Program - Symposium KK: Plasmonic Materials and Metamaterials

Controlling light-matter interactions on a subwavelength scale is a pivotal focus of nanophotonics. Advances in this field have enabled powerful applications including near-field microscopy, surface enhanced spectroscopy, biosensing, and solar energy harvesting. To date, most research efforts have focused on engineering the interaction of matter with the electric field of light. It has recently been discovered, however, that closely packed metal nanoparticle assemblies can support both electric and magnetic modes at optical frequencies. Establishing methods to engineer the optical magnetic modes and their interplay with electric modes will be crucial for advanced nanophotonic applications. In this work, we utilize fully rigorous generalized Mie theory calculations to explore the near and far field properties of silver nanoparticle trimers. We theoretically demonstrate Fano-like interference effects between the fields radiated by the electric and magnetic modes of symmetric nanoparticle trimers (radius = 30 nm, interparticle spacing = 2 nm). Breaking the symmetry of the trimer system leads to a strong interaction between the modes, which can be readily understood in terms of group theoretical arguments. The near and far field electromagnetic properties of the broken symmetry trimer are tunable across a large spectral range. We exploit this Fano-like effect to demonstrate spatial and temporal control of the localized electromagnetic hotspots in the plasmonic trimer. Displacing one of the spheres in the symmetric nanoparticle trimer by 1 nm results in a 30-fold increase in the near field enhancement and changes the relative phase by π/8 at the junction between the other two spheres. Our experimental efforts towards biologically templated assembly of nanoparticle clusters and their characterization with polarization-selective single particle spectroscopy will also be discussed.

Lithographic photopatterning to fabricate 2D and 3D structures is typically a static process in the sense that a solid block of photoresist is exposed to spatially varying light intensity that is constant in time. The ability to grow inorganic materials whose morphologies respond dynamically to changing illumination conditions can enable the development of complex architectures with tailored optical responses. We have developed a method for the light-mediated formation of nanoscale lamellar patterns during the electrodeposition of photoresponsive selenium-tellurium (Se-Te) alloys. While no pattern is observed for Se-Te films prepared in the dark, those deposited under illumination display a lamellar structure that is continuous over the entire growth substrate. The angle and polarization of the incident light determine the growth direction of the lamellae and their orientation on the substrate, respectively. The illumination wavelength controls the lamellar size and pitch, where the periodicity can be varied continuously from ~150 nm for ultraviolet illumination to ~350 nm for near-infrared light. Because the patterns are dynamically responsive, structural complexity can be built into the films by changing the illumination conditions during the electrodeposition process. For instance, branching is induced in the lamellae by switching the illumination wavelength. We attribute the light-induced pattern formation to interference between scattered fields at the surface of the Se-Te film, which produce a periodic modulation in light intensity. Full-wave simulations of the interference patterns formed by varying illumination wavelengths correspond well with the observed lamellar periodicities. The mechanism by which the local light intensity affects the growth of the SeTe alloy is a subject of further investigation. This light-mediated growth technique opens up the possibility for optimizing the absorption and transmission characteristics of a material by using light in a feedback loop to direct the film’s structure and morphology.

With recent advancements in nanoscale fabrication, significant effort has been focused on developing large-area plasmonic optical sensors such as those for surface-enhanced Raman scattering (SERS). Most of these efforts have focused upon making periodic or aperiodic arrays of structures via electron beam lithography or nanoimprint lithography and then coating via traditional methods such as electron beam evaporation or sputtering of silver or gold. These efforts have led to large-area, highly reproducible and uniform enhancements, which for SERS have surpassed 1x10⁸ with typical variations on the order of 15-30% across a large-area array. However, when plasmonic nanoparticles are placed in close proximity to one another (<20 nm) the plasmonic field intensities between the nanoparticles are increased dramatically due to interparticle near-field plasmonic couping, which are the cause of SERS 'hot-spots'. Additionally, a red-shift in the surface plasmon resonance (SPR) condition is observed. It has long been anticipated that if large-area structures of coupled plasmonic nanoparticles could be achieved that the most sensitive detectors could be created. This implies that a very dense network of nanoparticles be created, with each nearest neighbor being spaced at less than 20 nm gap from its neighbor following metal deposition, something that is difficult for most nanolithography approaches and extremely difficult for most standard metal coating techniques. However, the recent development of atomic layer deposition (ALD) of silver has now enabled the fabrication of such structures. Using Ag ALD, we have demonstrated that large area arrays (20-60 um on a side) of Ag-coated, Si nanopillars with controllable, reproducible and uniform interpillar gaps as small as 2 nm can be achieved using Si nanopillar templates with interpillar gaps (50-100 nm) that are attained using standard ebeam lithography techniques. Using a combinatorial approach, a complete diameter and gap dependence, with the post-metal diameters ranging from 196-397 nm and the interpillar gaps ranging from 2-199 nm was fabricated. SERS measurements were carried out on a self-assembled monolayer of thiophenol to ensure an accurate accounting of the number of molecules could be ascertained. Enhancement factors in the 10⁷-10⁸ range were observed for the best responding structures with interpillar gaps >20 nm. In all cases, as the interpillar gap was reduced, a slight decrease in SERS intensity was observed, until the gap was reduced below 20 nm, where the onset of plasmonic coupling is anticipated. At this point, over an order of magnitude increase in SERS intensity was observed over the uncoupled arrays. It should be noted that in the case of the uncoupled arrays, this translated to 1 sec acquisition times, while acquisition times of 0.1-0.5 sec, depending on incident wavelength, were required for the coupled arrays to ensure saturation of the detector did not occur.

