Surface Plasmon Resonance Frequency Research Articles

Voltage-induced color-selective absorption with surface plasmons

A phenomenon of voltage-induced color-selective absorption at metal/liquid crystal interface with surface plasmons is reported. When a white light is incident at a metal/liquid crystal interface, those photons in surface plasmon resonance frequency range are totally absorbed and the reflected light shows the complementary color. If a voltage is used to change the refraction index of the liquid crystal, then the surface plasmon resonance frequency will change, and the reflected light will also show the color change. The reflectance spectrum is calculated from a well-known solution to the boundary value problem in optics. Good agreement with the observations is obtained. This phenomenon could lead to a new generation of display devices.

Modification of the optical properties of glass by sequential ion implantation

The linear and nonlinear optical properties of a series of samples formed by the sequential implantation of Ti, O and Au in high purity silica are examined. Energies of implantation for each ion were chosen using TRIM calculations to insure overlap of the ion distributions. The Ti was implanted with nominal doses of 1.2 and 2 × 10 17 ions/cm 2. The samples were implanted with oxygen to the same nominal dose as the Ti. Au was then implanted with a nominal dose of 6 × 10 16 ions/cm 2. The samples were subsequently annealed in oxygen at 900°C for two hours. The Ti is incorporated into the host network, whereas the Au forms nanosize colloids. The presence of the Ti in the substrate causes a shift in the surface plasmon resonance frequency of the Au metal colloids as well as increase in the nonlinear response of the composites. The results are interpreted using effective medium theory.

Optical scattering and absorption by metal nanoclusters in GaAs

Optical extinction by a dilute dispersion of metal nanoclusters in GaAs is calculated using the optical theorem and Maxwell–Garnet theory with complex dielectric functions for Cr, Fe, Ni, Cu, Ag, Au, Er, and As. The large dielectric function of the semiconductor host shifts the surface plasmon resonance frequencies from the ultraviolet to the near infrared. The noble metals have well-defined resonances with significant absorption and Rayleigh scattering at photon energies compatible with diode lasers and semiconductor electro-optic modulators. Interband transitions in metals such as As, Cr, Fe, Ni, and Er strongly damp the surface plasmon modes, quenching the resonant absorption by the particles, but providing significant absorption to wavelengths longer than 1.5 μm. Metal-semiconductor composites may arise during growth or processing of materials, such as GaAs:Er for fiber-optic applications, and GaAs:As in which metallic precipitates of As form in GaAs after low temperature growth of GaAs using molecular beam epitaxy.

Scanning near-field optical microscope for the imaging of magnetic domains in optically opaque materials

We describe a scanning near-field optical microscope capable of magnetic imaging in optically opaque materials. The near-field probe is a ∼30 nm Ag particle, optically excited near the surface plasmon resonance frequency. The separation of the probe and sample is regulated by a Newton ring interferometer. The polarization rotation of the scattered light due to near-field magneto-optic interactions is measured. A two-dimensional image is presented of thermomagnetically recorded magnetic domains in a perpendicularly magnetized Co/Pt multilayer material with 0.5-μm-diam domains at one micrometer spacing. Images suggest a resolution of <100 nm.

Cubic nonlinearities in small-particle composites: local-field induced giant enhancements

We discuss methods for enhancing the cubic nonlinear susceptibility χ e of a composite material, starting from an exact relation between χ e and the fourth moment of the electric field in the related linear composite. At zero frequencies, in a random metal-insulator composite, χ e can be greatly enhanced near a percolation threshold. Similar enhancement is obtained when a linear fractal is embedded in a nonlinear host. In a random Drude metal-insulator composite χ e is greatly enhanced near surface-plasmon resonances; the enhancement shows very strong structure which is nearly undetectable in the linear response. In a suspension of spheres coated with a nonlinear material, χ e / χ coat ⪢ 1 near the surface-plasmon resonance frequency of the core particle.

Numerical studies of the nonlinear properties of composites.

Using both numerical and analytical techniques, we investigate various ways to enhance the cubic nonlinear susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{e}}$ of a composite material. We start from the exact relation ${\mathrm{\ensuremath{\chi}}}_{\mathit{e}}$ =${\mathit{tsum}}_{\mathit{i}}$${\mathit{p}}_{\mathit{i}}$${\mathrm{\ensuremath{\chi}}}_{\mathit{i}}$〈(E\ensuremath{\cdot}E${)}^{2}$${\mathrm{〉}}_{\mathit{i},\mathrm{l}\mathrm{i}\mathrm{n}}$/ ${\mathit{E}}_{0}^{4}$, where ${\mathrm{\ensuremath{\chi}}}_{\mathit{i}}$ and ${\mathit{p}}_{\mathit{i}}$ are the cubic nonlinear susceptibility and volume fraction of the ith component, ${\mathit{E}}_{0}$ is the applied electric field, and 〈${\mathit{E}}^{4}$${\mathrm{〉}}_{\mathit{i},\mathrm{l}\mathrm{i}\mathrm{n}}$ is the expectation value of the electric field in the ith component, calculated in the linear limit where ${\mathrm{\ensuremath{\chi}}}_{\mathit{i}}$=0. In our numerical work, we represent the composite by a random resistor or impedance network, calculating the electric-field distributions by a generalized transfer-matrix algorithm. Under certain conditions, we find that ${\mathrm{\ensuremath{\chi}}}_{\mathit{e}}$ is greatly enhanced near the percolation threshold. We also find a large enhancement for a linear fractal in a nonlinear host. In a random Drude metal-insulator composite ${\mathrm{\ensuremath{\chi}}}_{\mathit{e}}$ is hugely enhanced especially near frequencies which correspond to the surface-plasmon resonance spectrum of the composite. At zero frequency, the random composite results are reasonably well described by a nonlinear effective-medium approximation. The finite-frequency enhancement shows very strong reproducible structure which is nearly undetectable in the linear response of the composite, and which may possibly be described by a generalized nonlinear effective-medium approximation. The fractal results agree qualitatively with a nonlinear differential effective-medium approximation. Finally, we consider a suspension of coated spheres embedded in a host. If the coating is nonlinear, we show that ${\mathrm{\ensuremath{\chi}}}_{\mathit{e}}$/${\mathrm{\ensuremath{\chi}}}_{\mathrm{coat}}$\ensuremath{\gg}1 near the surface-plasmon resonance frequency of the core particle.

