aqueous environments was developed. Greater than three-fold fluorescence intensity enhancement was also demonstrated for a dye layer adsorbed to the surface of a filter when the excitation angle was tuned to match the GMR condition. 2-D metallic nano-dome arrays were also fabricated and showed sensitivity to the gap between adjacent nano-domes and to the smoothness of the gold coatings. The experimental (spatially averaged) SERS enhancement factor was calculated as being greater than 1.35 x 10⁵ for samples with ~15 nm nano-gaps. The 2-D aluminum NHA for microarray applications exhibited more than a three-fold improvement in fluorescence enhancement of Cy3 dye when compared to a leading commercial microarray glass slide in various sandwich bioassays. The periodic structure of the NHA helps to couple excitation light into vertically-confined propagating surface plasmons at both the aluminum-air and the aluminum glass interfaces. The specific hole geometry helps to focus light into the base of the nanoholes via the localized surface plasmon resonance effect. When fluorescently labeled molecules are immobilized within these regions, they are excited by a much greater intensity of light per unit area, and emit a much greater fluorescent signal, leading to the observed enhancements.

We have demonstrated a surface-enhanced Raman scattering (SERS) substrate capable of detecting 1 pM of benzenethiol (BT) and developed better understanding of its enhancing mechanism by varying the fabrication process and also performing detailed simulation studies. The extreme enhancement originates from the three dimensionally fabricated and optimized substrate, which is made of Au-nanoparticle (NP) clusters stacked using vertically standing ZnO nanowires (ZnO-NW) as skeletal frames that completely dissolve away during the synthesis procedure and leave vertical light passages among clusters, allowing light to reach deeper into the stacks for higher sensitivity.
Our straightforward two-step-fabrication process involves (1) the hydrothermal synthesis to create ZnO-NWs perpendicularly standing on the substrate and (2) the liquid phase deposition (LPD) to create Au NPs on ZnO-NWs: (1) to grow ZnO-NWs, we coat a Si substrate in a ZnO-seed solution consisting of zinc acetate dihydrate in ethanol and anneal at 350 °C, followed by immersion in a ZnO-NW precursor solution made of zinc nitrate hydrate, hexamethylenetetramine, and polyethylenimine in DI water at 95 °C; and (2) to grow Au-NP clusters using the LPD process, we submerged the ZnO-NW substrate in a Au-NP precursor solution consisting of sodium tetrachloroaurate(III) dihydrate, sodium citrate dihydrate and sodium hydroxide in DI water at 90 °C. The LPD process was repeated 1-8 times to systematically vary the thickness of the Au-NP layers.
After the ZnO-NW synthesis, we used the scanning electron microscopy (SEM) to examine the substrate and observed 1 μm-tall perpendicularly synthesized ZnO NWs. We also observed after the Au-NP deposition process that 20nm-diameter Au NPs were uniformly coated on the surface of ZnO NWs, and increasing the iterations of the LPD process resulted in thicker Au-NP-cluster stacks.
To characterize SERS performance, we incubated the substrates with 1-8 iterations of the Au-NP synthesis in 1mM BT solution. The SERS intensity of BT consistently increased with the increasing number of LPD iterations up to 5 and then saturated due to the increase in the effective sensing area and plasmonic interactions among NPs. After 5 iterations, the most laser power was absorbed in the upper layers of the Au-NPs, and the additional layers had almost no effect. If Au-NPs were cumulated on Si substrates without ZnO-NWs, the SERS intensity never saturated even after 8 iterations and was about half the intensity from the 3D stacked substrates. Numerical simulation also showed good agreement with our experimental results and provided helpful insight into the SERS mechanism, including the roll of the ZnO-NWs. The substrate also generated clear BT Raman peaks down at 1 pM, and its enhancement factor was ~10⁸. Because of the excellent SERS performance and simple two-step wafer-scale fabrication using wet chemistry, our approach will prove to be very useful in various sensing applications.

Metallic structures can efficiently couple light into a region of subwavelength size. In these regions, large local field enhancements can occur and enable an increased absorption of molecules which are placed in these so called "hot spots". Arrays of metallic structures which are designed for surface enhanced infrared absorption spectroscopy (SEIRA) enable the detection of molecular vibrations with high sensitivity [1,2].
Metallic strip gratings can provide a significant field enhancement at the edges along the strips. In a previous publication we showed the spectral tuning of gold strip grating resonances by varying the grating period and demonstrated significant enhancement of a PMMA absorption band at a wavelength of 5,7 µm [3]. Such grating resonances can also be spectrally tuned by varying the angle of incidence and thus be matched to absorption bands of interest [4].
In contrast, infrared metallic antenna arrays are usually tuned to specific resonance wavelengths by e.g. varying the antenna length or the refractive index of the substrate material [2,5]. Nevertheless, by changing their periodicity, resonances of infrared antenna arrays can be improved in terms of field enhancement by realizing collective excitations [1]. In principle, this means that the antenna resonances should be close to, but still at higher wavelength compared to the spectral position of the first grating oder. Fulfilling this condition, the local field enhancement can be increased by one order of magnitude and thus enables even higher sensitivity for SEIRA [1].
In our work [6], we use the angular range of a Schwarzschild-objective for a tuning of the antenna array performance in order to optimize the enhancement of two adjacent vibrational bands of a self-assembled thiol-monolayer. Varying the incident angle shifts the spectral positions at which collective excitation and peak field enhancement of the antennas occur. This allows a spectral tuning of the array resonance without changing geometry or surrounding material of the antennas. Two adjacent molecular absorption bands are examined and the enhancement factor is improved by a factor of up to 1.75 compared to usual angle-averaged measurements.

