Illumination of metal nanoparticles at the plasmon resonance produces enhanced evanescent fields on the nanoparticles’ surfaces. The unusual strength of the field make it a target for exploring photoinduced phenomena at the nanoscale, if efficient functionalization or coating of the nanoparticle surface with appropriate chromophores is possible. One direction is to use cyanine dyes that form monolayers of J-aggregates on the surface of noble metal nanoparticle colloids. The unique, collective electronic properties of J-aggregates produce excitons with enormous extinction coefficients that are of interest for their efficient energy transfer, electron transfer, and nonlinear optical properties. In that vein, we report our results on time-resolved spectroscopy and near-field scanning optical microscopy (NSOM) of J-aggregate exciton dynamics on Ag and Au nanoparticle colloids. Ultrafast transient absorption studies show that J-aggregate exciton lifetimes on Ag nanoparticles are much longer than on Au nanoparticles, with a 300 ps lifetime that is two orders of magnitude longer than the electronic processes in the nanoparticles themselves. Complementary NSOM studies of the colloids show that fluorescence from the J-aggregates on the Ag nanoparticles is induced by the scanning probe. These results may be significant for improving the nanophotonic performance of hybrid materials for nanoscale applications.

The third-order nonlinear optical response of materials composed of noble metal nanoparticles embedded in a dielectric matrix is large around the surface plasmon resonance frequency, due to local electric field enhancement in the particles. This response can be described by both nonlinear refraction and absorption, related to the complex third-order susceptibility, χ⁽³⁾, of the composite material. χ⁽³⁾ is linked with the intrinsic metal particle susceptibility, χ⁽³⁾_m, whose value is ruled by interband and intraband transitions. Depending on the incident pulse power and duration, very high conduction electron temperatures can be reached subsequent to the pulse absorption, and can result in a modification of the nonlinear response ("hot electron" effect). The χ⁽³⁾ real and imaginary parts of Au:SiO₂ thin films, synthesized by radio-frequency sputtering, are measured simultaneously by the z-scan technique, with both nanosecond and femtosecond laser pulses at 560 nm. Comparing the results obtained in both regimes, we show, by using a simple thermal model, that the "hot electron" phenomenon which is significant when exciting with ultrashort pulses, not only reduces the modulus of χ⁽³⁾ by three orders of magnitude, but also greatly affects its phase.

Incoherent: diffuse and depolarized,component of the enhanced third harmonic generation (THG)intensity is associated with the third-order hyper-Rayleigh scattering (HRS)in 2-D random ensemble of silver nanoparticles. A comprehensive analysis of the linear and nonlinear light scattering in combination with the results of atomic force microscopy revealed the fractal nature of the silver island films.

In this contribution, we present measurements of the ultrafast dephasing time T₂ of surface plasmon polariton excitation in gold nanoparticles by means of persistent spectral hole burning. T₂ is an essential parameter that does not only reflect the role of different dephasing and deexcitation mechanisms but also allows one to determine the field enhancement factor that is of great importance for many applications of nanoparticles. In our experiments gold nanoparticles were first fabricated in ultrahigh vacuum on sapphire substrates by deposition of atoms, followed by diffusion and nucleation, i.e. Volmer-Weber growth. Subsequently, systematic measurements of T₂ in the size range between r = 7 nm and 14 nm were carried out. The most essential among the numerous results is the observation of the influence of the reduced dimension on the dephasing time. While T₂ = 14 fs has been measured for r = 12 nm which is, within the error bars, consistent with the damping contained in the bulk dielectric function, the value of T₂ shrinks to, for example, T₂ = 11 fs for r = 7 nm. This reduction of T₂ can be attributed to surface scattering of the electrons. Further experiments are in progress to confirm the predicted 1/r law for the variation of T₂.