Nanoscience has supplied us with a wealth of new strategies to construct high performance devices such as solar cells, thermoelectrics, sensors, transistors, and transparent electrodes via a bottom-up synthetic approach. This fabrication route offers the benefits of large-scale chemical synthesis, high throughput and circumvents the need for etching processes that waste material and create surface defects. One of its greatest challenges is finding suitable assembly and contacting procedures that allow for complex device fabrication without costly patterning steps. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into complex interconnected networks. The small gaps that form naturally at nanowire junctions cause local light focusing and heating predominantly at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a connection between the wires is made. At each junction point where nanowelding occurs, the bottom nanowire recrystallizes epitaxially onto the top nanowire, consistent with the simulated heat generation profile. This talk will use results from full-field simulations, electron microscopy, optical scattering and single junction resistance measurements performed before and after optical illumination to understand this self-limited plasmonic nanowelding process. Large-area nanowire mesh networks have also been fabricated and tested as transparent electrodes, connecting the microscopic to macroscopic properties and demonstrating one potential application. Moving beyond transparent electrodes to complete solar cells, the ability to control light and heat at the nanoscale becomes even more important. We have already demonstrated that these metal nanowire transparent electrodes can be integrated into organic solar cells and have used laser beam induced current (LBIC) measurements to map out their local effects. By optimizing the optical and electrical interactions, we hope to use these metal nanowire transparent electrodes as both efficient current collectors and active light focusing elements to enhance solar conversion efficiency.

Optical imaging and manufacturing at deep sub-wavelength scale promises vastly new applications in nano-electronics, metrologies, and single-molecule biology. The diffractive nature of the light, however set a fundamental limit in optical resolving power. Approaches including near-field microscopy and plasmonic lens (PL) have been developed to circumvent the diffraction limits by accessing the electromagnetic field at near-field, but confronted the formidable limitations due to the momentum mismatch induced trade-off between resolution and energy coupling efficiency. Through adiabatic transformation, converting the propagating surface plasmon (PSP) modes efficiently into localized surface plasmon (LSP) has been theoretically proposed. Here we report a new high-throughput optical focusing scheme based on a compound multi-stage plasmonic lens (MPL) that is capable of compressing the optical energy to deep sub-wavelength scale with high efficiency through progressive coupling of both PSPs and LSPs. Combined with a pulsed laser source and an advanced airbearing surface (ABS) technology, the MPL provides an unprecedented spatial resolution with extremely high speed writing at the order of 10 m/sec. As a result, 22nm half-pitch features are successfully recorded on an inorganic resist, which corresponds to 1/16 of the wavelength.

Zero refractive index materials are materials where light can propagate without phase delay. In this paper, the relationships between internal coupling effects and zero refractive index in a metallodielectric composite are studied in detail. The proposed composite is made of metal pieces and spiral wires periodically arranged in an epoxy dielectric matrix. Full wave simulation was performed to obtain the dispersion relationship (refractive index versus frequency) and field distributions of the structure. Near zero refractive index is observed at the range between 2~3GHz. In this range, as shown by the field distribution graph, negative electric/magnetic polarizations are induced by currents and charges vibrating out-of-phase with incident EM waves. By careful analysis of the dispersion curve, it is found that the bandwidth can be significantly enhanced when adjacent metallic elements are coupled transversely rather than longitudinally, while frequency (at which n=0) is easily to be shifted by adjusting lengths and curvatures of spiral wires. Based on these results, the structure was applied as a zero index superstrate over a patch antenna, whose directivity is subsequently improved by 14 times. It is also indicated that the structure can be scaled down into micrometers for Terahertz uses.

Gold nanoparticles have good localized surface plasmon resonance (LSPR) in the visible and near-infrared region of spectrum, which offering them potential applications to optoelectronics. Those LSPR can be finely tuned by the nanostrucutres with controllable sizes and shapes. Gold nanorods with controllable LSPR can be easily achieved by synthesizing nanorods with different aspect ratio. However, the structure gold nanorods with large surface-to-volume ratio are not stable and will reshape to from nanospheres at high temperature due to minimization of surface free energy. It is believed that gold nanorods cannot survive after annealing at 250 degree Celsius for 30 minutes. This low thermal stability of gold nanorods may not be compatible with high temperature characteristics of semiconductor processing and limit their applications. Here, we report that the thermal stability of gold nanorods can be greatly enhanced by silica-coating. After thermal annealing at 600 degree Celsius for 1 hr, silica-coated gold nanorods still remain their rod structure with minor transformation into shorter and thicker rods. In comparison, bare gold nanorods all turn into gold nanospheres after thermal annealing. The spectral data also confirm the structural changes observed by scanning electron microscope. We also report the recent progress in application of plasmonic gold nanorods to thin film photovoltaics to improve absorption through scattering and electromagnetic field localization effects.

The fabrication of periodic metallic structure has gained many interests due to its surface plasmon resonance behavior in recently years. By coupling a radiation dipole with a surface plasmon generated on the surface of noble metals, energy can transfer effectively from the dipole into the surface plasmon resonance. Localized surface plasmon resonance is strongly dependent on the size, shape, surrounding environment and metal features. In this study, we develop a simple and low cost solution process to fabricate a novel Ag coated monodispersive silica colloid monolayer substrate to enhance Raman scattering signals of various organic dyes. For SERS substrate fabrication process, we first synthesized amorphous monodispersive spherical silica particles by using the sol-gel method. Tetraethylorthosilicate (TEOS) was dissolved in ethanol and the solution was held and stirred with a mechanical stirrer in water bath at 30 °C for 30 min. Then ammonia solution was added into the TEOS solution and stirred for 2 h. After 2 h of reaction, a monodispersive silica colloid solution was obtained. Next, we changed the parameters, such as the concentration of monodispersive silica colloid solution, solvent type, spin-coating speed, to obtain the optimal monodispersive silica colloid monolayer. After monodispersive silica colloid was self-assembled on the silicon wafer, Ag was deposited on the silica monolayer by thermal evaporation to obtain Ag coated silica monolayer. The surface morphology of SERS substrate was studied by atomic force microscopy and scanning electron microscopy; the extinction spectra was measured by using a spectral micro-reflectometer equipped with an optical microscope. In order to confirm the effect of surface-enhanced Raman scattering of organic compounds on the SERS substrate, methyl red, methyl orange and methylene blue were used as model compounds. The compound was spin coated on the SERS substrate and evaluated its Raman scattering respectively. A large enhancement of Raman scattering was observed when the SERS and SPR were correlated in the surface morphology of the SERS substrate at the exciting wavelength of 632.8 nm. The optimal SERS substrate shows the largest Raman scattering signal enhancement of up to 44,500 times. This study provides a simple process to fabricate highly sensitive SERS substrate that can enhance the intensity of Raman spectrum of organic compound by tuning process parameter easily. The observed SERS in this study will be beneficial for the design and fabrication of functional devices and sensors.