[1] R. Adato, A. A. Yanik, J. J. Amsden, D. L. Kaplan, F. G. Omenetto, M. K. Hong, S. Erramilli, and H. Altug, P. Natl. Acad. Sci. Usa 106, 19227–19232, 2009.
[2] F. Neubrech, A. Pucci, T. W. Cornelius, S. Karim, A. Garcia-Etxarri, and J. Aizpurua, Phys. Rev. Lett. 101, 157403, 2008
[3] T. Wang, V. H. Nguyen, A. Buchenauer, U. Schnakenberg, and T. Taubner, Opt Express 21, 9005–9010, 2013.
[4] J. W. Petefish and A. C. Hillier, Anal. Chem. 86, 2610–2617, 2014.
[5] J. M. Hoffmann, X. Yin, J. Richter, A. Hartung, T. W.W. Mass, and T. Taubner, J. Phys. Chem. C 117, 11311–11316, 2013.
[6] J. T. W. W. Mass and Thomas Taubner, ACS Photonics, doi:10.1021/acsphotonics.5b00399, 2015.

Current near-field spectroscopy techniques are limited by the ability to fabricate nanoscale probes that are robust, reproducible, and support high quality optical resonances. I will present our recent work on the synthesis and self-assembly of colloidal nanocrystals for the fabrication of resonant optical nanojunctions. Previously, we demonstrated that shaped colloidal nanocrystals can be organized into nanojunctions that possess intense "hot spots" due to electromagnetic field localization. When these nanocrystals are brought into contact with a metal substrate, they form a high quality optical cavity with a coupled resonance mode. Here, I will describe how colloidal nanocrystals can be assembled and used as scanning near-field optical probes for Raman nanospectroscopy. Nanocrystals are assembled onto commercially available silicon atomic force microscope cantilevers that can then be raster scanned across a surface during optical measurements. Nanocrystal size and shape can be used to modulate the optical response of the scanning probe. To the best of our knowledge, the nanocrystal probes demonstrated in our work exhibit the largest near-field enhancements reported for tip-enhanced Raman spectroscopy measurements under ambient conditions. The methods presented in our work have the potential to enable Raman nanospectroscopy as a powerful tool for chemical mapping of surfaces and nanomaterials.

During the last decade, resonant optics attracted enormous interest in science and technology. This not only relies on the fact that this research field provides deep insights in fundamental physics but also on an increasing number of applications ranging from filters over waveguides towards sensor devices. The latter aspect went along with the introduction of appropriate fabrication tools where e-beam lithography still plays a central role. Although excellent in resolution, accuracy and reproducibility this technology is mostly limited to flat surfaces implied by the preliminary required resist process step. Also, the fabrication of complex, free-standing and even overhanging 3D architectures is very complicated or even impossible via this technology. Both aspects together, unfortunately, limit the applications by means of rough or highly exposed surfaces regions and also prevents the fabrication of potentially interesting and promising 3-dimensional plasmonic architectures. Ideally, an additive manufacturing technique would be needed which allows direct-write fabrication on the nanoscale with appropriate materials. One such technology is focused electron beam induced deposition (FEBID) which has been used in the past for e.g. local modification of traditionally fabricated plasmonic structures via carbon deposits to tailor and fine-tune plasmonic activities. Regarding metal structures, FEBID had the long lasting problem of high carbon impurities up to 90 at.% which has recently been solved by a fast and simple post-fabrication process leading to pure and highly compact Pt and Au (nano)structures. In this contribution we demonstrate new pathways towards plasmonic 3D structures on practically any given surface by using FEBID as 3D nano-printer. After a brief introduction into this technology, we compare plasmonic activities of traditionally fabricated Au nano-discs with analogous FEBID structures via scanning transmission electron microscopy (STEM) based electron energy loss spectroscopy (EELS) measurements. Beside chemical purity, FEBID discs reveal same plasmonic edge and surface modes as found for ideally produced Au discs while slightly shifted in their energies. In the second part, we demonstrate FEBIDs unique 3D nano-printing capabilities for the fabrication of free-standing 3D nano-architectures with branch widths of less than 50 nm while lengths in the micron range are possible. This is complemented by STEM based EELS measurements to demonstrate the potential of FEBID as fabrication method for 3D plasmonic structures. Together with the additive direct-write character, it becomes evident how FEBID can contribute in the field of resonant optics by yet unknown 3D architectures in combination with accessibility to surface areas which have been considered as very challenging or even impossible in the past.

A new self-assembled plasmonic sensing element is proposed and demonstrated that combines a zero-mode-waveguide in an aluminum film with a gold nanoparticle to form an easily optically accessible gap plasmon with stable spectral performance and supporting large field enhancement factors. The structure is fabricated by nanosphere lithography which generates 100 nm diameter nanoholes in a thin Al film, followed by drop-coating of a colloidal solution of 60 nm Au nanoparticles. This produces ‘hole in one’ structures, consisting of partly submerged Au nanoparticles attached to the Al sidewall of the nanohole, as confirmed by scanning electron microscopy. Numerical simulations of the structure demonstrate that normal-incidence illumination can excite a gap plasmon at the contact point between the Au nanoparticles and the native-oxide-coated Al sidewall. This is in sharp contrast with the more common particle-on-a-mirror geometry in which normal incidence illumination cannot excite the lowest order gap plasmon underneath the particle. Several hole-in-one nanostructures are investigated in single particle microscopy and spectroscopy measurements. Excellent correspondence between experimental and simulated scattering and transmission spectra provides strong support for the presence of the predicted gap-plasmons in the sample, both showing a strong plasmon resonance peak at ~650 nm and a polarization dependent optical response that matches the off-center location of the nanoparticle as observed in SEM images. The rationale behind the selected structural parameters and materials choices will be discussed, and implementation in Surface Enhanced Raman Scattering sensor arrays will be discussed.

