梯度颗粒复合介质的光学双稳

重点研究了组分的梯度构形对带壳球形颗粒复合介质的光学双稳特性的影响.其中球形颗粒是由非线性核和介电函数具有梯度分布的线性壳组成.对于壳层介电函数具有幂指数分布的情况,通过求解麦克斯韦方程,得到各区域的势能分布函数,从而求得核内电场的数学表达式.数值研究发现,该复合介质的光学双稳阈值和区域与壳层的厚度及壳层的介电幂指数有关,随着壳层厚度增大或幂指数增大,双稳阈值将变宽.此外,还研究了正入射情况下复合材料体系的反射系数随外电场的变化情况, 发现其关系曲线是一条回线. 关键词： 梯度颗粒 复合介质 光学双稳     

弱非线性复合体中的高阶非线性响应

利用谱表示理论和微扰展开法，从理论上给出了适合于一般微结构复合体系的有效非线性响 应的一般表示式，并结合有效媒质近似（EMA），在弱非线性条件下研究了由三阶非线性组 分（体积分数为p）和线性组分构成的非线性复合体系的有效非线性响应，讨论了复合 体系的有效介电常数ε～e=εe+χe|E0|2+ηe|E0| 4中的有效三次非线性响应χe和有效高次非线性响应ηe与体积分数p和 退极化因子L之间的关系，分析了非线性组分的介电常数为复数情形时体系的有效高阶非线 性响应，从理论上说明了组分的高次非线性响应对整个复合体系的有效介电常数的影响.  关键词： 非线性复合介质 有效非线性响应 谱表示理论     

具有各向异性的梯度颗粒复合材料的有效介电响应

本文研究了具有梯度壳层椭球颗粒复合体系的有效介电响应，在稀释条件下用准静态近似方法, 推导了颗粒壳层具有任意介电梯度形式的椭球颗粒复合体系的有效介电常数和部分共振条件的一般表达式。并以壳层的介电常数为主轴的幂函数形式为例，得出了通过调节壳层的介电梯度形式、颗粒的结构和形状,可以提高该体系的有效介电常数和实现部分共振的结论。     

光学复合体系中有效介质理论的适用条件

以纳米晶掺杂复合体系为例，研究该类多组分光学材料的有效介电系数随复合体系结构和组分介电系数的变化，进而分析有效介质理论的适用条件。假定纳米晶被周期性掺杂到背景晶体中，且均为各向同性材料。借助转移矩阵理论研究发现，当纳米晶和背景晶体的介电系数相差较大时，有效介电系数会表现出波长依赖性。此外，发现复合体系在其光学能隙处没有有效介电系数，说明此时基于等效介质思想的有效介质理论均不适合描述复合体系的光学性质，这是因为在物理上均一的光学材料无法形成光学能隙。     

半导体/绝缘体复合材料的介电特性和光谱特性

研究了半导体/绝缘体复合材料的介电特性和光学吸收特性随外加场频率和半导体颗粒形状的变化规律,利用有效介质近似理论(EMA)进行数值模拟,当外加场的频率发生改变时,复合材料的介电特性有一个峰值存在,随半导体颗粒体积分数的增大,峰值也随之增强;而复合材料的光谱特性随频率的变化有两个峰值存在,随着半导体纳米颗粒的体积分数及形状的改变,相应的峰值也发生改变。     

金属/电介质颗粒复合介质的非线性交流响应

推广T矩阵法来研究二组分颗粒复合介质的非线性交流响应，解析推导出基频、三次及五次谐波频率下的有效线性介电常数及三阶和五阶非线性极化率的解析表达式.当组分颗粒的介电常数为实数时，表达式同以前微扰理论完全一致.进一步数值计算了金属-电介质复合介质的有效非线性交流响应，数值结果表明，体系的三阶、五阶非线性极化率在表面等离子共振频率附近有明显增强，而且，随着体积分数提高，共振峰增强且伴随着共振频率红移现象.还进一步讨论了维度效应对体系非线性交流响应的影响. 关键词： T矩阵方法 复合介质 非线性交流响应     

带壳颗粒复合介质光学双稳的温度效应

研究非稀释极限下带壳的球形颗粒复合介质体系光学双稳的温度效应.其中带壳颗粒是由具有线性响应的核和非线性响应的壳层组成.由于复合体系中的空间局域场是不均匀的,所以应用自洽平均场理论,解析推导核内电场平方的空间平均值与外电场的关系表达式,并对不同温度及不同结构参数下的光学双稳进行了数值计算.结果表明,双稳与温度T、结构参数k和体积分数f有关.在k和f一定时,随温度的升高,双稳效应因上阈值减小、下阈值增大而逐渐减弱,直至双稳消失;在T和f一定时,随结构参数的增大,上、下阈值都增大,而上阈值的增速较大从而导致双稳区域宽度加大. 关键词： 带壳颗粒 复合介质 光学双稳 温度效应     