The effects of Au or Ag nanoparticles on optical absorption enhancement of organic photovoltaics based on blended poly(3-hexylthiophene):phyenyl-C61-butylric acid methyl ester (P3HT:PCBM) were investigated using a finite-difference-time-domain (FDTD) method. The various shaped nanoparticles such as sphere, plate and semi-spheroid were chosen in this study. First, the spherical metal nanoparticles were embedded in a buffer layer of 20 nm thickness and their size was varied from 10 nm to 50 nm. The metal nanoparticles with 10 ~ 20 nm diameter offered negligible absorption enhancement in an active layer. Unlike those short metal nanoparticles, the incorporation of the taller metal nanoparticles than the buffer layer led to a significant absorption enhancement by plasmonic resonance especially in case of Ag nanoparticles. Ag nanoparticles gave broader and stronger absorption enhancement in the active layer than Au nanoparticles. 34 % enhancement in the optical absorption of the active layer was observed with Ag nanoparticles with 50 nm diameter at 10 % coverage. The electric field distributions around spherical metal nanoparticles, their self-absorption, and the active layer thickness dependence on the absorption enhancement were also discussed. Second, the influence of Ag nanoparticles of plate/spheroid shape on optical absorption of the active layer was studied. Nanoplates of disc, square, triangular plate and oblate semi-spheroid shape were embedded in the buffer layer, and their height was set for 20 nm of the same thickness as the buffer layer. Embedding Ag nanoplates of optimal lateral dimension in the buffer layer provided substantial optical enhancement in the active layer. The resonance wavelength of metal nanoplates was able to be tuned by adjusting their lateral dimension, which was one of the keys to enhancing optical absorption of the active layer.

This work reports on the utilization of silver and gold nanocubes as substrates for the surface-enhanced Raman scattering (SERS) detection of a wealth of pesticides. The nanocubes were obtained via the chemical reduction of Ag+ or AuCl4- ions in solution. Such nanostructures were then employed as substrates for the SERS detection of organochloride compounds, such as 2,6-dichloro-4-nitroaniline (Dicloran) and 2,4-dichloro-6-nitrophenol. In all cases, the Ag and Au nanocubes displayed high performances as SERS substrates and the characteristic Raman signals of the probed molecules displayed good signal to noise ratios at the micromolar range. In addition, density functional theory (DFT) calculations for normal Raman and SERS spectra were performed to obtain a reliable analysis of the specific molecule–surface interactions over the silver and gold substrates. Our results show that the prepared Ag and Au nanocubes can serve as versatile platforms for the development of new sensing techniques for the ultrasensitive analysis of various pesticides based on the SERS effect. Reference Costa, J. C. S.; Ando, R. A.; Sant’ana, A. C.; Rossi, L. M.; Santos, P. S.; Temperini, M. L. A.; Corio, P. Phys. Chem. Chem. Phys. 2009, 11, 7491–7498.

By tuning the radiative coupling of localized surface plasmons to diffracted orders, we demonstrate how tunable stop-gaps may be opened in the dispersion diagram of plasmonic crystals of nanorods. The stop-gap arises from the mutual coupling of Surface Lattice Resonances (SLRs), which are collective Fano resonances associated with counter-propagating surface polaritons. The different field symmetries of the high and low frequency coupled SLR bands lead to pronounced differences in light extinction over narrow spectral regions. We observe that standing waves of very narrow spectral width compared to localized surface plasmon resonances are formed at the high frequency band edge, while subradiant damping leads the low frequency band into darkness. We show how the dispersion of coupled bright and dark SLRs, the frequency width of the gap, and the in-plane momentum width of the standing waves, can all be tailored by tuning the form factor of the array, i.e., the dimensions of the nanorods which determine their polarizability. We elucidate the physics in terms of a coupled oscillator analog to the plasmonic crystal. Our model serves to estimate very high quality factors for SLRs, and relates the tunability of the stop-gaps to the coupling strength of the surface modes involved.

The advent in engineering plasmonic resonances in noble metal nanostructures led in recent years to a broad range of novel concepts, with applications in sensing, energy harvesting, or nonlinear optics, among others. In this work we probe non-concentric gold NRs (NC AuNRs) deposited on a Si3N4 substrate, which are based on geometries of concentric AuNR dimers we fabricated and characterized experimentally and theoretically. Finite difference time-domain simulations for NC AuNRs will be discussed in detail, regarding effects of structure size and substrate, as well as variation of the offset from the center of the AuNR, among other parameters that influence the optical response. Implications for application of this class of materials will be suggested.

By altering the electromagnetic density of states near an optical emitter, optical cavities can modify the emission properties of the emitter, either enhancing or suppressing emission depending on the degree of detuning between the emission frequency of the emitter and the resonance frequencies of the cavity. In the visible and near-IR part of the spectrum, such optical cavities are typically fabricated in dielectric material systems, where the low inherent loss of the materials and the ability to make highly reflective mirrors has enabled fabrication of cavities of very high quality. Recently, progress has been made in fabricating metal-based optical cavities containing coupled emitters. These cavities are capable of tightly confining light, leading to an electromagnetic density of states that is greatly modified spatially as opposed to spectrally. Here, we present measurements from a metal-based optical cavity containing coupled optical emitters that greatly modifies both the spectral and temporal characteristics of the coupled emitters. Our structure utilizes plasmonic hybridization between a silver nanowire and a planar silver substrate to tightly confine electromagnetic energy in the nano-scale gap between the nanowire and substrate. A layer of optical emitters within the gap (either organic dye molecules or colloidal nanocrystals) strongly interacts with the confined electric field, leading to a 1000-fold enhancement of the spontaneous emission rate of the coupled emitters. These results suggest that metal-based optical cavities can allow quantum cavity electrodynamics of intrinsically broad emitters such as colloidal quantum dots and organic dyes.

