一维光子晶体带隙结构研究

在考虑介质色散的基础上,研究了介质层厚度对光子晶体带隙结构的影响.利用传输矩阵法,计算了以LiF和Si两种材料组成的一维光子晶体带隙结构.结果表明,介质层厚度的增加会引起禁带的红移,厚度减小会引起蓝移.分析了含空气缺陷层、金属缺陷层的光子晶体结构,发现空气缺陷层对带隙结构的高反射区域变化不大,而在低反射区域,反射系数为零的波带之间出现了两边反射系数增加,中间反射系数减小的情况.在金属缺陷层的带隙结构中,金属对整个波长范围光的吸收作用不同,金属对低反射区1.6 μm、1.85 μm处透射率较大的透射光吸收作用明显,而在1.28~1.38 μm处透射率波长区间,几乎无吸收.     

异质三周期光子晶体的光子带隙特性

利用传输矩阵法研究了由TiO₂和SiO₂两种材料构成的异质三周期一维光子晶体的传光特性,发现该结构能在波长为200~ 2 000 nm的范围内形成7处比较明显的光子带隙,并且波长越大,带隙的宽度越大。重点研究了这种结构的光子晶体的透射谱线与入射角度、介质层数及介质层厚度的关系,发现该结构形成的光子带隙的大小和位置对介质层的循环周期数不敏感,但对入射角度和介质层的厚度很敏感。     

含左手材料的一维三元准周期光子晶体带隙特性研究

选用LiF、Si和左手材料为介质构成了一维三元光子晶体,并在考虑其色散关系和几何厚度随机误差的基础上,利用传输矩阵法,计算了该光子晶体的带隙结构.结果表明:各介质厚度的变化率相同时,主带隙的相对宽度的变化率也相同,而折射率最大的介质厚度的变化对带隙绝对宽度的影响最大.光线入射角度的变化对主带隙的绝对宽度和相对宽度的影响较小.     

一维缺陷光子晶体温度的测量

采用Si和SiO₂两种介质材料构造一维缺陷光子晶体，缺陷层介质为Si，利用传输矩阵法对带有缺陷的一维光子晶体的传光特性进行了理论分析，并得到其带隙特性.由于缺陷的存在，使得光子晶体的透射谱中产生缺陷峰.当被测温度变化时，根据两种介质的热光效应和热膨胀效应，光子晶体介质和缺陷层的光学厚度和折射率发生变化，透射谱缺陷峰产生漂移，由缺陷峰的中心波长漂移量得到被测温度的大小.构建了一维缺陷光子晶体测量温度的实验系统，实验结果表明缺陷峰中心波长与光子晶体所受的温度呈线性关系，测量灵敏度为0—2     

异质单周期内对称光子晶体的光子带隙特性

利用传输矩阵法,理论上对由TIO₂和SiO₂构成的异质单周期内对称光子晶体的透射谱进行仿真,分别改变入射光的角度、光子晶体介质层数和BA两层的厚度比,观察其透射谱,研究发现该结构形成的光子带隙的位置大小对介质层数的变化不敏感,但对入射角和BA两层厚度比的变化很敏感,这一研究对于光子晶体的设计具有重要意义.     

各向异性金刚石结构的光子晶体

基于平面波展开法,理论分析了填充率、介质各向异性的程度对单轴金刚石结构三维光子晶体禁带的影响.结果表明,当填充率和介质各向异性取合适的值,单轴各向异性金刚石结构光子晶体存在着完全带隙.介质各向异性的引入使该晶体的带隙变窄甚至完全关闭.在各个布里渊区域里,带隙率和带隙宽度随介质各向异性程度的变化而变化.各向异性的引入为调整光子禁带提供了一种方法.     

由单负材料组成的一维对称型光子晶体中的隧穿模

由电单负材料A和磁单负材料B构成了一维对称型光子晶体,数值计算表明其带隙中出现了一隧穿模. 材料层数增加,隧穿模宽度急剧变窄,而其位置不变.隧穿模的位置和宽度对入射角的变化都不太敏感. 材料的几何厚度减小,隧穿模的位置蓝移,而其宽度不变. μ_A, ε_B增加,隧穿模的位置红移,宽度减小. 利用隧穿模的以上特性可以实现对电磁波传播的动态调控.     