Wavelength tunability of the surface plasmon resonance of nanosize metal colloids in glass

It is demonstrated that the wavelength of the surface plasmon resonance of nanosize metal colloids embedded in silica can be changed by the alloying of metal ions using sequential implantation of different metal elements. Cu and Ag ions were sequentially implanted in silica to doses of 6 and 12×10 16 ions/cm 2. Both Ag- and Cu-rich metal alloy colloids ∼ 20 nm in diameter were formed in the silica matrix. The position of the surface plasmon resonance frequency was shifted to a frequency that depends on the elements implanted and their relative concentrations. These results have implications for the tunability of non-linear optical waveguide devices and switches.

Raman scattering by high-density dispersions. I. Formulation of theory and application to a pair of coated particles

The methods developed in our earlier papers, dealing with Raman scattering from small isolated, spherical particles, uniformly coated with radially uniaxial molecules, are extended to include scattering driven by applied multipolar fields. Such fields are generated by the action of the incident electromagnetic radiation on extraneous material, such as other particles belonging to the same high-density dispersion. In this paper we treat inelastic scattering associated with the Raman dipole matrix elements of the adsorbed molecules ignoring other contributions. Analytical expressions are developed for calculating the primary Raman–Stokes moments given the amplitudes of the applied multipolar fields. As a test of the practicability of the procedure, a full calculation for parallel polarization has been performed for two identical contacting, CO-coated Ag particles oriented with their line of centers parallel to the direction of polarization of the incident radiation. For this case, the effect of interparticle coupling is to broaden and intensify the excitation spectrum, extending the region of intense Raman scattering to much lower frequencies. Intensity enhancements approaching 106 are calculated for frequencies as low as 80% of the single-particle surface-plasmon resonance frequency.

Frequency shifts of molecules at rough metal surfaces.

The frequency shifts for dipolar transitions of molecules at rough metallic surfaces are studied in a phenomenological model following the approach of our previous work [Phys. Rev. 8 36, 4664 (1987)], with the surface roughness modeled by a shallow grating profile in most cases. The main findings here are as follows: (1) Such surface-induced shifts are indeed observable for molecular frequencies away from the surface-plasmon resonance frequency of the metal; (2) the presence of surface roughness can either enhance or suppress the flat-surface-induced shifts, leading to extra morphology-dependent resonances originated from the radiative coupling between the molecular emission and the substrate surface plasmon; and (3) the e6'ects in the perfectly reflecting limit can be worked out analytically with numerical results showing interesting features that are unique for this case. Correlations are made with respect to previous experiments on vibrational shifts as well as to recent calculations from a microscopic approach.

Surface plasmon effects on the optical reflectivity of adsorbed molecular multilayers

We calculate the change in optical reflectivity due to the presence of thin films formed by a few layers adsorbed in a metal using a discrete point-dipole model, and we compare our results with those of the usual Fresnel theory. We find a considerable enhancement of the differential reflectance near the surface-plasmon resonance frequency of the vacuum-metal interface. In the case of p-polarization, the contributions coming from the images of molecules lying closer to the metal can be singled out if the angle of incidence is chosen to be the Brewster angle of the vacuum overlayer interface.

Influence of the Electron Charge Distribution on Surface-Plasmon Dispersion

The effects of the spatial variation of the electron density on the surface-plasmon dispersion relation are investigated. We show that measurements of that relation are a useful probe of the electron density in the surface region. Previous calculations on homogeneous materials have predicted a linear or quadratic dependence of the frequency on momentum parallel to the surface. We find that the usual surface-plasmon resonance frequency at first decreases with increasing momentum and then increases with further increases in momentum. This behavior agrees with the experimentally observed dispersion. Additional higher-frequency surface modes, similar to those observed in laboratory plasmas, are identified.

Some Surface Effects in an Electron Gas

The effects of various external probes on a gas of interacting electrons confined to a foil are examined in an approximation for the response functions which reduces to the random-phase approximation for a bulk sample. Some band effects are included in a phenomenological manner. Surface corrections to the bulk properties are found which cannot be obtained in a wave-number-independent treatment nor in the Born approximation. The momentum dependence of the surface plasmon's resonant frequency is found and the response of the system to fast incident electrons and electromagnetic radiation is calculated.