Surface plasmons on metallic surfaces have been widely studied due to their attractive optical properties. One of the key issues has been the inherent absorption loss in the metal, which often limits device performance. Here we show that this strong absorption can in fact be beneficial, by harvesting the high concentration of hot-electrons created within the metal. This principle is demonstrated in simple metal-insulator-metal (MIM) devices, where the hot electrons are extracted from the top metal by tunneling through the extremely thin insulator barrier, and collected in the lower electrode. Here we report plasmon-enhanced photodetection through hot carrier collection in grating structure MIM devices that don’t suffer from the limited bandgap range in semiconductors. The key factor in realizing this is to properly design features on the metal surface to excite surface plasmons at any specific wavelength. The grating devices are optically excited under different polarizations to demonstrate surface plasmon excitation is responsible for the photocurrent, and current enhancement of more than 360% at 596 nm is observed. Linear dependence of the current on light power indicates a single-photon process during excitation of electrons.

Colorimetric sensors that selectively detect environmental pollutants in real time are becoming increasingly important areas of research. In particular, the design of materials that detect and discriminate between pollutants with similar molecular structures are in high demand. We have developed a series of colorimetric sensors based on silver (Ag), and gold (Au) metallic nanoparticles, and Ag/Au bimetallic nanoparticles. Particle size and shape were controlled through wet-chemical synthesis. The quality and the structure of the surface of the nanoparticles were found to play an important role in the detection process. The interaction of the nanoparticles with the pesticides ethion, malathion, parathion, fenthion and paraoxon was examined. We found that with proper control of particle size and composition, these nanoparticles are highly selective toward OP pesticides, giving specific changes in optical signal. The sensors can be tuned to have up to ppb detection limits. The presentation will demonstrate the rational choices in substituent selection for selective discrimination between organophosphorus compounds.

Scattering near-field scanning optical microscopy called ANSOM or sSNOM has been applied to look at plasmonic distribution. Unfortunately, the probes that need to be used in order to effectively scatter the plasmonic signal have significant perturbation on the plasmonic propagation because of the need to use probes with high dielectric constant to obtain effective signal to noise in such scattering experiments. In this paper, we will demonstrate the application of our development of multiprobe scan probe microscope technology for effective localized illumination of plasmonic structure with an apertured NSOM probe which produces all k-vectors. The propagating plasmons are imaged with an active fluorescent material embedded in a glass probe [1] with minimal perturbation of the plasmonic propagation. The results indicate that localized apertured NSOM illumination and active apertureless monitoring of plasmons has significant potential for investigating plasmonic structures. 1. A. Lewis and K. Lieberman, "Near-field Optical Imaging with a Non-evanescently Excited High-brightness Light Source of Sub-wavelength Dimensions," Nature 354, 214 (1991).

Extremely large nonlinear absorption was obtained from three-dimensional (3D), bowtie-shaped Au nanoantennas. Nonlinear light absorption can be substantially increased by localized electric fields excited around metal nanoparticles. We show that linear transmission spectra supported by FDTD calculations exhibit the strongest field enhancement at the LSP wavelengths. The imaginary part of the third-order nonlinear susceptibility (Im χ⁽³⁾), characterized by open-aperture z-scan measurement, for the 3D bowties embedded in a dielectric material was measured to be 10^-4 esu. The LSP-assisted nonlinear absorption of these 3D bowtie nanoantennas exceeded the reported values found in other metal nanoparticle-dielectric composites by more than two-orders of magnitude. These 3D nanoantennas can be used as a key element to increase the functionality for nanoscale nonlinear optical devices.

This article presents a systematic approach for design of 2.5 dimensional plasmonic crystals for enhanced light-nanomaterial interaction. A comprehensive study on plasmon resonance and the resulting near field enhancement in periodic arrays of gold and silver nanopillars as well as their dependence on geometry of nanopillars (diameter d, spacing s, and height h) as well as the excitation wavelength and angle was conducted utilizing 3D-finite difference time domain (FDTD) analysis. The study was utilized to design a plasmonic crystal (nanopillar arrays in square lattice with d, s, and h values of 310 nm, 620 nm, and 70 nm) with maximum plasmonic activity at 980 nm to couple with a thin layer of poly(methyl methacrylate) (PMMA) dispersed with upconversion nanocrystals of NaYF4: 3%Er, 17%Yb. The designed gold plasmonic crystal was fabricated by deposition of a 100 nm gold layer on a glass substrate followed by deposition of another 70 nm gold layer through electron beam patterned holes in PMMA and a following lift-off process. A PMMA layer dispersed with nanocrystals of NaYF4: 3% Er, 17%Yb with thickness of 105 nm was then deposited on the plasmonic crystal and spectroscopic measurements were conducted on the samples. A minimum in the measured reflection spectrum at 980 nm showed an excellent agreement between the design and measurement and confirmed the maximum plasmonic activity of the gold plasmonic crystal at 980 nm. The upconversion spectra of the upconversion nanopartciles in the PMMA matrix was obtained by a confocal microscope using 980 nm probe laser with 6 mW illumination intensity. The measurements revealed that the plasmonic crystal has led to an over 16X enhancement in the upconversion efficiency of the NaYF4: 3%Er, 17%Yb nanocrystals.

Constructing sophisticated plasmonic nanogap nanostructures with highly strong and quantitative surface-enhanced Raman scattering (SERS) signals and a narrow distribution of enhancement factors (EFs) is of significant importance in many research areas such as nanomaterials, plasmonics, Raman, chemical and biological sensing. Here, we extensively studied relationships between single-molecule SERS intensity, EF distribution over many particles, interparticle distance, particle size/composition and excitation laser wavelength using single-particle Raman measurement and 3D finite element method-based electromagnetic calculation with two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) dimer designs. Two GSND probes include “GSND-I” to study the effect of change in inter-particle gap distance from 4.8 nm to 0.2 nm or no gap and “GSND-II” to study the effect of change in Au core size with a fixed gap distance and shell thickness and change in excitation laser wavelength. We learned that synthesizing <1-nm gap (0.2 nm gap in this case) is a key to obtain high EF value (as high as 5.9x1013) with a narrow EF value distribution (between 1.9x1012 and 5.9x1013). In the case of GSND-II probes, a combination of >50-nm Au cores and 514.5-nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50-nm Au cores with 514.5-nm laser or GSND-II with 632.8-nm laser.