Nonlinear optical properties of granular magnetoresistive films are studied by second harmonic generation (SHG) technique and nonlinear magneto-optical Kerr effect (NOMOKE). For different types of granular structures - magnetic nanoparticles in non-magnetic metallic (Co-Cu) or in a dielectric host material (CoFe-Al₂O₃), a clear correlation between GMR and NOMOKE is observed, which manifests itself in the appearance of a local maximum in both the GMR and NOMOKE for the same concentration range of magnetic metal prior to the percolation threshold. This correlation apparently occurs on the macroscopic level due to the similar influence of the nanogranules structure on surface magnetization of granules which determines NOMOKE, and spindependent conductivity of the films.

The Ag nanoparticle based localized surface plasmon resonance (LSPR) nanosensor yields ultrasensitive biodetection with extremely simple, small, light, robust, and low-cost instrumentation. Using LSPR spectroscopy, the model system, biotinylated surface-confined Ag nanotriangles, was used to detect less than one picomolar up to micromolar concentrations of streptavidin. Additionally, the monitoring of anti-biotin binding to biotinylated Ag nanotriangles exhibited that the system could be used as a solution immunoassay. The system was rigorously tested for nonspecific binding interactions and was found to display virtually no adverse results. These results represent important new steps in the development of the LSPR nanobiosensor for applications in medical diagnostics, biomedical research, and environmental science.

Films consisting of self-assembled gold nanoparticles cross-linked with alkane-dithiols were prepared by a filtration method and studied with scanning electron microscopy to determine the structure of the films and spectrophotometry and ellipsometry to ascertain their optical properties. The structural characterization showed the existence of nanometer sized voids within the films. This previously unmentioned feature is responsible for the previous difficulties in modelling the optical properties with effective medium models.This can be remedied, using a two-tiered hierarchical effective medium model, which takes into account the existence of the voids. Using this model we were able to fit the experimental data,with only the void volume fraction to be determined by the overall fit, while the gold volume fraction in the linker medium is fixed by the wavelength of the resonance peak. Our model should be applicable to all such films, when the deposition method, which determines the microstructure, is properly taken into account.

We present a detailed description of the apparatus and techniques that we have utilized in our experimental study of individual plas on resonant nanoparticles,along with a brief description of some major results. The apparatus consists of a spectroscopic system combined with a modified darkfield microscope, which enables the user to sequentially select individual resonant nanostructures in the microscopic field of view for spectroscopic study. Plasmon resonant nanostructures scatter light elastically,and typically have very large scattering cross-sections at their resonant optical wavelengths. In general, spectra can be obtained with acquisition times between .1 to 30 seconds,and color images can be captured using consumer digital color cameras. Spheres,tetrahedrons,and pentagonal platelets were fabricated using colloidal chemistry techniques. To produce highly anisotropic structures such as nanorods and "barbells", templates were used. Many of these nanostructures have been individually spectroscopically characterized,and their spectra correlated with their shape and size as determined by transmission electron icroscope (TEM). The unique shape,size, composition,and dielectric surroundings of the individual plasmon resonant nanostructures determine their plasmon resonant behavior. We will show how the composition of the substrate on which the particles are immobilized and the dielectric of the surrounding medium have a significant effect on the plasmon resonance of the individual particles.

High SERS sensitivity for protein detection has been accomplished with semicontinuous silver films. Specifically, an insulin surface density as low as 80 fmol/mm² and 25 amol in a probed area has been readily detected.

Metal film over nanosphere (MFON) electrodes are excellent substrates for surface-enhanced Raman scattering (SERS) spectroscopy. These surfaces are produced by vapor deposition of a metal film over nanospheres that are assembled in a hexagonally close packed arrangement. The efficiency and reproducibility of AgFON electrode as SERS substrates are confirmed by the repeatability of the electrochemical surface enhanced Raman scattering spectra of pyridine and the Ru(bpy)₃³⁺/Ru(bpy)₃²⁺ complexes adsorbed on AgFON electrodes. The Raman signal for AgFON electrodes is observed to be extremely stable even at extremely negative potentials in both aqueous and nonaqueous electrolytes. Recent reports have indicated that SERS enhancement factors of up to 14 orders of magnitude can be achieved, providing the sensitivity requisite for ultra trace level detection of target analytes. For this reason, we are developing a method for bacterial endospore SERS detection based on the endospores marker -- dipicolinic acid (DPA). The SERS spectra of dipicolinic acid in aqueous solutions are reported. The dipicolinate vibrational features could be observed in the SERS spectra at the concentration as low as 8 × 10^-5 M in 5 minutes. These limits of detection are entirely controlled by the thermodynamics and kinetics of DPA binding to the AgFON surface.

