Coupling coefficients of gratings with graded refractive index waveguides

In this paper, the effective coupling coefficient κeff and the self-coupling coefficient ζ1 are introduced to describe the characteristic of gratings in a resonant situation when the effects of radiation and other partial waves coupling are considered. The dependence of these two coupling coefficients on grating tooth shapes and depths and the dimensions of graded refractive index (GRIN) waveguides is numerically analysed. The results show that the gratings with linear GRIN waveguides have the largest |κeff|. The possibility of realizing a complex-coupled DFB laser, even a pure gain or loss coupled DFB laser, employing only a real refractive index coupled grating is also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Optical properties of aperiodic thin-layer structures: Effective refractive index

Simple analytical expressions for the effective refractive index of an aperiodic thin-layer structure have been obtained and analyzed. The expressions correctly describe the optical properties of the structure in a wide range of wavelengths. The optical properties that are general for the structures of this class have been established. The validity of the results is shown by a numerical experiment.     

Low-loss amorphous Si waveguides with gradient refractive index cladding structure

Amorphous Si waveguides with gradient refractive index cladding structure are proposed and fabricated using plasma-enhanced chemical vapor deposition method.Compared with 6 dB/cm for ridge waveguide without gradient cladding,the propagation loss of the gradient cladding waveguides is less than 1 dB/cm with both TE and TM polarizations.     

Low refractive index contrast porous silicon omnidirectional reflectors

We report on the fabrication and characterization of a porous silicon omnidirectional reflector formed by periodic substructures stacked together. For these substructures, a low refractive index contrast has been used, resulting in substructures without omnidirectional reflectivity band. The use of a low refractive index contrast involves the reduction of the requirements to obtain omnidirectional reflectors. We demonstrate the existence of an omnidirectional reflectivity range with a high gap-to-midgap ratio by means of reflectivity spectra measurements for a range of incidence angles. The results are in good agreement with a theoretical model of the reflector. The fabricated structure is the first reported porous silicon reflector suitable for 1.55 μm applications.     

Ellipsometric study of refractive index anisotropy in porous silicon

Porous Si layers of different thicknesses were prepared by anodising p⁺-type Si substrates with a resistivity of 0.01 Ω cm. The porosity of the samples ranged from 23% to 62%. The refractive index values for the ordinary and extraordinary rays were determined by multiple angle of incidence ellipsometry, from which an optical anisotropy parameter varying from 13% to 20% was obtained. The porous Si layers were modelled as uniaxially anisotropic films on an isotropic substrate, with an optical axis perpendicular to the sample surface. The morphological anisotropy which is typical for the p⁺-type porous Si with a predominating cylindrical geometry is responsible for these optical properties. All the porous Si layers studied were found to be optically negative.     

Sensor properties of structures with porous silicon layer

We have measured at room temperature current-voltage and noise characteristics of structures with a porous silicon (porosity 80%) layer at adsorption of gases ammonia, propane and butane mixture, and ethyl alcohol vapor. It was obtained that the largest change in CVC and low-frequency noise is observed under action of ammonia gas on the structure. Physical reasons of sensor properties of studied samples are discussed.     

变波长逆WKB方法确定单模渐变平面光波导的折射率轮廓

将渐变波导模式方程(WKB积分方程)化为分段积分,以波导某一模式在不同波长下的转折点为分段点,当波长相差很小时,相应的转折点相差也很小,可在各个分段积分中作折线近似,从而从理论上推出确定波导轮廓数据的递推式.以所得轮廓必须满足光滑条件为判据,最后定出波导的轮廓.该方法尤其适用于单模渐变波导,而且无需事先假设待定轮廓的函数形式.本文对双曲止割和抛物线轮廓的理想波导进行了计算机模拟,结果证明该方法的精度达到10~(-3)甚至于更高.而且理论上具有分割愈密,精度愈高的优点.     

Light amplification and attenuation in stratified structures with a complex refractive index

We report a new kind of laser-threshold resonance in electromagnetic wave propagation inside a layered medium. Specifically, for large values of the gain coefficient, the wave interaction with only the first few lattice interfaces results in transmission and reflection values of much greater than unity. We also show that the duality phenomenon between losses and gains on the field amplitude decay is due to the existence of counterpropagating waves on reflection from the layer interfaces.     

Light amplification and attenuation in stratified structures with a complex refractive index

Abstract

We report a new kind of laser-threshold resonance in electromagnetic wave propagation inside a layered medium. Specifically, for large values of the gain coefficient, the wave interaction with only the first few lattice interfaces results in transmission and reflection values of much greater than unity. We also show that the duality phenomenon between losses and gains on the field amplitude decay is due to the existence of counterpropagating waves on reflection from the layer interfaces.     

Low-dispersion characteristics of silica-based graded refractive index lens for laser display optical system

We report the characteristics of silica-based graded refractive index lens fabricated by sol-gel method. Lens diameter is precisely controlled to be 125 ??m ± 0:5 ??m. Chromatic dispersion is defined as the difference of refractive index distribution coefficient, and concluded to be sufficiently low against ordinary achromatic lenses. We also calculated the difference of focal distance among RGB wavelengths, the difference was from 25 to 35 ??m almost regardless of the lens length. Divergence angle dispersion was suppressed less than 10 per cent, demonstrating excellent achromaticity for raster-scanning laser display devices.     