Significant research in nanotechnology has occurred recently in all areas of science, including biomedical, electronics, and consumer products. However, little is still known about the fate of nanoparticles as they transition from engineered to environmental systems. With an increasing rate of nanomaterials being used, concern has risen about their potential human or environmental harm. An important factor to consider in these systems is surface functionality, as this is one of the main contributors to particle stability and end use. In light of this, an investigation was conducted on gold particles to compare the response of different ligands on the surface of nanoparticles. The goal of this study is to measure the hierarchy of binding constituents, rate of ligand attachment and displacement, and a soil retention study in order to understand the interactions of different surface ligands and the kinetics of ligand exchange on plasmonic nanoparticles. These values are determined by measuring the exchange of non-fluorescent with fluorescent ligands on the surface of plasmonic nanoparticles. In this study gold nanoparticles are used as they have well-defined “quenching” of the photoluminescence of dye molecules at the surface. We utilize this “quenching” as tool to measure the rate of ligand exchange on the surface by observing the change in fluorescence of bound fluorophors using FRET. In order to understand how naturally occurring ligands will interact in both aquatic and soil systems, a wide array of fluorescently labeled common stabilizing ligands was synthesized via NHS chemistry. The displacement of these ligands from the surface of the gold particles could then be tracked by an increase in their photoluminescent signal. Calibration curves were created for each ligand so that the concentration of displaced ligands could be determined during reactions. The exchange of ligands were measured using three techniques; introducing fluorescently tagged ligands to a system of Au nanoparticles, introducing free non-fluorescent ligands to a system of Au nanoparticles with bound fluorescently tagged ligands, and using a combination of the first two with two different fluorescent molecules. In the first scenario the exchange can be measured by observing a decrease in fluorescent intensity upon binding. The second allows fluorescent intensity to increase as surface ligands are replaced by free ligands. Finally, in the third scenario the fluorescent intensities of the different molecules changes depending on which ligands bind to the surface of the nanoparticles. This investigation has provided a hierarchy of ligands and their exchange kinetics. The knowledge gained in this study will help in predicting the behavior nanoparticles will exhibit when they enter natural systems. Knowing how nanoparticles will behave in these systems may provide further insight into any harmful effects on both humans and the environment, as well as how to manage these particles.

Ideal plasmonic nanojunctions occur when high-curvature metal surfaces are separated by small nanometer-sized gaps, producing intense “hot spots” due to electromagnetic field localization within the gaps. While direct-write techniques such as electron-beam lithography are able to generate complex nanostructures with impressive spatial control, these methods encounter difficulties in fabricating gaps on the order of a few nanometers and in the scalable manufacturing of nanojunction arrays. Here, we fabricate nanojunctions by organizing polymer-grafted nanoparticles directly within a supported polymer thin-film matrix. We engineer the non-specific nanoparticle interactions that modulate the relative strengths of attractive van der Waals and repulsive steric forces by addressing simple parameters such as polymer chain length, rigidity, and grafting density. We demonstrate this by fabricating a nanoparticle thin-film that switches from one nanoparticle orientation (edge-connected) to another (face-connected), producing a stimulus-responsive change in the plasmonic response that is consistent with electric field calculations.

We report a new concept to tune localized surface plasmon resonance (LSPR) band of two dimensional (2D) crystalline metallic nanoparticle (NP) sheets in combination with layer-by-layer structures on metal substrates and nano-domain formations in mixed monolayers. In principle, the multilayerd 2D AgNP crystalline sheets fabricated by the Langmuir-Schaefer method keep the LSPR band position at the same wavelength (λmax ~ 465 nm) on quartz [1], however, they change their colors (wavelength and intensity) drastically on Au or Ag substrates depending on the number of layers (1-5 layers). The response of the LSPR bands was absolutely non-linear, which exhibits the maximum absorption at the layer number of 2 or 3, while the LSPR band position shifted linearly to the longer wavelength. We investigated the mechanism of interlayer coupling by use of AuNPs sheet (λmax ~ 630 nm) as a marker. The data revealed the complexity of electromagnetic interaction within the layered films; i.e., depending on the layer position of the integrated AuNPs monolayer, the color of the multilayered films changed largely, even though the total layer numbers are the same (e.g. AgNPs sheet : AuNPs sheet = 4:1). Another way to tune the LSPR band is nano-phase segregation in mixed films. When AgNPs and AuNPs were spread at air-water interface from the mixed solution, the AuNPs always formed island-like domains in the AgNPs matrix phase. The color of the mixed film varied in a wide range due to the size of domains determined by the AgNPs/AuNPs mixing ratio. These phenomena were reasonably interpreted by FDTD calculation in consideration of domain size effect and interparticle coupling between AgNP and AuNP. [1] Toma M, Tamada K, et al, Phys. Chem. Chem. Phys. 13, 7459 (2011).