Plasmon-enhanced optical trapping is being actively researched as a means for stable trapping of nanoscale objects, which cannot be addressed by conventional diffraction-limited laser tweezers. In this approach, plasmonic nanoantennas are illuminated to generate highly localized and enhanced electromagnetic field in the vicinity of the nanoantenna. The highly localized and enhanced electromagnetic field creates much stronger optical gradient forces and tighter potential wells for confining particles than in conventional optical tweezers, thus providing a means to trap nanoscale objects and molecules. However a long standing problem in this field is how to rapidly load the potential well without relying on Brownian diffusion. Conventional design rely on Brownian diffusion to load the trap, which is very slow and could take several minutes to hours depending on the concentration of the nanoscale objects. Furthermore since the plasmonic trapping sites are pre-patterned on a substrate, current plasmonic nanotweezers suffer from the problem of lack of dynamic control over the particles in the trap, which has limited their performance. Recently we have addressed these challenges by introducing a novel design paradigm known as the Hybrid Electrothermoplasmonic Nanotweezer (HENT)¹, where the intrinsic photo-induced heating of the plasmonic nanoantenna is combined with an applied AC electric field to induce a large scale microfluidic vortex on-demand. The microfluidic vortex enables rapid delivery of suspended nanoparticles to an illuminated plasmonic nanoantenna where they are trapped within a few seconds. In this talk I will discuss the working principle of HENT, as well as HENT-based nanotweezers incorporating apertures in metal films made of plasmonic titanium nitride and gold. I will also discuss how to ‘print’ trapped particles on plasmonic hotspots thus providing additive nanomanufactuing functionality on-chip. The HENT device holds promise for numerous applications including selection, and analysis of nanoparticles, label free biosensing, single nanoparticle spectroscopy such as Surface Enhanced Raman Scattering (SERS), and deterministic coupling of quantum emitters to plasmonic cavities to enhance their emission properties.
References:
1. Ndukaife, J. C. et al. Long-range and rapid transport of individual nano- objects by a hybrid electrothermoplasmonic nanotweezer. Nat. Nanotechnol. (2015). doi:10.1038/NNANO.2015.248

Very large spontaneous-emission-rate enhancements (∼1000) are obtained for quantum emitters coupled with tiny plasmonic resonance, especially when emitters are placed in the mouth of nanogaps formed by metal nanoparticles that are nearly in contact. This fundamental effect of light emission at subwavelength scales is well documented and understood as resulting from the smallness of nanogap modes. In contrasts, it is much less obvious to figure out whether the radiation efficiency is high in these gaps, or if the emission is quenched by metal absorption especially for tiny gaps a few nanometers wide; the whole literature only contains scattered electromagnetic calculations on the subject, which suggest that absorption and quenching can be kept at a small level despite the emitter proximity to metal. A deep understanding of the emission processes involved in such systems is central in the design of modern plasmonic nanoantennas and yet is still missing.
In this talk, I will clarify through analytical derivations in the limit of small gap thickness why quantum emitters in nanogap antennas offer good efficiencies, what are the circumstances in which high efficiency is obtained, and whether there exists an upper bound for the maximum efficiency achievable. To support these theoretical predictions, I will provide a comprehensive numerical analysis of nanocube-type antennas in the limit of small gap and review distinct behaviors achievable in such an architecture family, see also [same authors ACS photonics accepted and Nanoscale Horiz.].

Despite more than a decade of intensive scientific exploration, new plasmonic phenomena continue to be discovered, including quantum interference of plasmons, observation of quantum coupling of plasmons to single particle excitations, and quantum confinement of plasmons in single-nm scale plasmonic particles. Simultaneously, plasmonic structures find widening applications in integrated nanophotonics, biosensing, photovoltaic devices, single photon transistors and single molecule spectroscopy. Decay of surface plasmons to hot carriers is a new direction that has attracted considerable fundamental and application interest, yet a theoretical understanding of ultrafast plasmon decay processes and the underlying microscopic mechanisms remain incomplete. Ultrafast experiments provide insights into the relaxation of non-equilibrium carriers at the tens and hundreds of femtoseconds time scales, but do not yet directly probe shorter times with nanometer spatial resolution. Theoretical calculations can access these scales and complement such experiments, but have so far been primarily restricted to free electron models applicable to simple metals.

Here we report the first ab initio calculations of phonon-assisted optical excitations in metals, which are critical to bridging the frequency range between resistive losses at low frequencies and direct interband transitions at high frequencies. We will discuss calculations for multiplasmon and nonlinear processes in the ultrafast regime from the mid-IR to visible and in different geometries.

Finally, we combine first principles calculations of electron-electron and electron-phonon scattering rates with Boltzmann transport simulations to predict the ultrafast dynamics and transport of carriers in real materials.
In particular, we calculate the distributions of hot carriers generated by plasmon decay and their transport in metallic nanostructures which guide material selection and geometry design for plasmonic energy conversion devices. We also predict the evolution of electron distributions in ultrafast experiments on noble metal nanoparticles from the femtosecond to picosecond time scales.

Since the extraordinary transmission by the metallic nano-hole arrays which enables to filter the specific wavelength of incident light, this phenomenon is studied for various applications, recently. Once the hole arrays are fabricated with specific geometry conditions, the filtered light spectrum form is determined by some factors, the type of metal and the optical properties of materials surrounding the filter structure. Therefore, the static structure is hard t o be utilized as more useful applications. To give a dynamic characteristic to the metallic hole array, the combination of liquid crystals (LCs) is one of powerful solutions. Due to the anisotropic optical property of LCs, it can modify the filtered light spectrum form of metallic hole arrays as modulating the dielectric constants. Thus, we can observe the distinct wavelength shifts and color changes by applying the electrical bias to LCs.