颗粒复合介质在高温下的光学双稳特性

利用Maxwell-Garnett近似，结合谱表示方法，理论研究了金属/绝缘颗粒复合介质的光学双稳特性随环境温度的依赖关系.数值结果表明，体系的光学双稳区域随环境温度的升高而逐渐变窄，甚至消失.还进一步研究了复合体系的折射系数和消光系数随温度的变化关系. 关键词： 颗粒复合介质 光学双稳     

用椭偏术研究与接触介质有关的贵金属的介电函数

为了研究接触介质对金属光学性质的影响，实验中使用具有不同折射率的梯形棱镜作为衬底，将金和银蒸发到棱镜底部，用椭偏术分别测量了薄膜在金属-空气、金属-衬底界面的介电函数.结果表明：无论在Drude区，还是在带间跃迁区，金属-衬底界面处薄膜介电函数不仅与金属-空气界面处的测量值不同，而且随衬底折射率改变而改变.在固体接触条件下获得的结果与其他作者在液体接触条件下获得的结果相一致，但尚难被现有机制所解释. 关键词：     

梯度分布球颗粒复合介质的介电特性和光吸收特性

研究球形颗粒复合介质沿颗粒半径方向有梯度分布的有效介电响应和光吸收性质,利用微分有效偶极距近似方法计算有梯度分布球颗粒的等效介电常数.数值计算结果表明,对于一般的球形粒子,介电常数随半径按幂指数规律变化时,系统的有效介电常数随指数n的增大而减小;而对金属球形粒子,则存在等离子体共振吸收峰,且随参数mw和mv的增大,其位置将发生红移.这为复合材料的制备提供了一种新的理论依据.     

Double-resonance plasmon-driven enhancement of nonlinear optical response in a metamaterial with coated nanoparticles

By means of a simple analytical model, we show the possibility of giant double-resonance enhancement of nonlinear cubic optical response of metamaterial containing layered (coated) nanoparticles with nonlinear dielectric core covered by metallic shell. Such nanoparticles support two surface plasmons of dipole type with different eigenfrequencies depending on volume portion of nonlinear dielectric. We demonstrate that giant enhancement of nonlinearity takes place under condition of double resonance when the fundamental frequency of light wave and its third harmonic simultaneously coincide or close to the frequencies of surface plasmons.     

Surface plasmon polariton at the interface of dielectric and graphene medium using Kerr effect

We theoretically investigate the control of surface plasmon polariton(SPP) generated at the interface of dielectric and graphene medium under Kerr nonlinearity. The controlled Kerr nonlinear signal of probe light beam in a dielectric medium is used to generate SPPs at the interface of dielectric and graphene medium. The positive, negative absorption, and dispersion properties of SPPs are modified and controlled by the control and Kerr fields. A large amplification(negative absorption) is noted for SPPs under the Kerr nonlinearity. The normal/anomalous slope of dispersion and propagation length of SPPs is modified and controlled with Kerr nonlinearity. This leads to significant variation in slow and fast SPP propagation. The controlled slow and fast SPP propagation may predict significant applications in nano-photonics, optical tweezers, photovoltaic devices, plasmonster, and sensing technology.     

Quasi-Rayleigh light scattering by internal elements of a metamaterial in a light-carrying core of a single-mode optical fiber based on the Veselago effect

At present, single-mode optical fibers composed of metamaterials—so-called “left-handed” optical media—for the far- and mid-IR ranges have already been created. In the near future, left-handed singlemode optical fibers for the visible and near-IR ranges will be created, light-carrying cores of which will be composed by an ordered structure of dielectric elements, the dimensions of which will be much smaller than the light wavelength, while the effective refractive index of the structure will be negative; i.e., the structure will possess the so-called “Veselago effect.” We show that, because the dimensions of these dielectric elements many times exceed the dimensions of molecules of optical media, the elements should strongly scatter light, with this scattering considerably exceeding the Rayleigh (molecular) light scattering that occurs in conventional quartz single-mode optical fibers. We propose to term this phenomenon the quasi-Rayleigh light scattering. Numerical estimates of the quasi-Rayleigh light scattering for left-handed single-mode optical fibers at a light wavelength of λ = 1.55 μm have been made.     