We demonstrate the controlled linear assembly of silver-poly-N-isopropylacrylamide (Ag-PNIPAM) hybrid core-shell particles via a wrinkle assisted deposition method [1]. The particles were deposited on glass substrates by a spin-release process from poly-dimethylsiloxane (PDMS) templates with different wavelengths yielding linear assembled particle arrays. The assemblies show a high degree of order on cm scale, which is already visible by the naked eye due to strong iridescent colors caused by interference of the incident light with the periodic particle arrays. The ordering is also confirmed by laser diffraction experiments, where diffraction is observed up to the fourth order. Structural investigations employing SEM reveal anisotropy of the assemblies on two length scales, macroscopically guided through the wrinkle structure and locally due to deformation of the soft polymer shell leading to smaller inter-core separations as compared to assembly on flat substrates without confinement. Additionally, radial distribution functions (RDF) are shown, clearly highlighting the impact of confinement on nearest neighbor distances and symmetry. These results are compared to results obtained from wrinkle assisted assembly of hard spheres[2]. Monte-Carlo simulations confirm that the observed symmetries for hard-core/soft-shell particles are attributed to the soft interaction potential. In addition, results from polarization dependent UV-vis spectroscopy indicate Plasmon coupling of the silver cores. The presented method is a fast, cost effective technique to prepare anisotropic structures on large areas. [1] M. Müller, M. Karg, A. Fortini, T. Hellweg, A. Fery, in preparation [2] N. Pazos-Pérez, W. Ni, A. Schweikart, R. A. Alvarez-Puebla, A. Fery, L. M. Liz-Marzán, Chemical Science, 2010, 1, 174-178.

Emerging plasmonic and photovoltaic applications benefit from effective interaction between optical antennas and unidirectional incident light over a wide spectrum. In this study, we propose a honeycomb array of plasmonic nanoantennas with broken symmetry for obtaining a unidirectional radiation pattern over a wide spectrum. The honeycomb nanoantenna array is based on a hexagonal grid with periodically arranged nanostructure building blocks. To analyze the far-field optical distribution and spectral behavior of the plasmonic antenna honeycomb, a two-dimensional Wigner-Seitz unit cell is used to represent the nanostructure building block of broken symmetry. When combined with periodic boundary conditions, superposition of the fields from a single asymmetric building block generate far-zone optical fields lead to constructive or destructive interference in different directions. The constructive interference along the array's normal direction engenders unidirectional radiation. Consequently, due to the broken symmetry of the Wigner-Seitz cell, multiple resonances are supported by the plasmonic antenna honeycomb array over a broad spectrum.

It is well established that plasmonic waves can propagate along chains of closely spaced metallic nanoparticles. Their dispersion relations are readily calculated within the quasistatic approximation, using the tight-binding method[1], and even including higher multipole moments[2]. In this work, we present a calculation of these dispersion relations in the presence of either an external magnetic field, or an anisotropic environment such as a nematic liquid crystal. With an external magnetic field applied parallel to the chain, we show that a linearly polarized plasmonic wave is Faraday-rotated as it propagates along the chain, and we calculate both the rotation angle and the depolarization of this wave per unit chain length, within the quasistatic approximation, including single-particle damping. If the chain is immersed in a nematic liquid crystal with principal axis parallel to the chain, we show that both the width and the center of the plasmonic band are modified. We calculate the modified dispersion relations using the tight-binding method, again including single-particle damping. Both calculations are carried out using a generalized depolarization tensor formalism to compute the dipole field of a single metallic grain in the presence of these external perturbations[3]. These results may be useful in developing nanoscale optical devices. For example, because the dielectric tensor of a nematic liquid crystal depends on both temperature and applied electric field, the dispersion relations of plasmonic waves propagating along a chain of metal particles may be controllable by temperature and electric field. [1] M. L. Brongersma, J. W. Hartman, and H. A. Atwater, Phys. Rev. B62, R16356 (2000). [2] S. Y. Park and D. Stroud, Phys. Rev. B69, 125418 (2004). [3] S. Y. Park and D. Stroud, Appl. Phys. Lett. 85, 2920 (2004).

We present critical coupling of electromagnetic waves to plasmonic cavity arrays fabricated on Moiré surfaces. The critical coupling condition depends on the superperiod of Moiré surface, which also defines the coupling between the cavities. Complete transfer of the incident power can be achieved for traveling wave plasmonic resonators, which have relatively short superperiod. When the superperiod of the resonators increases, the coupled resonators become isolated standing wave resonators in which complete transfer of the incident power is not possible. Dark field plasmon microscopy imaging and polarization dependent spectroscopic reflection measurements reveal the critical coupling conditions of the cavities. We image the light scattered from SPPs in the plasmonic cavities excited by a tunable light source. Tuning the excitation wavelength, we measure the localization and dispersion of the plasmonic cavity mode. Dark field imaging has been achieved in the Kretschmann configuration using a supercontinuum white light laser equipped with an acoustooptic tunable filter. Polarization dependent spectroscopic reflection and dark field imaging measurements are correlated and found to be in agreement with FDTD simulations.

Coupling effects in plasmonic nanostructures are of tremendous interest due to the potential to actively modulate the plasmon resonances relative to individual nanoparticles and the strong field enhancements that are observed in small gaps between adjacent nanoparticles. In this report, we demonstrate the synthesis and surface assembly of nanorod dimers and arrays. Gold nanorod dimers and arrays, with rod diameter and length of 50 and 100 nm respectively, were synthesized in porous anodic alumina templates through electrodeposition allowing for fine control (<2 nm) of the nanorod length and the size of the gap between rods. High-resolution dark field hyperspectral imaging was used to image and measure polarized UV-Vis scattering spectra from individual dimer pairs fabricated with varying gap size (ranging from 2 to 20 nm). The polarized UV-Vis spectra were able to clearly resolve the transverse and longitudinal plasmon peaks and demonstrated large red shift in longitudinal peak position (nearly 200 nm) as the dimer gap was reduced from 20 to 2 nm. The shift in the longitudinal peak with decreasing gap size correlated well with the expected results based on discrete dipole approximation simulations. These results demonstrate the ability to fabricate and homogeneously assemble over large areas nanorod dimers and arrays with programmable control over plasmonic properties through fine control over rod length and gap size. Moreover, this approach is a low-cost complement to traditional top-down approaches that can easily be extended to asymmetric multi-metal plasmonic systems. Finally, the results demonstrate the ability to precisely image and characterize the plasmonic properties from an individual dimer pair using high-resolution hyperspectral imaging.