一维铁磁材料光子晶体的光学传输特性

利用光学传输矩阵方法,模拟计算了由铁磁材料组成的一维光子晶体的光学传输特性.研究了能带结构随介质层厚度h(O ≤h≤0.5λ0 λ0=1.2×10-6m)变化的规律,并与相同周期结构的普通光子晶体的传输特性进行了比较.结果表明,当O.15λ0＜h＜0.165λ0时,铁磁光子晶体的带隙宽度略小于普通光子晶体的带隙宽度;在h取其他值时铁磁光子晶体的带隙宽度远远大于普通光子晶体的带隙宽度,并且在h=0.25λ0时存在最大值.这种结构的光子晶体通过掺杂有望在多通窄带滤波技术中得到广泛的应用.     

全介质双腔法布里-珀罗结构的共振模研究

利用传输矩阵法研究了光在全介质双腔法布里-珀罗(F-P)结构中的传输,得到了透射率的一般表达式。能在禁带中出现共振模的双腔F-P结构共有12种组合方式,并且要求两腔之间的介质层数必须小于其外侧的介质总层数。当微腔的光学厚度是λ0/4的奇数倍时,其两端接触的首层介质必须是异种介质;而当光学厚度是λ0/4的偶数倍时则必须是同种介质。当两腔长度一定时,共振模的位置主要取决于两腔之间的介质层数,而谱线宽度则主要取决于两腔外侧的介质层数。当双腔F-P结构呈左右对称时共振模的透射率等于1。     

远红外窄带滤光片厚度的鲁棒性分析

鉴于在制备过程中存在介质层厚度的随机扰动,计算了远红外窄带滤光片厚度的鲁棒性。结果表明:当随机度小于0.05时,窄带滤光片全向带隙和带隙率变化不大,但缺陷层的透射率只有原来的1/3左右;当随机度大于0.1时,随着厚度的随机扰动逐渐增大,窄带滤光片的全向带隙和带隙率都逐渐减小;当随机度达到0.2时,窄带滤光片的第一个全向带隙消失,第二个全向带隙向短波方向偏移明显,缺陷层的透射率也只有原来的1/3左右;当随机度达到0.5时,窄带滤光片的两个全向带隙全部消失。制备了一种远红外窄带滤光片,其结果较好地验证了厚度的随机扰动对滤光片的影响。     

Design of an omnidirectional mirror using one dimensional photonic crystal with graded geometric layers thicknesses

We propose a flexible design for one-dimensional photonic crystals (1D-PCs) with a controllable omnidirectional band gap covering the optical telecommunication wavelengths which are 0.85 μm, 1.3 μm and 1.55 μm. We used for this design the chirped grating. Chirping is applied to geometric thicknesses of layers. It takes two forms, one is linear and the other is exponential. We exploit this technique to have the suitable omnidirectional band gap covering the maximum of optical telecommunication wavelengths. With a quarter wave structure, we can have an omnidirectional band gap generating only one of these wavelengths. With graded structure, we obtain, as is reported in this paper, an omnidirectional band gap which covers the wavelengths 1.3 μm and 1.55 μm at the same time with a large bandwidth. We also achieve an omnidirectional band gap containing the wavelength 0.85 μm and which is obviously larger than that of the quarter wave stack.     

Branching features of photonic bandgaps in Fibonacci dielectric heterostructures

We propose branching features of photonic band gaps in one-dimension Fibonacci dielectric heterostructures with arbitrary generation orders and thicknesses based on the gap map diagram, which is determined by the band edge formalism to avoid numerical instability. We find that the gap map diagram can be divided by the half-wave lines into several regions, each of which has similar gap pattern. The branching rules including the existence condition and the frequency range of each major gap as well as the number of all minor gaps in each region for systems with arbitrary thicknesses and generations are proposed. The number of major gaps in each region for the Fibonacci systems equals two rather than one for traditional binary layers. Moreover, the conditions of maximum gapwidth for the Fibonacci systems are not near the quarter wavelength, although the ones for traditional binary layers are at the quarter wavelength.     