Strong Raman signals have been observed in various molecules attached to rough metal film surfaces or nano silver/gold particles. This phenomenon is denoted as surface enhanced Raman scattering (SERS). Recent experiments have shown that the effective cross sections of Raman scattering can reach the same level that of fluorescence of good laser dyes, making SERS a promising single-molecular detection tool. The commonly used substrates for SERS consist of colloidal Ag/Au particle aggregates, where SERS active sites, called “hot spots”, are only found by chance and not controllable. The poor repeatability and controllability of these SERS substrates have prevented SERS from viable industrial applications, therefore it is imperative to design and fabricate optimized "hot spots" with desired plasmon resonance frequency in a controllable fashion. In this paper, we present a new class of composite nano particles, which is consisted of stacked alternative metal/dielectric layers, called nanoburger. We study optical properties of these nanoburger particles by using discrete dipole approximation method. The numerical results show that nanoburger particles possess many advantages over single layered particles, including high brightness or scattering intensity, high local field enhancements, and more freedom of tuning plasmon resonance wavelength. Another important merit of the nanoburger particles is that they can be fabricated with traditional micro/nano lithography techniques, and thus are integrable with techniques such as lab-in-a-chip.

Novel gold nanoparticle aggregates have been synthesized using simple colloidal chemistry techniques. The electronic absorption spectra of the aggregates can be manipulated by controlling the synthetic conditions. The aggregates have been demonstrated for the first time to exhibit strong activity for surface-enhanced Raman scattering (SERS). SERS studies were performed using rhodamine 6G (R6G), a molecule which normally does not show SERS enhancement on gold surfaces, showed an enhancement factor on the order of 10⁹, which is similar to or better than most ensemble averaged SERS enhancement factors reported to date. The results demonstrate that these gold nanoparticle aggregates are promising for SERS applications in detection and analysis of molecules.

Nano gold particles interact strongly with visible light to excite the collaborative oscillation of conductive electrons within nano particles resulting in a surface plasmon resonance which makes them useful for various applications including bio-labeling. In this paper, we study the effect of particle sizes with particle plasmon resonant wavelength and the coupling between pair of elliptical metallic disks and ellipsoid particles by simulations and experiments. The red-shift resonant peak wavelength of coupled particles to that of single particle is due to particle plasmons near-field coupling. The shift decays is approximately exponentially with increasing particle spacing, and reaches zero when the gap between the two particles exceeds about 2.5 times the particle short axis length. It is also found that the exponential decay of peak shift with particle gap is size independent but shape dependent.

Recent theoretical works have suggested the possibility of constructing a diffraction-free lens by using a negative refractive index medium (NRIM). The key theoretical proposition is that evanescent waves can be greatly enhanced by increasing the thickness of the NRIM. We present here experimental evidence on enhanced transmission of evanescent waves via surface plasmon at a thin silver film operating near surface plasma resonant frequency. We found the transmission of evanescent waves rapidly grows with the film thickness up to about 50 nm, after which it decays as loss becomes significant. These experiments also demonstrated the broadening of enhanced transmission spectrum as photon energy approaches plasma resonance ε_Ag = -1 condition. These findings represent the first step toward the understanding and realization of a diffraction-free lens by using NRIM.