Here, we fabricated nano-hole arrays on aluminum thin films. After fabricated, this arrays and indium thin oxide (ITO)-glasses are used as a top and a bottom elect rode, respectively. Polymer dispersed liquid crystal(PDLC) and LCs are used to dielectric constant changeable materials. In order to get homogeneously aligned LCs, top and bottom electrodes are coated with polyimide layers. Also, the gap between top and bottom electrodes was controlled by silica spacers. As a result, when the bias voltage was applied to our devices, dielectric constant was changed. This causes noticeable resonance wavelength shifts according to the change of dielectric constants of LCs. It can pave the way for dynamic color displays or electrical security letter.

Heat-assisted magnetic recording (HAMR) uses laser heating to allow writing on high anisotropy media such as FePt. It promises to increase the areal density of hard disk drives by as much as 100 folds. For HAMR technology to be successful, one of the requirements is that the energy delivered by the laser diode through a plasmonic waveguide is absorbed by the magnetic layer efficiently [1]. If a significant portion of light is reflected, it increases the energy consumption, complicates heat dissipation and more importantly, significantly reduces the lifetime of the write head.
In this work, we report the design of a metal-dielectric-metal resonance nanocavity superabsorber structure for the recording media to significantly enhance the light absorption, thus greatly minimize the detrimental effects due to light reflection [2]. To test the principle, we first designed and fabricated Al/Al₂O₃/Pt trilayer structures. The thickness of Pt layer is fixed at 10 nm, with thickness of Al₂O₃ tuned from 50 to 200 nm. Our simulation showed that the absorption is >85% at the wavelength of 800 nm. Since Al is a good reflector in the visible to near-infrared regime and Al₂O₃ is a lossless layer with very high transmittance, most of the absorption is due to the light dissipation in the 10 nm Pt, enhanced by the interference effect from the nanocavity. Our experimental findings are consistent with simulation results, and the absorption maximum can be continuously tuned from 400 to 800 nm. Al/Al₂O₃/FePt trilayer structures are also fabricated, and their optical absorption will be measured. We expect that this concept can be adapted to HAMR media fabrication.
[1] Dieter Weller, Gregory Parker, Oleksandr Mosendz, EricChampion, Barry Stipe, Xiaobin Wang, Timothy Klemmer, Ganping Ju, and Antony Ajan, IEEE Trans. Magn., 50, 1, (2014).
[2] Haomin Song, Luqing Guo, Zhejun Liu, Kai Liu, Xie Zeng, Dengxin Ji, Nan Zhang, Haifeng Hu, Suhua Jiang, and Qiaoqiang Gan, Adv. Mater., 26, 2737-2743, (2014).

Dielectric-based high contrast metasurface filters integrated on backside illuminated CMOS image sensor (BSI CIS) photodiodes can replace conventional color filters and micro-lenses by thin layer nanostructures. Unique characteristics of metasurface like polarization insensitivity, large angular operation and control of phase profile allow smaller CIS pixel size with reduced optics thickness on the photodiodes. Metasurface filters fabricated on glass substrates confirm the feasibility of high transmittance color filters down to 1um pixel size. Optical simulation of the filter design on BSI Si photodiode demonstrates high performance for low/high lights and the possibility of sub-um pixel CIS. Further applications such as high resolution 3D sensor, mini-spectrometer and hyperspectral imaging are also anticipated.

Plasmonic nanocrystals hold great promise for chemical sensing due to their highly tunable optical resonances based on judicious tailoring of their size, shape, and orientation. We demonstrate large-area chemical detection using the plasmon gap-mode resonance of film-coupled Ag nanocubes for a surface enhanced Raman scattering (SERS) sensor. The ability to design nanocrystal SERS substrates using entirely bottom-up assembly techniques allows for parallel and scalable fabrication on a multitude of various materials. We demonstrate the utility of such SERS sensors for 2D chemical mapping large-area surfaces, with high spatial resolution with uniform enhancement over substrates that are approximately 2 x 10⁴ μm² in area.

Metallic nanostructures with well-defined resonances are a key building block for nanoscale photonic “meta-devices”, that offer a variety of nonlinear and switchable properties. The post-fabrication control over the nanostructures’ resonances is fundamental for enabling reconfigurable devices based on metamaterials. These reconfigurable structures allow for all-optical switchable nanophotonic circuits, imaging, sensors, data storage and displays. Commonly, a hybrid design including metal resonators and an active medium allows for these functionalities [1].
Here we present the tuning of the spectral position of the nanostructure resonance frequency by locally addressing phase-change material (PCM) thin-films with single laser pulses on a nanosecond timescale. PCMs offer a huge contrast in the refractive index n due to a phase transition from their amorphous to their crystalline state [2]. By optically induced local phase transitions, the effective refractive index of the PCM medium, which affects the resonant structures resonance, can be precisely influenced and tuned over a broad range.
We use a simple home-made laser setup, with which we are able to vary the switching of a PCM layer over a diameter range from 0.5 to 4.3 μm. In turn this allows us to alter the nanostructure resonance frequency of a 600 nm long rod-shaped antenna between 1650 and 2100 cm^-1 in steps down to about 15 cm^-1 [3]. The local switching of the PCM layer is reversible and is realized on nanosecond time scales. In contrast to our previously presented work on large-scale femtosecond resonance tuning [4], the local addressing enables a multi-step tuning of single nanostructures pointing towards actively controllable metasurfaces. Our concept can be combined with many different hybrid metamaterial designs, e.g. chiral structures for polarization control [5]. Since the structural and with that the dielectric properties of PCMs are stable even at elevated temperatures, the presented non-volatile concept is well-suited to be included in imaging or storage devices.

[1] Zheludev, N. I. and Kivshar, Y. S. Nat. Mater. 11 (2012).
[2] Wuttig, M. and Yamada, N. Nat. Mater. 6 (2007).
[3] Michel, A. U.; Kalix, T.; De Rose, A.; Chigrin, D. N.; Wuttig, M. and Taubner, T. in preparation
[4] Michel, A. U.; Zalden, P.; Chigrin, D. N.; Wuttig, M.; Lindenberg, A. M. and Taubner, T. ACS Phot. 1 (2014).
[5] Yin, X.; Schäferling, M.; Michel, A.U.; Tittl, A.; Wuttig, M.; Taubner, T. and Giessen, H. Nano Lett. 15 (2015).