Optical control of the edge of the band gap of semiconductor layered Fabry-Perot type structures

A mechanism of controlling light is studied, which is based on the change of refractive indices of narrow-gap layers in a semiconductor layered periodic Fabry-Perot type structure (superlattice) caused by the contribution of nonequilibrium charge carriers produced by controlling radiation. By choosing the wavelength of controlling light near the edge of intrinsic absorption of narrow-gap layers and by choosing the wavelength of controlled light near the edge of the band gap of a structure, one can obtain for 1.5-μm controlled radiation the modulation depth above 90%, with controlling radiation intensity of ～6 kW/cm².     

Accuracy design of ultra-low residual reflection coatings for laser optics

Refractive index inhomogeneity is one of the important characteristics of optical coating material, which is one of the key factors to produce loss to the ultra-low residual reflection coatings except using the refractive index inhomogeneity to obtain gradient-index coating. In the normal structure of antireflection coatings for center wavelength at 532 nm, the physical thicknesses of layer H and layer L are 22.18 nm and 118.86 nm, respectively. The residual reflectance caused by refractive index inhomogeneity(the degree of inhomogeneous is between -0.2 and 0.2) is about 200 ppm, and the minimum reflectivity wavelength is between 528.2 nm and 535.2 nm. A new numerical method adding the refractive index inhomogeneity to the spectra calculation was proposed to design the laser antireflection coatings, which can achieve the design of antireflection coatings with ppm residual reflection by adjusting physical thickness of the couple layers. When the degree of refractive index inhomogeneity of the layer H and layer L is-0.08 and 0.05 respectively, the residual reflectance increase from zero to 0.0769% at 532 nm. According to the above accuracy numerical method, if layer H physical thickness increases by 1.30 nm and layer L decrease by 4.50 nm, residual reflectance of thin film will achieve to 2.06 ppm. When the degree of refractive index inhomogeneity of the layer H and layer L is 0.08 and -0.05 respectively, the residual reflectance increase from zero to 0.0784% at 532 nm. The residual reflectance of designed thin film can be reduced to 0.8 ppm by decreasing the layer H of 1.55 nm while increasing the layer L of 4.94 nm.     

近红外波段锑基铋掺杂薄膜厚度对光学常数与光学带隙的影响

采用磁控溅射法制备了不同厚度的锑基铋掺杂薄膜,用X射线衍射(XRD)和透射电子显微镜(TEM)研究了薄膜结构随厚度的变化。利用椭圆偏振法测定了样品薄膜在近红外波段的光学常数与光学带隙,研究了膜厚对样品薄膜光学常数和光学带隙的影响。结果表明,膜厚从7 nm增加至100 nm时,其结构由非晶态转变为晶态。在950～2200 nm波段,不同厚度薄膜样品的折射率在4.6～8.9范围,消光系数在0.6～5.8范围,光学带隙在0.32～0.16 eV范围。随着膜厚的增加,薄膜的折射率和光学带隙减小,而消光系数升高;光学常数在膜厚50 nm时存在临界值,其原因是临界值前后薄膜微观结构变化不同。     

向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

High efficiency antireflection coating in MWIR region (3.6–4.9 μm) simultaneously effective for Germanium and Silicon optics

Antireflection coatings have critical importance in thermal imaging system working in MWIR region (3–5 μm) since optics of high refractive index materials are used. Germanium (Ge) and Silicon (Si) optics are used extensively in the MWIR thermal systems. In this paper a study has been carried out on the design and fabrication of multi-substrate antireflection coating effective for Germanium and Silicon optics in MWIR (3.6–4.9 μm) region. The wave band 3.6–4.9 μm is chosen for the reported work because detector system used in MWIR region has a band selection filter effective in the same wavelength region and atmospheric transmission window in MWIR region is effective in 3–5 μm spectral band. Comprehensive search method was used to design the multilayer stack on the substrate. The coating materials used in the design were Germanium (Ge), Hafnium oxide (HfO₂) and Y-Ba-Fluoride (IR-F625). The fabrication of coating was made in a coating plant fitted with Cryo pump system and residual gas analyzer (RGA). The evaporation was carried out at high vacuum (2–6 × 10^?6 mbar) with the help of electron beam gun system and layer thicknesses were measured with crystal monitor. The result achieved for the antireflection coating was 98.5% average transmission in 3.6–4.9 μm band for Germanium and Silicon optics. This work will be helpful in reducing the plant operation time, material and power consumption, as two different kinds of optics are simultaneously coated in a single coating cycle.     

