Optical absorption in ultrafine gold particles

Optical absorption of ultrafine, gas evaporated gold particles (diameters 3—4 nm) has been measured in the wavelength interval 0.3—2.5 μm. The data agree well with the Maxwell-Garnett theory in conjunction with optical constants for bulk Au, provided these are modified to incorporate the effect of size dependent electron scattering.     

Optical characteristics of a cluster of closely-packed dielectric spheres

We employ the pseudospectral time-domain technique to simulate light scattering by a cluster of mono-disperse dielectric spheres. The total scattering cross-section (TSCS) spectrum of the cluster is obtained and analyzed. Research findings show that the TSCS spectrum exhibits characteristics of the cluster geometry as a whole, as well as characteristics of the constituent dielectric spheres. Furthermore, an optical signature indicative of the constituent sphere size is identified, suggesting the possibility to obtain microscopic geometrical information of a closely-packed geometry from multiply scattered light.     

Observation of surface mode absorption in oxide coated metal spheres

The infrared absorption coefficient of 5 μm zinc spheres coated with ZnO of variable thickness is reported and compared with theory. The peak absorption by surface phonons in the oxide shell shifts from the long wavelength longitudinal optical mode frequency for thin oxide shells to the Fröhlich frequency for thick oxide shells.     

Optical bistability of planar metal/dielectric nonlinear nanostructures

We study theoretically a nonlinear response of the planar metal/dielectric nanostructures constituted from periodical array of ultra thin silver layers and the layers of Kerr-like nonlinear dielectric. We predict hysteresis-type dependences of the components of the tensor of effective dielectric permittivity on the field intensity allowing the change in material transmission properties from transparent to opaque and back at extremely low intensities of the light. It makes possible to control the light by light in all-optical nanoscale devices and circuits.     

Optical absorption of lithium metal vapor at high temperatures

Experimental results are reported for a unique spectroscopic device called the Plasma Spectroscopy Cell. Optical absorption of lithium metal vapor was observed at high density and temperature. Absorption spectra are analyzed using theoretical calculations of absorption cross sections for lithium-helium interactions, and singlet and triplet state transitions of diatomic lithium in the visible spectral range. This is believed to be the most complex example yet calculated in which absolute bound-bound, bound-free, free-bound, and free-free contributions for all possible optically allowed transitions are all included, in quite respectable agreement with experiment.     

Optical transmission through metal/dielectric multilayer films perforated with periodic subwavelength slits

The transmission of p-polarized plane wave through Ag/SiO₂ multilayer films perforated with periodic subwavelength air slits is investigated by using the finite-difference time-domain (FDTD) method. The results show that the optical transmission property is mediated by the interference among the propagating coupled-SPP modes along the lateral direction inside the SiO₂ layers and the conditions of Fabry-Pérot-like resonance along the longitudinal direction together. When some geometric parameters are suitably initialized, the high transmission peaks can split into more peaks as the functional layer (metal/dielectric/metal sandwich stack) number increases, and the wavelength of the same-order transmission peak exhibits a red shift as the grating period increases.     

Morphology-dependent resonances in dielectric spheres with many tiny inclusions

The morphology-dependent resonances (MDRs) in a dielectric sphere that contains many tiny inclusions are studied by use of a recently developed degenerate perturbation method. Degenerate MDRs in the sphere split into multiplets because of the loss of spherical symmetry and manifest themselves as broadened spectral lines in the scattering cross section. Furthermore, the distribution of MDRs in a multiplet is found to obey Wigner's semicircular theorem.     

Characterization of dielectric spheres by spiral imaging

We study the spiral spectra scattered off transparent dielectric spheres when probed by different Laguerre-Gaussian light beams, carrying nested topological wavefront dislocations. We show that such scattering data may be employed to determine geometrical properties of the spheres, such as their position. The technique is a generalization of standard Mie scattering, and it can be extended to study and to characterize nanospheres.     

Optical activity of dielectric superlattices with defects

Natural optical activity of a nonideal 1D multilayer system is considered phenomenologically and the concentration dependence of its specific rotation angle is simulated numerically. As a model system, the two-sublattice SiO₂ LC structure was chosen. Specific gyrotropy features caused by the corresponding disordering types of the studied systems were revealed. Based on the developed phenomenological theory, the frequency dependence of the specific rotation angle of the polarization plane of linearly polarized light was studied for the case of a molecular-crystal multilayer system whose layers contain pointlike defects. This creates additional possibilities for simulating optically active multilayer composite materials.     