We design, fabricate, and test metamaterial building blocks functioning as optical nanocircuits in the NIR regime. We explore arrays of nanorods with rectangular cross sections, made of Transparent Conductive Oxides (TCOs), such as indium tin oxides (ITO). Using the equivalent circuit theory and FDTD (Finite-difference time-domain) simulations, we design and analyze the nanoscale circuit element functionalities of such building blocks. We show that we can control the functionality of these metatronic circuits by tailoring the cross sectional dimensions and the pitch of the TCO nanorods arrays. Furthermore, such nanocircuits can also function differently for different polarization of the incident E-field, thus making such circuits “stereo-circuits”. When nanorods are illuminated by E-field vector parallel to the rods, the array should function as the “parallel” combination of elements, and the parallel L-C circuit may act as a bandpass filter. However, when the E-field vector is polarized perpendicular to the rods the array may behave as “series” combination and it indeed acts as a bandstop filter. We have fabricated and tested several samples of such nanorods made of ITO, and have shown the agreement of the measurement results for the spectral response of the fabricated ITO nanorods arrays with the calculations and simulations. In this presentation, we will present our theoretical and experimental results for NIR filter metamaterials using the TCO nanorod arrays functioning as the optical nanocircuits.

It is well known that metallic nanostructures have the ability to concentrate light into deep subwavelength volume due to surface plasmon resonance. Semiconductor nanostructures, such as Si/Ge nanowires could also have strong interaction with light due to excitation of dielectric resonances. In our work, it is shown that by carefully combining metal and semiconductor materials within subwavelength volume, such hybrid nanostructures could have novel light-matter interactions due to drastically different materials properties of metals and semiconductors. We have demonstrated a gold coated silicon nanowire could act as an "invisible" photodetector due to the process known as "plasmonic cloaking". We could further show such hybrid metal/semiconductor with interesting scattering and absorption properties that differ from both metal and semiconductor nanostructures alone due to the coupling of resonances excited in each component. These results could shine light on the design of next generation of optoelectronic devices with complex functionalities.

We use spectroscopic imaging to investigate the enhancement of infra-red to visible upconversion in rare-earth doped nano-particles (NaYF4:Yb:Er) supported on nano-fabricated plasmonic substrates, including: Ag nano-wires synthesized by wet chemistry methods, and lattices of Au nano-pillars fabricated by electron beam lithography, the latter of which are designed to support a surface plasmon polariton at frequencies which are near-resonant with the rare-earth ion (Yb3+) absorption. We observe a systematic enhancement in the efficiency of upconversion associated with the interaction of the co-doped nano-particles with the plasmonic substrate. Spectrally-resolved imaging provides a massively parallel means of assessing the range of achievable enhancement and its relation to the specific configuration of the substrate / upconverting nano-particle system. Spectrally-resolved reflectivity of the plasmonic substrates confirms the role of the surface plasmon polariton in the upconversion enhancement. Experimental results are compared to Finite Difference Time Domain simulations of the field distributions near the metallic nanostructures and their frequency-dependent reflectivity.

We consider a graded-index (GRIN) photonic crystal (PC) as an input and output coupler for a photonic crystal waveguide (PCW) and investigate the enhanced coupling efficiency figures. We show that the designed GRIN PC supports modified versions of the Hermite-Gaussian modes. An analogy in between the examined GRIN PC and the quadratic GRIN medium is shown to be useful in understanding the propagation of the electromagnetic waves inside the GRIN PC. We numerically and experimentally demonstrate that by employing the GRIN PC it is feasible to focus spatially wide input pulses into the narrow entrance of the PCW, hence improving the input coupling efficiency as high as 8.27 dB with an insertion loss -1.62 dB. At the same time, the highly diverging beam exiting from the PCW end with circular wave fronts are transformed into planar wave fronts. As a result, the divergence angle of the beam is substantially reduced from 70 degrees to 11 degrees. Accordingly, the confinement of the out-coupled beam resulted in a 90% reduction in the half-power beam width (HPBW) values. A directional beaming efficiency up to 76% is reported in the accompanying microwave experiments. HPBW values down to 7 degrees are measured.

In the limiting case of the directional selectivity such a device that would allow (nearly) total transmission in one direction and no transmission in the opposite direction within the same propagation channel could be considered as the electromagnetic counterpart of a diode. The conventional approach to achieve the unidirectional transmission in passive devices is based on the use of the anisotropic or nonlinear materials. In particular, the strongly pronounced unidirectional transmission has been demonstrated for the one-dimensional photonic crystals (PCs) and for the stacks of the two-dimensional PCs, in which anisotropic materials were utilized. Directional waveguides have been realized in PCs with broken time-reversal symmetry. In this paper, we investigate the directional selectivity in the PC gratings in the microwave regime at beam-type illumination. The simulations and the microwave experiments are performed for a wide frequency range that involves the first five PC bands (Floquet-Bloch waves), which are distinguished in terms of their respective dispersion features. The presented results include the transmission spectra of the examined structures for the plane-wave illumination, the frequency response of the transmittance for Gaussian-beam and horn antenna illuminations, and the angular distributions of the transmittance, at a proper value of the angle of incidence. We have demonstrated unidirectional transmission in the PC gratings with one-side echelette-type corrugations at beam-type illumination. Simulation results obtained for the plane-wave and Gaussian-beam illuminations, and the experimental results for the microwave horn antenna illumination were presented and analyzed. We have observed a good connection between the features detected at plane-wave, Gaussian-beam and horn antenna illuminations.

We developed plasmonic resonant cavities based on vertical metallic nanowire arrays for enhancing the efficiency of photoluminescent dyes. Gap plasmon modes are excited in the space between two adjacent nanowires when the roundtrip phase change is a multiple of 2Ï€. We demonstrate continuous tuning of plasmon resonances in the visible (400 to 800 nm) by adjusting the geometrical dimensions of the nanowires (radius, height) as well as the refractive index of the materials immersing the nanowire array. An increase in dye absorbance is observed when the plasmon resonance is aligned with extinction maximum of the dye. The material luminescence is also enhanced when the plasmon resonance is aligned with the emission wavelength. We further discuss the possibility to fabricate of a two resonance plasmon laser that employs surface plasmons to enhance both absorbance and emission for a more efficient pumping of the gain medium.