We report design principles for and experimental realization of narrowband color filters for CMOS image sensors based on plasmonic guided mode resonances. We describe designs yielding narrow bandwidth and maximal transmission amplitude of finite-sized resonant color filters based on guided mode resonance (GMR) structures consisting of a high index waveguide and a periodic slit array that serves as a light incoupler. The bandwidth of the resonance can be tuned by tailoring the distance between the waveguide and the coupler, which varies the coupling between modes of the dielectric waveguide and periodic plasmonic slit array. Our nominal structure consists of a 120 nm thick SiN_x waveguide layer on a SiO₂ substrate capped by an aluminum periodic slit array which separated from SiN_x waveguide by a thin SiO₂ film. Incident plane wave illumination from the bottom substrate efficiently couples to the waveguide modes of the SiN_x slab which scatters into air through the slit array. The resonant frequency is determined by the slit periodicity and the dispersion of the SiN_x waveguide. To analyze the dependence of the resonance on periodicity and spacer layer thickness, several periodic arrays were fabricated with periods varying from 280 nm to 480 nm and spacer thicknesses from 0 to 100 nm. Transmittance of 60-70% was achieved through each array with a varying bandwidth of 15 nm to 80 nm depending on the spacer layer thickness. Counterintuitively, our mode coupling data and measurements demonstrate that higher radiative loss is desirable from the standpoint of achieving high transmission for smaller pixel sizes, in spite a broader transmission bandwidth. These insights enable one to develop a general strategy to determine the optimum filter design, based on the required bandwidth, pixel size and the transmission peak values.

Semiconductor nanostructures are at the heart of modern-day electronic devices and systems. Due to their high refractive index, they also provide a myriad of opportunities to manipulate light. When properly sized and shaped, they can support strong optical resonances that boost light-matter interaction over bulk materials and enable their use in controlling the flow of light at the nanoscale. In this presentation, I will discuss the use of individual, resonant nanostructures and dense arrays thereof (metafilms) in a variety of optoelectronic devices and illustrate how the performance of these devices can be improved by engineering the constituent nanostructure, size, shape, and/or spacing. I will illustrate the different types of optical functionalities that can become available in moving from metafilms constructed from ‘non-resonant’ nanostructures to metafilms assembled from ‘resonant’ structures.

Infrared Frequency Selective Surfaces (FSS) type metasurfaces have diverse applications ranging from thermal management to advanced detectors and sensors. The science of controlling radiation with metasurfaces is rapidly maturing and design lesions can be learned from engineering or the electromagnetic response at microwave frequencies. The major obstacle to implementing metasurfaces for practical applications is the cost of their fabrication. While nanofabrication tools developed for producing integrated circuits are sufficient for prototyping applications, this approach is too expensive for large area manufacturing. In this work we present results using Microsphere Photolithography (MPL) for producing infrared (IR) metal-insulator-metal (MIM) metasurfaces. MPL uses a self-assembled array of microspheres as to focus UV radiation. The microspheres produce a high aspect ratio sub-diffraction limited photonic jet in the photoresist. After development this creates a hexagonal close packed (HCP) lattice of holes in the resist. This can be used with evaporation and lift-off can be used to deposit metal disks. This HCP disk array can be optimized to control the spectral emittance from the surface. In this work we optimize the metasurface to be a near perfect absorber using a frequency domain finite element package. Surfaces are fabricated and measured using FTIR. These results agree with the simulation. The entire process is performed in a non clean-room laboratory environment. One of the most interesting features of the fabrication procedure is that it produces a polycrystalline metasurface characterized by defects and grain boundaries. These can be characterized using simple optical diffraction measurements. For spectrally absorbing metasurfaces, the polycrystallinity does not adversely affect the device performance and may prove to be advantageous for cutting off diffraction at IR wavelengths. The resonant frequency of the metasurface can be adjusted by changing the exposure does which permits the patterning of complex spectrally selective features. After demonstrating near perfect absorbing metasurfaces we show how the MPL technique can be expanded to more complicated devices. Finally the steps toward scaling up to square meter scale fabrication are discussed.

In this talk, we present the first demonstration of a metasurface for light manipulation where the performance is truly achromatic, surpassing a major barrier in metasurfaces functionality. To achieve a dispersionless design, from filter circuit theory, a constant time delay must be implemented. This is realized by plasmonic nanoantennas in IR and all-dielectric patterns in visible. The constituting layered elements, with each layer being a capacitive or an inductive structure, behave like low-pass circuit filters with almost linear phase response over the designed spectrum. Chromatic aberration is successfully avoided. The plasmonic metasurface operates well in 5.5–7.5 μm, and the all-dielectric case has the desired performance over almost the entire visible band. Metasurfaces for light focusing and bending are presented. The performance is highly efficient (close to 85%) and in transmission mode, further independent of frequency. The structure is ultra-thin offering the full 360 phase change as well. The proposed concept can pave path for various unique applications in optics based on metasurfaces and where the characteristics need to be independent of wavelength. The theory and full wave modeling of the large graded pattern metasurfaces are described along illustrating some novel results.