Multi-spectral antireflection coating on zinc sulphide simultaneously effective in visible,eye safe laser wave length and MWIR region

In recent years multi-spectral device is steadily growing popularity. Multi-spectral antireflection coating effective in visible region for sighting system, laser wavelength for ranging and MWIR region for thermal system can use common objective/receiver optics highly useful for state of art thermal instrumentation. In this paper, design and fabrication of antireflection coating simultaneously effective in visible region (450–650 nm), Eye safe laser wave length (1540 nm) and MWIR region (3.6–4.9 μm) has been reported. Comprehensive search method of design was used and the number of layers in the design was optimised with lowest evaluated merit function studied with respect to various layers. Finally eight-layer design stack was established using hafnium oxide as high index layer and silicon-di-oxide as low index coating material combination. The multilayer stack had been fabricated by using electron beam gun evaporation system in Symphony 9 vacuum coating unit. During layer deposition the substrate was irradiated with End-Hall ion gun. The evaporation was carried out in presence of oxygen and layer thicknesses were measured with crystal monitor. The result achieved for the antireflection coating was 85% average transmission from 450 to 650 nm in visible region, 95% transmission at 1540 nm and 96% average transmission from 3.6 to 4.9 μm in MWIR region.     

Marine environment compatible antireflection coating with nanotop layer on silicon optics

Antireflection coating on silicon optics have crucial importance in thermal device working in 3.6–4.9 μm wavelength region. When the thermal device is used in marine environment, the optics face harsh saline weather condition compared to normal field environment. This deteriorates coated optics and to improve mechanical strength of the coating, a nanotop layer on the antireflection coating has been developed. In this paper a study has been carried out to improve marine environment compatibility by employing a nanolayer on the top of antireflection coating on silicon optics. Optimac synthesis method was used to design the multilayer stack on the substrate with germanium and IR-F625 as high/low refractive index respectively and the layer number was restricted to four layers. The top nanolayer was 60 ± 2 nm thick hafnium dioxide layer developed with ion assisted deposition (End–Hall) on the optics during coating process. The deposition of multilayer coating was carried out inside the coating plant fitted with cryo pump and residual gas analyzer. The evaporation was carried out at high vacuum (2–6 × 10⁻⁶ mbar) using electron beam gun and layer thicknesses were measured with crystal monitor. The average transmission achieved was 97% in the spectral band of 3.6–4.9 μm with a hardness of 9.7 GPa on the coated optics.     

Recording of dynamic gratings in the nonlinear optical coating of a planar waveguide

The possibility of controlled energy exchange between interfering waveguide modes in a singlemode planar waveguide with a nonlinear optical coating is analyzed. As the coating, a suspension of bacteriorhodopsin D96N was used, which makes it possible to realize two spectrally separated mechanisms of recording and controlling dynamic gratings, i.e., the spatial modulation of the trans-cis excitation rate and the spatial modulation of the cis-trans relaxation rate. The method of phase-modulated beams was used to implement the energy exchange. The dynamic gratings in the coating were recorded by using both radiation with a wavelength within the absorption band of the trans state (630 nm) and radiation with a wavelength within the absorption band of the cis state (440 nm). Efficient control of the energy exchange between the waveguide modes by means of uniform exposure of their interference region to radiation with another wavelength was observed. A completely integral geometrical layout for optically controlled energy exchange was realized. The results obtained are compared with known data on energy exchange between beams in the bulk of a similar nonlinear medium.