Despite recent successes in making left-handed materials in the microwave frequency range, there has been little progress in achieving same for infrared frequencies. A novel approach to making a material with negative index of refraction using photonic crystals made of dielectric components with a small (of order minus one)negative dielectric permittivity has recently been proposed. Periodic structures with negative-epsilon dielectrics support surface waves which can have a negative group velocity.The nature of these surface waves depends on the dielectric components:they are surface plasmons for plasmonic materials (such as metals) or surface phonon polaritons for polar crystals (such as SiC,ZnSe,GaP) with the reststrahlen band.The advantages of using phononic materials (long phonon lifetime,scientifically important frequency range)will be illustrated. Depending on the photonic lattice (square or hexagonal), the resulting meta-material can be either isotropic,or strongly anisotropic. Another application of the negative-epsilon materials is nano-lithography.As was suggested earlier (Pendry 2000,Shen and Platzman 2002), any material with (formula available in paper) can be used to significantly enhance near-field imaging. It is shown that a thin slab of SiC is capable to focus the 10.55 micron radiation of a CO₂ laser to several hundred nanometers, thus paving the way for a new nano-lithographic technique: Phonon Enhanced Near Field Lithography in Infrared (PENFIL). Analytic calculations of the fields in the focus of such slab are presented, and parametric dependence on the slab width and phonon lifetime explored.

The optical properties of metallic nanoshell systems are investigated using the Finite Difference Time Domain (FDTD) method. The method provides a convenient approach for calculating several physical properties of nanoshells based structures including the optical absorption and scattering cross sections as well as the local electromagnetic fields near the nanoshell surfaces. The method is applied to silver and gold nanoshells and nanoshell dimers. Comparisons with classical Mie scattering are presented.

In this paper we report an experimental and theoretical study of the optical properties of metallodielectric gratings with subwavelength gaps in the thin metal limit. A mask-free method of fabrication for large area submicron silver gratings on silica substrates has been developed using soft-lithographic techniques. By measuring the zeroth-order transmission of these gratings, both an edge anomaly associated with the Rayleigh wavelength and a resonant anomaly associated with the excitation of surface plasmons (SPs) are observed. A crossed grating configuration is studied: the presence of the additional crossed grating results in a dramatic widening of the plasmonic band gap relative to that of a 1D grating.

The paper deals with the theoretical investigation of plane, normally incident electromagnetic wave transmission through the flat metal folm whose dielectric constant has small periodical sinusoidal modulation in one dimension parallel to the projection of the electric field onto the film surface. The dependencies of the film transmittancy on the parameters of the problem (frequency, modulation depth and absorption) are examined. It is shown that the film transmittancy increases considerably when the conditions for resonance interaction of an incident electromagnetic wave with surface plasmon polaritons (SPPs) are met. It is found that for small but finite absorption there are two frequencies in the vicinity of which the transmittancy can achieve the values of the order of unity due to resonances on symmetric and antisymmetric (relative to the mean plane) SPP modes. It is shown that for each value of absorption there exists a certain optimal modulation depth, which maximizes the resonance transparency.

The results of analytical and numerical investigation of the surface plasmon-polaritons (SPP)dispersion relation on double periodical high reflecting surfaces (two-dimensional photonic crystals)are presented. The formalism is developed for gratings formed by the modulation of either optical properties or the relief of the medium. The coupling between SPP existing on the non-modulated boundary leads to the mini-gaps arising at the Brillouin-zone boundaries. The dependence of the dispersion relation upon the parameters of the problem (amplitude of the modulation, an angle between the elementary translations,etc.) is calculated for different types of symmetry that corresponds to the coupling from two to six polaritons. The specific values of the parameters corresponding to existence of the standing polariton modes, vanishing of the polariton group velocity are found. The distribution of surface charges for corresponding polariton modes is presented. The ratio between the polariton dispersion relation and the light diffraction under the condition of the polariton excitation is discussed as well. The results obtained can be used to design the two-dimensional photonic crystals with specific and given properties.