The extraordinary optical transmission through an array of subwavelength holes in a metal film is well known. We experimentally demonstrate here that a continuous gold film on a periodically textured substrate exhibits extraordinary transmission phenomena, even though no holes were etched in the film.
A periodically corrugated array of tapered nanocups of period ~750 nm was fabricated on a thin polystyrene film using soft lithography and double replication. We started with a master pattern consisting of a periodic array of tapered nanocups with sub-micron pitch patterned on polycarbonate substrate. The inverse of this pattern on polycarbonate was transferred to a PDMS mold by soft lithography. A mixture of PDMS prepolymer and curing agent was poured directly onto the master substrate. After curing, PDMS was peeled off from the master substrate to expose the inverse pattern on the PDMS surface. We used the patterned PDMS surface to transfer the pattern onto polystyrene films by imprinting the film with the PDMS mold, under elevated pressure and temperature. A thin continuous non-conformal gold film was then sputter deposited on polystyrene nanocup array with angle directed deposition, and a zero-order transmission spectrum was measured.
Measurements exhibited the extraordinary transmission peak at λ₁ ~ 700 nm in addition to the surface plasmon resonance peak of gold at λ_sp ~ 504 nm. The transmission was enhanced ~ 2.5X as compared to the area fraction of the film occupied by the nanocups. Scattering matrix simulations employing tapered nanocups on gold-coated polystyrene film demonstrated a similar behavior exhibiting extraordinary transmission peak at a wavelength slightly smaller than the period. The electric field intensity was enhanced by a factor >100 near the bottom of the corrugated structure that has ultrathin gold film. Simulations indicated generation of surface plasmons at the air-gold interface. The varying thickness of the gold film on the corrugated substrate enables spatial regions of high transmission through ultrathin regions of the metal film, in conjunction with negligible transmission through thick regions of the film, thus functioning similar to a nanohole array. Conventional ways of fabricating the hole array structures involve advanced microelectronics facilities and procedures, and are difficult to implement for larger area samples. We demonstrate a particular simpler method to fabricate a subwavelength array structure using simple soft lithographic procedures that does not require any nanofabrication facilities, and may be easily achieved in a workbench without high vacuum facilities.

Optical invisibility is highly sought-after for its wide range of applications from display technologies to military operations. Previous optical cloaks mainly used the quasi-conformal mapping technique to conceal an object by restoring the wavefront as if it were reflected from a flat surface. However, this technique necessitates refractive index modulation over a large volume, leading to a bulky cloak which scales unfavorably to macroscopic sizes. Here we demonstrate an ultrathin metasurface cloak working at visible wavelengths using the concept of reflection phase manipulation with metasurface nanoantennas. Our conformal cloak conceals a three-dimensional arbitrarily-shaped object by complete restoration of not only the wavefront, but also the phase of the reflected light. We believe the ultrathin image manipulation capability demonstrated here can lead to practical applications in flexible optoelectronics, 3D display and military technologies.

We present an experimental demonstration and theoretical analysis of a new, monolithic dielectric resonator metasurface design that yields high quality factor Fano resonances. Our approach utilizes perturbations of high-symmetry resonator geometries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple “bright” dipole modes to “dark” dipole modes whose emission is suppressed by local field effects. Our approach is widely scalable from the near infrared to radio frequencies. We report simulations for a germanium-based resonator metasurface that achieves a quality factor of ~ 1300 at ~ 10.7 mm, and we present two experimental demonstrations of a near-infrared Fano metasurfaces. The first utilizes silicon as the resonator material and exhibits a quality factor of ~ 350 at 999 nm. The second utilizes GaAs and achieves a quality factor of ~ 600 at 970 nm. We envision that combining such high quality factor modes with nonlinear and active/gain materials such as GaAs will lead to a new class of optical devices.

Portions of this project were funded through Sandia National Laboratories LDRD program. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Plasmonic nanostructures, which are formed formed by rectangular metal patches (nanobricks) placed on the top of a thin dielectric spacer supported by a thick metal film, support the propagation of gap surface plasmons (GSP) that can be efficiently reflected at the nanobrick terminations, forming thereby GSP resonators. The dimensions of GSP resonators can be deeply subwavelength for small spacer (gap) thicknesses that also strongly influence the balance between the scattering and absorption cross sections of the GSP resonators. Our research has shown that periodic arrays of GSP resonators can be advantageously exploited for controlling the phase and amplitude of the reflected light, practically independently for orthogonal linear polarizations, by choosing the nanobrick dimensions. The understanding of light reflection by arrays of GSP resonators led us to a number of potentially application-relevant developments, including the design of highly efficient and background-free (i.e., no diffraction and no scattering into other polarizations) plasmonic metasurfaces that provide strong phase gradients in the reflected optical fields, diffracting thereby different polarizations into different directions that can be adjusted independently. This approach was further extended for realization of efficient polarization-controlled coupling to surface plasmon polariton modes. We have also developed a design strategy for independent control of phase and amplitude of the reflected radiation that is applied to realize Fourier optics based analog computing, including differentiation and integration of optical fields. Very recently, we have developed metagratings, which consist of three interweaved GSP-based metasurfaces, that would allow one to easily analyze an arbitrary state of light polarization by conducting simultaneous (i.e., parallel) measurements of the correspondent diffraction intensities, revealing immediately the Stokes parameters of the polarization state under examination.

Actively tuning the resonant frequency of plasmonic structures plays an important role towards plasmonic applications. Most of the demonstrations in this field rely on changing the optical property of surrounding active dielectrics, or physically manipulating the shape of plasmonic structures. In this study, we show that direct charge injection into metallic nanoparticles can also efficiently modulate their resonant frequencies and field enhancement factors. The device is realized by dispersing monolayer gold nanoparticles onto a wafer-scale graphene film. Due to the extremely low density of states in graphene and the corresponding weak screening effect, when the graphene film is charged by a back gate electrode, the Fermi level of gold nanoparticles can be shifted up to 20%, leading to a 60nm blue-shift of resonant wavelength and dramatic increase of scattering area. Furthermore, we found that the chemical property of the charged gold nanoparticle is radically changed as well. When the nanoparticles are negatively charged, their activation energy can be reduced by 50%. This understanding paves a method to utilize metallic particles in designing and optimizing sustainable photocatalysts with tunable catalytic activities. In this talk, a new intercalation method to fabric large mono- atomic layer metal film for plasmonic applications will also be discussed.