Investigation of light reflection at Brewster angles from a nonlinear optical film on a magnetoelectric substrate

We investigate the light reflection from a nonlinear optical dielectric film on a bigyrotropic magneto-electric substrate magnetized parallel to the interface. The reflection matrix has been obtained and reflection coefficients are investigated with respect to the incidence angle for both polarizations of the incident light. We analyzed the optical nonlinearity and the magnetoelectric coupling contributions to the reflection of the electromagnetic waves and the appearance of so-called pseudo-Brewster angles.     

Nonlinear susceptibility of a metal-dielectric nanocomposite caused by induced dipole-dipole forces between particles

The optical coefficients of a nonlinearity for a macroscopic ensemble of aggregates of metal nanoparticles that is caused by deformation of the spatial structure of clusters in a strong light field are calculated for the first time. For a continuous medium that consists of a nonabsorbing dielectric containing aggregated silver nanoparticles that do not possess an intrinsic optical nonlinearity, coefficients of nonlinear absorption are obtained in relation to the intensity and frequency of incident light. It is shown that, at intensities of up to a few megawatts per square centimeter, the addition to the absorption of a nanocomposite to be modeled is adequately described by a third-order nonlinearity. The magnitude and sign of the nonlinear absorption coefficient agree well with the previously obtained experimental data for aggregated silver and gold colloids under nanosecond excitation.     

Giant Enhancement and Sign Inversion of Optical Kerr Nonlinearity in Random High Index Nanocomposites near Mie Resonances

High-index dielectric nanoantennas excited at Mie-type resonances have exhibited enormous enhancement of optical nonlinearity. Such nanostructures have been actively studied by researchers in recent years. Here, a numerical analysis study of the optical Kerr effect of nanocomposites consisting of high refractive index (GaP) spheres at the wavelength of 532 nm is presented. This is done by means of 3D finite-difference time-domain simulations. The effective nonlinear refractive index of 0.8 µm thick nanocomposites and metasurfaces is evaluated. It is shown that the optical Kerr nonlinearity of the nanocomposites rises by orders in proximity to Mie resonances and may exceed the second-order refractive index of the bulk material. It is revealed that the sign of the effective optical Kerr coefficient is inverted near the Mie resonances. This effect may be of interest in developing nonlinear optical metadevices.     

A facile high‐performance SERS substrate based on broadband near‐perfect optical absorption

Plasmonic systems based on metal nanoparticles on a metal film with high optical absorption have generated great interests for surface‐enhanced Raman scattering (SERS). In this study, we prepare a broadband‐visible light absorber consisting Au nanotriangles on the surface of a continuous optically opaque gold film separated with a dielectric SiO₂ layer, which is a typical metal‐insulator‐metal (MIM) system, and demonstrate it as an efficient SERS substrate. The MIM nanostructure, prepared using nanosphere lithography with a very large area, shows a broadband with absorption exceeding 90% in the wavelength regime of 630–920 nm. We observe an average SERS enhancement factor (EF) as large as 4.9 × 10⁶ with a 22‐fold increase compared to a single layer of Au nanotriangles directly on a quartz substrate. A maximum SERS EF can be achieved by optimizing the thicknesses of the dielectric layer to control the optical absorption. Owing to the simple, productive, and inexpensive fabrication technique, our MIM nanostructure could be a potential candidate for SERS applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Relativistic effects of light in moving media with extremely low group velocity

A moving dielectric medium acts as an effective gravitational field on light. One can use media with extremely low group velocities [Lene Vestergaard Hau et al., Nature (London) 397, 594 (1999)] to create dielectric analogs of astronomical effects on Earth. In particular, a vortex flow imprints a long-ranging topological effect on incident light and can behave like an optical black hole.     

Self focusing of laser beams in a degenerate nonparabolic semiconductor: Kinetic approach

Using rigorous kinetic treatment the steady state self-focusing of laser beams in a degenerate nonparabolic semiconductor (e.g. n-InSb) has been investigated beyond the perturbation limit. The nonlinearity arises due to the energy dependence of the electron mass in the conduction band. The energy loss of electron energy is assumed to be due to polar optical phonons and momentum transfer due to ionised impurity scattering/polar optical phonon scattering. It is found that nonlinearity in the dielectric constant or the self-focusing is more pronounced in the case of ionised impurity case as compared to polar optical phonon case. The effect of absorption is to suppress the self-focusing but the latter is also enhanced by degeneracy.