Optical reflection from dielectric layers containing metal particles formed by ion implantation

Dielectric layers with silver nanoparticles, which are synthesized in a soda-silicate glass by implantation of 60-keV ions with a dose of 7.0×10¹⁶ Ag⁺/cm² at an ion current density of 10 μmA/cm², are analyzed. The depth of silver distribution was measured by Rutherford backscattering. Data on optical characteristics of composite layers were obtained from the transmission spectra and from the reflection, which were measured both from the side of an implanted glass surface and from the unimplanted side. To calculate reflection spectra, a multilayer plane-parallel film structure was considered, which was modeled on the basis of the matrix method using complex Fresnel coefficients. Dielectric functions of separate layers were determined using the Maxwell-Garnet theory of an effective medium. A qualitative agreement between the experimental and the model optical spectra was obtained taking into account a nonuniform depth distribution of metal nanoparticles in a composite material.     

The role of electron-hole pair excitations in the optical absorption of metals,particularly metal spheres

It is shown that electron-hole pair excitations can dominate the optical absorption of a small metal sphere over the entire frequency range up to the bulk plasma frequency. The limited size has the effect of giving more surface to volume, thereby giving a larger relative weight to the single particle excitations, predominantly produced in the surface region, as compared to the planar case where they make up for a smaller part of the total absorptance.     

Log-normal size distributions of ultrafine metal particles

We have produced particles with median diameters well below 5 nm of Al, Fe, Co and Sn, using inert gas evaporation. The logarithm of the particle volume has a Gaussian distribution which can be explained by a theoretical model for coalescence of liquid particles. The standard deviations of these log-normal distributions, as inferred from electron microscopy, always lie close to a common value regardless of the kind of metal or method of evaporation.     

Non compact single-layers of dielectric spheres electromagnetc behaviour

Single layer of dielectric spheres is a recognized model for the basic understanding of some aspects of photonic crystals. Here we present a systematic study of the effect of compacting in the electromagnetic transmission of dielectric spheres monolayers. Experiments were performed in the microwave domain (from 10 GHz to 30 GHz) with glass spheres of high dielectric permittivity ε = 7. Time Domain Finite Integration (TDFI) calculations were also accomplished. Experimental data and TDFI calculations agreement provides a double check on the lack of experimental artefacts and the correctness of simulation settings. Following the evolution of the lower frequency spectral peak with layer compacting ratio, we established three different electromagnetic regimes. For the higher and lower compacting ratio regimes, the peak frequency matches isolated sphere pure resonances, while for intermediate values of compacting, some transition between these two modes takes place. Extending the study to the complete frequency range, we find that sphere single layers transmission spectra become closer to isolated sphere scattering calculations as the compacting ratio is decreased. However as the agreement remains imperfect even for our lowest compacting measurable layer, we conclude that some structure contribution cannot be neglected even for low compact layers.     

Plasmon frequencies of small metal spheres

The frequencies of the plasmons of small metal spheres are calculated using hydrodynamic theory. The effects of spatial dispersion and surface diffuseness, which are usually neglected, are included in the derivation. It is found that each of these effects by itself can cause a significant shift of the surface plasmon frequencies. The shifts are, however, in opposite directions, so that the combined influence of both effects is relatively small.     

Effective dielectric constant of dilute polydisperse suspensions of spheres

We study the effective dielectric constant of a dilute, polydisperse suspension of spheres embedded in a uniform background. We consider a frequency region where the dipole polarizability of individual spheres exhibits a resonance. We evaluate the effective dielectric constant to second order in the volume fraction employing the dipole approximation, which in previous work has been shown to be applicable in resonance. We apply our results to suspensions of spheres with identical and uniform dielectric constant, assuming a log-normal distribution of sphere radii.     

Optical properties of a suspension of hard spheres

The optical properties of suspensions are studied in a wide range of concentrations. An expression for the polarization operator is obtained taking into account the contributions of two-and three-particle correlations. The extinction length l and the transport length l* are calculated in terms of a model of hard spheres. A detailed comparison of the results of calculations with experimental data is performed. In calculations, the structure factor is determined in the Percus-Yevick approximation, while the form factor is taken into account in the Rayleigh-Gans approximation and in terms of the Mie theory. It is shown that taking into account the contribution of three-particle correlations improves the agreement of the theory with experiment. It is found that, in the range of high suspension concentrations, the optical parameters are more sensitive to the choice of the model for the structure factor than for the form factor.