含双负缺陷的一维光子晶体耦合腔的杂质带特性

利用转移矩阵方法研究了含双负缺陷的一维光子晶体耦合腔的透射谱.计算结果表明，如果改变缺陷的折射率，缺陷模之间的耦合作用将发生改变，使得能隙中的杂质带也随之改变.若这个折射率取适当值，则可以在禁带中同时出现几个尖锐的透射峰和较宽的通带，这种结构可同时用于多通道窄带滤波和宽带滤波. 关键词： 光子晶体 缺陷模 杂质带 滤波     

Imaging properties of uniaxially anisotropic negative refractive index materials

The imaging properties of uniaxially anisotropic negative refractive index materials have been investigated theoretically. Based on an accurate theoretical formulation, we have discussed in detail whether and under what conditions the slabs of uniaxially anisotropic negative refractive index materials will have the power to focus perfectly all Fourier components of an image. It was shown that if the effects of absorption and dispersion can be neglected, under some particular conditions the slabs of uniaxially anisotropic negative refractive index materials will have the power to focus perfectly all E-mode or all H-mode or both E- and H-mode Fourier components of an image. It was also shown that perfect focus cannot occur in the presence of absorption and dispersion, but if the effects of absorption and dispersion are rather small, excellent focus can still occur.     

冲击压缩下Z-切石英的弹性响应特性和折射率

利用平面对称碰撞和“原位”测量实验方法,对Z-切石英的弹性响应特性和窗口光学特性进行了研究.结果表明,在至少10 GPa内,Z-切石英为弹性单波响应,且透光性良好,可作为速度剖面测试系统的光学窗口.10 GPa内Z-切石英的冲击波速度和粒子速度之间满足线性关系.1550 nm入射光条件下,其窗口速度修正采用指数形式较为精确,线性修正仅适用于特定范围;而折射率随密度的变化关系则可采用线性函数进行较好描述.以上结果为Z-切石英应用于基于1550 nm光源激光干涉测速系统的光学窗口提供了依据. 关键词： Z-切石英')" href="#">Z-切石英 速度修正 折射率 弹性响应     

Determination of the effective refractive index of porous silicon/polymer composite films

The equation for calculating the effective refractive index of porous silicon inserted polymer was obtained by three-component Bruggeman effective medium model. The dependence of the effective refractive index of porous silicon/polymer composite films on the polymer fraction with various initial porosity was given theorically and experimentally respectively. The porous silicon and polymer polymethylmetacrylate based dispersive red one (PMMA/DR1) composite films were fabricated in our experiments. It is found that the measured effective refractive index of porous silicon inserted polymer was slightly lower than the calculated result because of the oxidization of porous silicon. The effective refractive index of oxidized porous silicon inserted polymer also was analyzed by four-component medium system.     

具有缓变折射率的太赫兹宽带增透器件

用高阻硅制作的光学元件是太赫兹系统里常用的器件, 但是其高达3.42的相对折射率所引起的阻抗失配严重影响了太赫兹系统的功率, 因此研究人员尝试了各种各样的方式在高阻硅表面镀上有效的增透膜. 在太赫兹波段, 缺乏合适的材料是增透研究中亟需解决的一个重要问题. 介绍一种结构新颖的硅材料增透器件----- 三维光子倒置光栅. 与普通高阻硅片相比, 当结构周期为15 μm时, 该器件在0.2---7.3 THz范围内对太赫兹波具有明显的增透作用, 且覆盖了大部分太赫兹波段. 此外, 该器件的使用不受太赫兹偏振方向限制, 适用于大入射角情形, 并具有高达116.3%的相对3 dB带宽.     

Antireflective properties of disordered Si SWSs with hydrophobic surface by thermally dewetted Pt nanomask patterns for Si-based solar cells

We have theoretically and experimentally investigated the antireflective properties of the disordered subwavelength structures (SWSs) with a hydrophobic surface on silicon (Si) substrates by an inductively coupled plasma (ICP) etching in SiCl₄/Ar plasma using thermally dewetted platinum (Pt) nanopatterns as etch masks for Si-based solar cells. The Pt thin films on the SiO₂/Si surface were properly changed into the optimized dot-like nanopatterns via the thermal dewetting by rapid thermal annealing process. The antireflection properties were definitely affected by the etched profile of SWSs which can be controlled by the conditions of etching process. For the tapered Si SWS with a high average height of 724 ± 78 nm, the reflectance was significantly reduced below 5% over a wide wavelength range of 350-1030 nm, leading to a relatively low solar weighted reflectance of 2.6%. The structure exhibited reflectances less than 14.8% at wide incident angles of 8-70°. The hydrophobic surface with a water contact angle of 113.2° was obtained. For Si SWSs, the antireflective properties were also analyzed by the rigorous coupled-wave analysis simulation. These calculated results showed similar behavior to the experimental results.