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.     

周期排列的电介质小球所诱发的金属-电介质表面上的表面等离子激元的光学性质

研究了金属板的上下表面附近各放置一层按周期排列的电介质小球的体系的光学性质.用多重散射法计算的结果显示金属上侧的周期性排列的电介质小球可诱发金属-电介质表面上的表面等离子激元.这些表面等离子激元的存在可通过非常尖锐的吸收峰反映出来.对于无限厚的金属板，这些吸收峰的峰值位置主要与电介质小球的周期有关，且与解析理论符合得相当好.在有限厚度的金属板中，金属板的两侧表面会产生对称和反对称的两种表面等离子激元，从而使原来在无限厚的金属表面上所出现的单一频率的表面等离子激元劈裂为双频率.由于对称和反对称的表面等离子激元模式在金属板的两侧表面均有相当强的电磁场，因而它们可导致强的电磁波穿透.通过在金属板的下侧加入玻璃球层可将表面等离子激元的电磁场引导出金属，并产生透射波.用多重散射法计算的结果证实，在此体系中由表面等离子激元所引起的透射可达到相当的强度. 对该体系中的物理机理进行了详细分析，从而能够通过调节该体系中的一些参数来控制表面等离子激元出现的频率，使强吸收峰或强透射峰出现在所希望的频率上. 关键词： 表面等离子激元 吸收谱 透射     

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 spectra of some transition metal thiophosphates

Near-infrared and visible absorption spectra at room temperature of MnPS₃, CoPS₃, CoPS₃ are reported. These materials show weak structures below their fundamental absorption thresholds, due to the 3d-3d transitions occurring on the transition metal ion. By comparison with other transition metal compounds, such excitations are interpreted on the basis of the ligand field theory. The appearance of well developed crystal field spectra is a further confirmation that these materials are considerably ionic.     

Optical absorption spectra of adsorbates on metal surfaces

Optical absorption spectra of adsorbates on metal surfaces were calculated on the basis of the Newns-Anderson model supplemented with screening due to the surface plasmons. An inspection of the overall line shape calculated by the simplest second-order perturbation with respect to both the charge transfer and the adsorbate-surface-plasmon interactions prompted us to investigate the local structures of the excitations near the singly and doubly occupied adsorbate levels and near the Fermi level. Separately, analytic features of the absorption threshold were investigated in detail. With the aid of the image screening effect, the possibility occurred that the doubly occupied excited adsorbate state becomes the lowest final state. Furthermore, the exchange effect on the absorption threshold of the singly occupied adsorbate state was also studied.     

嵌入电介质小球的金属薄片的电磁波透射特性

研究了金属板上下两个表面内侧都周期性地嵌入电介质小球体系的透射和吸收性质.结果显示：对于嵌入深度很小的电介质小球,电磁波可通过隧穿效应进入到电介质小球内,并以腔体本征模式的形式存在.当嵌入深度很小时,周期排列的电介质小球会对金属表面做有效的周期性调制,使体系在特定的频率出现金属表面等离子激元.当腔体本征模式与金属表面等离子激元模式的频率相近时,它们之间的耦合将使两种电磁模式大幅度地增强,从而使上层的电介质小球内具有非常强的电磁场.这些高强度的电磁场有相当一部分可通过隧穿效应进入到下层的电介质小球内,并通过 关键词： 腔体模式 表面等离子激元 透射     

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.     

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.     

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.     

Raman scattering enhancement by dielectric spheres

Raman scattering experiments were performed on Si(60 nm)/metal/substrate structures with and without silica microspheres (with a diameter between 0.5 and 5 µm) on top. Raman scattering from the thin Si layer exhibits enhancements (~20) due to the dielectric spheres, where the enhancement factors depend on the diameter of the spheres. The interaction between light and dielectric spheres has been simulated by finite difference time domain calculations (FDTD), wherein particularly the electric energy density (ED) distribution in the thin Si layer was of concern. For microspheres with a diameter less than ~3 µm, the transverse ED distribution (perpendicular to the incident light direction) within the Si layer is characterised by a single peak centered on the optical axis. For larger diameters, a multimodal transverse ED distribution develops where the maximum is not centered on the optical axis. Using an ad‐hoc approach for surface enhanced Raman scattering in combination with the FDTD calculations, the experimental Raman observations are well accounted for. Copyright © 2013 John Wiley & Sons, Ltd.     

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 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.     

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.     

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.     

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.