The electronic structure and optical properties of metallic nanoshells are investigated using a jellium model and the Time Dependent Local Density Approximation (TDLDA). An efficient numerical implementation enables applications to nanoshells of realistic size with up to a million electrons. We demonstrate how a frequency dependent background polarizability of the jellium shell can be included in the TDLDA formalism. The energies of the plasmon resonances are calculated for nanoshells of different sizes and with different dielectric cores, dielectric embedding media, and dielectric shell backgrounds. The plasmon energies are found to be in good agreement with the results from classical Mie scattering theory using a Drude dielectric function. A comparison with experimental data shows excellent agreement between theory and the measured frequency dependent absorption spectra.

A general approach involved template-engaged displacement reaction has been demonstrated to prepare metal nanostructures with hollow interiors by reacting solutions of appropriate salts with solid metal nanostructures. For example, silver nanostructures with various morphologies including triangular plates, cubes, spheres, rods and wires have been used as templates to react with an aqueous chloroauric acid solution. The reaction led to the formation of hollow nanostructures with shapes similar to that of silver templates. The void space, wall thickness, and crystalline structure of these hollow structures were determined with the silver templates, which were converted into soluble species during the displacement reaction. Elemental analysis and electron microscopic studies indicated that these hollow structures were made of gold/silver alloys. The capability and feasibility of this method have also been demonstrated by preparing nanotubes made of different metals (e.g., gold/silver, palladium/silver, and platinum/silver alloys). The hollow nanostructures of gold/silver alloys exhibited significantly different surface plasmonic properties from their solid counterparts. For instance, the extinction peaks of the nanoshells of gold/silver alloy with roughly spherical shape were considerably red-shifted as compared to solid colloids of silver or gold having approximately the same dimensions. The high extinction coefficient in the red and near infrared regimes should make these nanoshells particularly useful as components in fabricating plasmonic devices and labels in probing the desired biomolecules.

The optical properties of silver nanoparticle arrays are studied by T-matrix and discrete dipole approximation (DDA) methods. Arrays of spherical silver particles with a radius of 30 nm are investigated with particular emphasis on the influence of array disorder on optical response. We find that the dipole peak intensities decrease when the array becomes disordered and the plasmon resonance wavelengths generally exhibit smaller blue shifts compared to perfectly ordered arrays. Using an extended DDA method, we calculate the extinction spectra of ordered nanodisk arrays, examining the variation of plasmon resonance wavelength with interparticle spacing (at fixed particle size). The calculated blue shift of the plasmon wavelength with decreasing interparticle spacing is found to be in excellent agreement with recent experiments.

We predict and theoretically investigate the unique possibility to control distribution of ultrafast local optical fields in metallic nanosystems in space with nanometer resolution and in time on the femtosecond scale. While the spatial degrees of freedom of the optical radiation do not allow focusing of light on nanoscale, the temporal degrees of freedom, i.e., phases of excitation femtosecond pulses, are quite efficient functional degrees of freedom that permit one to coherently control the distribution of the energy of local fields, concentrating it at a desired location at certain times. We study both a specially designed V-shape nanostructure and a random planar nanocomposite. Several types of exciting pulses are investigated, which has allowed us to distinguish effects of phase modulation and spectral composition of the excitation pulse. Possible applications of this effect include energy supply and control of ultrafast optical computations in nanostructures, local optical probing of nanosystems, including nanosensors of chemical and biological agents, and nanomodification of surfaces (nano-lithography).

A novel high efficiency electro-optic polymer light modulator based on waveguide-coupled surface plasmon resonance (WCSPR) is presented. The modulator consists of a five-layer system: dielectric layer/metal film/electro-optic (E-O) polymer layer/metal film/air. By combining WCSPR based on attenuated total reflection (ATR) method and Pockels effect from poled E-O polymer, we demonstrate that this kind of modulator operated with less applying modulation voltage, less optical insertion loss, and easy alignment compared to other light modulation techniques. Also, in this paper the theoretical derivation of WCSPR, the optimum design concerning the relation between the efficiency of modulator and E-O layer thickness, and the fabrication process of the E-O polymer light modulator are presented. This modulator is shown to allow a greater degree of modulation for a given voltage with working point chosen near the midst of WCSPR mode in the visible range.