Hyperbolic materials are defined as having a dielectric function that is opposite in sign along orthogonal axes, inferring both metallic and dielectric-like optical behavior simultaneously. Hexagonal boron nitride (hBN) is a polar dielectric crystal that is naturally hyperbolic. Its crystal lattice, consisting of 2D layers stacked by van der Waals forces, can support mid-infrared surface phonon polaritons with picosecond lifetimes. These are orders of magnitude longer than the femtosecond lifetimes of surface plasmons in metals, making hBN an ideal candidate material for infrared nanophotonic devices. Spatial confinement of hBN within nanostructures gives rise to localized 'hyperbolic' phonon polariton modes with tunable wavelengths based on the aspect ratio of the structure geometry. To date, these 3D confined low-loss resonances have only exhibited electric field oscillations perpendicular to the lattice planes (k_z wavevector variations) upon conventional far-field excitation. Here, we use a near-field scanning technique called Photothermal Induced Resonance (PTIR) to investigate phonon modes in hBN nanocones with electric field oscillations parallel to the lattice planes, more closely aligned with propagating modes in hBN slabs. In PTIR, infrared absorption is measured via thermal expansion with nanoscale spatial resolution. The sample is excited with tunable laser pulses while an atomic force microscope (AFM) generates IR absorption images and spectra based on cantilever deflections in areas of the sample absorbing specific frequencies of light. Our experiments use topside illumination with near-grazing incidence as a gold-coated AFM tip is continuously in contact with the sample. PTIR spectra of hBN nanocones reveal phonon resonances that are spectrally shifted from the previously identified k_z modes. Theoretical maps of the electric field indicate that these tunable k_t (transverse wavevector) modes are launched from the AFM tip and have in-plane field oscillations. Moreover, higher-order k_t modes show inverse wavelength dependence with respect to their k_z counterparts. Our results demonstrate that narrow-band thermal emission can be controllably stimulated with high spatial precision from hBN nanostructures. These findings are highly relevant to the potential use of hBN and other polar dielectric materials in developing advanced technologies for infrared and terahertz applications.

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer clusters as the number of nanocrystals is systematically varied. Precise, hexagonally-ordered Au nanocrystal oligomers with 1–31 members are assembled via capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual clusters is determined with spatially resolved, polarized darkfield scattering spectroscopy. We observe a strong red-shift in plasmon resonance wavelength as the number of nanocrystals per cluster increases, in agreement with theoretical predictions. Simulations also elucidate the modes supported by the clusters, including electric dipole and magnetic dipole responses, their coherent superposition, and the excitation of higher-order modes. The progression of structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic metamolecules and extended two-dimensional arrays.

Metallic nanowires, which possess unique propagation characteristics of surface plasmon polaritons (SPPs), are believed to be a key enabling component in future nanophotonics. The wavelengths of nanowire SPP modes scale with the cross-section dimension of the wire, thus showing no cutoff behavior as for a dielectric waveguide. The SPP fields are highly confined, which drastically enhance the density of state near the metal surface and can thereby strongly enhance light-matter interactions and enable the down-scaling of plasmonic devices to the deep subwavelength region. Furthermore, different SPP modes supported by metallic nanowires have distinct features and when selectively excited, can be tailored for different devices and applications.

Among other candidates for deep sub-wavelength plasmonic waveguides, silver nanowires (AgNWs), which can be routinely chemically synthesized with high crystallinity and atomically smooth surfaces, are of particular interest due to their low ohmic damping and negligible scattering loss. Therefore, AgNWs have been a subject of intense study and demonstrated as an appealing nanooptical platform for photonic logic circuits, light sources, detectors and transistor functionalities. We have develop an interference technique to characterize SPP modes on free-standing AgNWs, by extracting information from the mode beating of scattered light from the nanowire tip. The mode index, propagation length and weight of the two lowest order modes supported by AgNWs with different diameters were analyzed under different excitation wavelengths. In particular, we found that for small-diameter AgNWs, the fundamental (m=0) SPP mode dominates, while for large-diameter AgNWs, the dipole-like mode (m=1) is preferred. This result suggested that SPP mode selection can be realized by diameter selection. The results agrees well with full-wave electromagnetic simulations, which help to verify the experimental result and explain the coupling process.

Aluminum is an excellent material for plasmonics, with an oxidation stability superior to that of silver and a tunability ranging from the ultraviolet into the near infrared parts of the spectrum. With its low cost and compatibility with CMOS manufacturing techniques, Aluminum has significant potential as a substrate in plasmon based technological applications. In this talk, I will discuss our recent work on Aluminum plasmonics including a discussion of its significant tunability and a method for the chemical synthesis of pure aluminum nanocrystals. I will also discuss some recent applications such as: the demonstration of its use as a colorimetric LSPR sensor; its use in surface enhanced spectroscopy applications such as SERS and SEIRA; its use as pixel elements in display technology; as an active material in photodetectors and hot electron devices, and as a plasmonic photocatalyst.

High-index dielectric nanoparticles support strong geometrical resonances that enable tunable control of light with applications in nano-scale sensors, lasers, and solar cells. Coupling between resonant nanoparticles enables the hybridization of eigenmodes which can give rise to enhanced directionality and confinement. A fundamental understanding of the resonant properties of nanostructures that arise from hybridization is essential to fully exploit the potential of dielectric resonators in next generation devices. Here, we combined dark-field (DF) spectroscopy with hyperspectral cathodoluminescence (CL) microscopy to directly image the hybridized mode profiles inside coupled silicon nanoparticles.