Preconcentration of a molecule can be considered as an essential step when the concentration of target is very low. Typically, preconcentration is time-, cost- and labor- consuming. Here we report simple yet innovative preconcentration method based on enthalpy-driven accumulation of molecules at fluid-fluid interface. Numerical calculation reveals that local concentration of molecules at water/oil interface can be up to 100 times higher than that in water by controlling the difference in the diffusivities of the molecules in two fluids. We test the feasibility of our method by applying to surface-enhanced Raman scattering (SERS). Even though as-made colloidal gold nanorod without any further treatment is directly used as a SERS-active probe, 1 nM of rhodamine 6G is readily detectable based on our preconcentration scheme. We believe that this preconcentration method can be easily applied to a molecular detection in ranges from molecular diagnostics to environmental monitoring.

Plasmonic nanoparticles are in the focus of interest because of their interesting electric and optical properties which are highly related to the specific particle size and shape. There are synthetic procedures which allow us to fine tune them in order to use them for a desired application. However, the lack of capability to form reproducible organized structures is still a very important challenge to solve in order to control the plasmonic intercoupling between particles which is of key importance. In this work we report novel methods to produce organized structures of plasmonic nanoparticles either at the Macroscale range, through self-assembly of gold nanoparticles upon solution-drying in a periodic confining structure forming linear parallel arrays of colloids.[1] Moreover, the good reproducibility of these structures among big areas, make them perfect candidates as ultrasensitive substrates for Surface Enhanced Raman Scattering (SERS) due to the controllable plasmon coupling. Providing high and uniform SERS enhancement over extended areas. Or, at the Nanoscale regime, through a controllable cluster formation with high coordination numbers. The plasmonic behaviour of these organizations was theoretically and experimentally investigated. Moreover, these structures, were effectively use for Surface-enhanced Raman scattering (SERS) spectroscopy. Reaching enhancing factors up to 30 times higher to that observed for gold dimmers. 1.Chem. Sci., 2010, 1, 174; Soft Matter, 2011, 7, 4093

The development of metamaterials, artificial materials made of structures smaller than the wavelength of the operating incident radiation, opened the path for intriguing applications such as cloaking devices or imaging beyond the diffraction limit. We have shown that, utilizing the concept of self-rolling strained layers [1,2], swiss-roll-like three-dimensional microtube metamaterials of multiple windings can be fabricated [3,4]. In this work we show by means of finite-integration technique simulations that arrays of rolled-up metal/semiconductor tubes with slightly more than one winding interact resonantly with the magnetic component of an incident electromagnetic field and exhibit a negative permeability at terahertz frequencies [5]. The small overlap region of alternating metal and semiconductor exhibits a small capacitance C, which, together with the inductivity L of the loop, leads to a LC-circuit-like arrangement with a resonance frequency ω₀=1/(LC)^1/2. The effective permeability of the micro tube array retrieved from the simulation data has the shape of a Lorentz oscillator exhibiting negative permeability. We show that the resonance frequency can be tailored to desired values by changing the winding number. On the other hand, we also demonstrate that this dependence can be removed by the incorporation of an additional slit into the metal layer. In an experiment such a slit can be prepared by lithographic means before the rolling process. The slit forms a second capacitance C_slit operating in series with the overlap capacitance C. Since C_slit << C, deviations of the overlap area do not significantly influence the position of the resonance frequency anymore. We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the Graduiertenkolleg 1286. [1] V. Y. Prinz et al., Physica E 6, 826 (2000). [2] O. Schumacher et al., Appl. Phys. Lett 86, 143109 (2005). [3] S. Schwaiger et al., Phys. Rev. Lett. 102, 163903 (2009). [4] I. V. Semchenko et al., Crystallography Reports 56, 366 (2011). [5] A. Rottler et al., Opt. Lett, accepted.

Chirality arises in many natural contexts ranging from biological molecules to electromagnetic fields. Circularly polarized light is the paradigmatic example of a chiral electromagnetic field; when propagating in dispersion-less media, both the electric and magnetic fields undergo a full rotation per period and wavelength. This rotation can be clockwise or anti-clockwise. The spatial trajectories of these waves form a chiral set of solutions to Maxwell’s equations. Recently, A. Cohen et. al. suggested that Maxwell’s equations support solutions of enhanced spatial rotation of the fields compared to regular circularly polarized light [1]. Complementarily, they proved that these “superchiral” electromagnetic fields could be used to increase the enantioselectivity in the excitation of chiral molecules [2]. In this contribution, we demonstrate that plasmonic nanostructures offer enhanced chirality of electromagnetic fields in their near-fields. We show that regions of enhanced chirality can be created in the near field of metallic nanostructures, but also prove that structural chirality is not needed either in the nanostructure or in the incident field to generate superchiral near field hot-spots (SC hot-spots). The only requirement to generate SC hot-spots is existence of both electric and magnetic electromagnetic modes in the nanostructure. Using the Boundary Element Method (BEM), we show that a simple metallic nanosphere satisfies this condition. The plasmonic character of the nanoparticle provides a resonant electric dipolar mode while a non-resonant magnetic dipolar mode is also excited due to the eddy currents that are excited by the incoming magnetic field. Furthermore, we show how the achievable near field chirality can be enhanced through geometric tuning. A continuous transformation of a silver 50nm radius sphere into a 5nm thick oblate ellipsoid (disk) increases the maximum chirality factor from 3 up to 700 when illuminated by linearly polarized light. Illuminating the structures with circularly polarized light preserves the superchiral behavior of the near fields, even though it modifies the spatial and spectral properties of the SC hot-spots. Based on recently reported experimental results [2,3], these yet unexplored near-fields promise new avenues in molecular spectroscopy as well as in the asymmetric synthesis of chiral molecules. [1] Y. Tang et. al., Phys. Rev. Lett. 104,163901, 2010 [2] Y. Tang et. al., Science, 332, 333-336, 2010 [3] E. Hendry et. al. Nature Nanotechnology 5, 783–787, 2010