We pattern the top Si layer of a SOI wafer into single cubes and dimers using electron-beam lithography and reactive-ion etching. The individual particles are 90 nm wide, 100 nm high, and the particle spacing is varied between 0–100 nm. Using DF spectroscopy we study the influence of particle spacing on the scattering spectrum in the 430–570 nm spectral range, and find strong spectral broadening due to hybridization for dimers with <25 nm spacing. Using finite-element modeling, we calculate the eigenmodes of the dimer. We identify the hybridized eigenmodes in the scattering spectrum as the longitudinal and transverse bonding modes of the magnetic dipole modes in the individual particles.

These mode profiles are directly mapped using hyperspectral CL imaging with deep sub-wavelength resolution. Using a 30 kV electron beam in a scanning electron microscope as a localized and broadband optical source, we excite both single and coupled nanostructures and collect the radiation. This allows for the spectral and spatial characterization of the hybridized optical eigenmodes with a resolution that is determined by the electron beam rather than the optical diffraction limit.

Using this technique, we map the modal field profiles of the magnetic and electric dipolar modes inside individual Si nanoparticles. We then focus on coupled particles and find strongly asymmetric modal field profiles inside the individual particles as a result of hybridization. Detailed comparison of the measured modal field profiles with the eigenmode calculations shows that the measured profiles correspond to the hybridized electric bonding and magnetic anti-bonding modes. Finally, we study dimers composed of large dielectric bars to explore the ability of CL imaging to map highly-complex hybridized field profiles inside resonant nanostructures.

Our results demonstrate the characterization of the complex resonant properties of coupled nanostructures. Based on these fundamental insights, nanostructures with accurately engineered field confinement and scattering profiles can be designed, paving the way for more complex applications and devices.

Recently, the search for alternative plasmonic materials has accelerated due to the weak mode confinement and the lack of tunability when using noble metals in the infrared. This work investigates properties of surface plasmons on doped metal oxides in the infrared and more specifically the mid- and long-wave regime (2-12 μm). Plasmonic properties in deposited Ga-doped ZnO (GZO) can be controlled by varying the fabrication techniques, via a fluctuating free carrier concentration and mobility, allowing multiple degrees of tunability. This tunability allows a control of the GZO plasma frequency and the correlated plasmonic mode confinement. With GZO also being CMOS compatible, it has high prospects for infrared plasmonics.
Presented are theoretical and experimental investigations pertaining to GZO plasmonics. Samples are fabricated on silicon substrates via pulsed laser deposition and characterized by infrared ellipsometry, fourier transform spectroscopy, and using table top sources and detectors. Complex permittivity spectra are presented, as well as analytically determined plasmon properties such as the field propagation lengths and penetration depths, in the infrared range of interest.
In addition to material characterization and determination of optical constants, specifically designed structures based on 1D-gratings and 2D-arrays are presented for investigation of plasmon modes excitation, extraordinary optical transmission, and light trapping. Also presented will be initial optical and electrical investigations of gate-biased GZO layers which will be critical in building a foundation for actively tunable GZO infrared plasmonics. The plasmonics GZO structures presented here can be tailored to the wavelengths of interest and utilized for capabilities in infrared sensing, enhanced sensitivity detection, and novel integrated components.

Plasmonic nanostructures serve as the main backbone of surface enhanced sensing methodologies, yet the associated optical losses lead to localized heating as well as quenching of molecules,
complicating their use for enhancement of fluorescent emission. Additionally, conventional
plasmonic materials are limited to operation in the visible part of the spectrum. We will elucidate how
nanostructures consisting of conventional and polar dielectrics can be employed as a highly
promising alternative platform.
Dielectric nanostructures can sustain scattering resonances due to both electric and magnetic Mie modes.
We have recently predicted high enhanced local electromagnetic field hot spots in dielectric nanoantenna
dimers [1], with the hallmark of spot sizes comparable to those achievable with plasmonic antennas, but with
lower optical losses. Here, we will present first experimental evidence for both fluorescence and Raman
enhancement in dielectric nanoantennas [2], including a direct determination of localized heating, and
compare to conventional Au dimer antennas. The second part of the talk will focus on the mid-infrared
regime of the electromagnetic spectrum, outlining possibilities for surface enhanced infrared absorption
spectroscopy based on polar [3] and hyperbolic [4] dielectrics.
[1] Albella et al, ACS Photonics 1, 524 (2014)
[2] Calderola et al, Nature Communications 6, 7915 (2015)
[3] Caldwell et al, Nano Letters 13, 3690 (2013)
[4] Caldwell et al, Nature Communications 5, 5221 (2014)

The field of photonics has attracted great attention due to its capability to overcome the limitation of integration with Si based photonic on-chip devices where the dimensions of optical components are constrained by optical diffraction. Addressing the requirement of chip-scale optical devices, noble metals such as gold (Au) or silver (Ag) have been extensively employed to demonstrate numerous plasmonic devices including waveguides, switches, and modulators. However, these metals have intrinsic drawbacks such as incompatibility with well-established processes for silicon-based products, and lack of tunability. Consequently, it is important to discover new materials within the field of plasmonics. Titanium nitride (TiN) is one of the potential metallic component for refractory materials which can sustain its optical properties at high temperature (up to 1100 °C in the atmosphere). Previously, TiN was paired with CMOS-compatible silicon nitride to enable a fully solid-state hybrid plasmonic waveguide which achieved a propagation length greater than 1 cm for a ~8 μm mode size at 1.55 μm. Transparent conducting oxides (TCOs) are the leading alternative plasmonic materials in the near infrared range. Our recent work has investigated optical tuning of AZO films by pump-probe spectroscopy, demonstrating a change in the refractive index of -0.17+0.25i at 1.3 μm with an ultrafast response less than 1 ps. Utilizing TCOs as a dynamic material, we are able to design on-chip modulator with the TiN/Si₃N₄ hybrid plasmonic waveguide. Simply by placing a thin layer of aluminum doped zinc oxide (AZO) on top of the waveguide structure, a modulator with very low insertion loss is achieved. A modulation of ~0.4 dB/μm is possible in the structure with