Bioinspired Si subwavelength gratings by closely-packed silica nanospheres as etch masks for efficient antireflective surface

We experimentally investigate the antireflective properties of various silicon (Si) subwavelength grating (SWG) structures using closely-packed silica nanospheres monolayers with different sizes as etch masks and a subsequent inductively coupled plasma (ICP) etching, together with theoretical calculations based on a rigorous coupled wave analysis method. The geometric structure of Si SWGs is optimized by changing the size of nanospheres and ICP etching parameters. The antireflective properties depend strongly on the period, height, and shape of the hexagonally ordered SWG structures, especially correlated with ICP etching parameters. For an optimized Si SWG structure with a rounded cone shape, the reflectance is significantly reduced, indicating a low reflectance of <4.4% over a wide wavelength region of 300–1100 nm. From theoretical analysis, the reflectance of rounded cone-shaped Si SWG structures is minimized with a period of ∼300–350 nm and heights of >750 nm, which is reasonably consistent with the experimental results. The angle-dependent antireflection characteristics are also discussed.     

Design and fabrication of antireflective GaN subwavelength grating structures using periodic silica sphere monolayer array patterning

We designed and fabricated gallium nitride (GaN) subwavelength grating (SWG) structures on GaN/sapphire via patterning using the periodic silica sphere monolayer array as an etch mask and a subsequent dry etching for efficient antireflection coatings. Theoretical optimization of GaN SWG structures was performed in terms of their geometrical parameters by the rigorous coupled-wave analysis simulation using a theoretical structural model. The bullet-like parabola-shaped SWGs with a large height-to-diameter ratio (R _H/D) yielded good broadband and wide-angle antireflective surface properties. Considering the R _H/D, the GaN SWG structure using 320-nm silica spheres theoretically and experimentally exhibited the most efficient antireflection property because it provided a linearly graded effective refractive index profile with relatively long relaxation length. For various geometries of the fabricated GaN SWGs on GaN/sapphire, the calculated reflectance results showed a similar tendency with the experimental results.     

Femtosecond laser-assisted etching of three-dimensional inverted-woodpile structures in fused silica

Three-dimensional inverted-woodpile (WP) structures were embedded in a microchannel by femtosecond laser direct-writing of fused silica followed by chemical etching with diluted hydrofluoric acid. We show the hole size is linearly dependent on laser-scanning depth for various pulse energies, permitting the control of laser exposures to facilitate close 5 μm periodic packing of uniform microcapillary arrays. Exposure compensation for depth-dependent etching rate and optical beam aberrations yielded stable and crack-free uniform inverted-WP structures. The direct formation of the inverted-WP structure together with microchannels in an all-fused silica substrate, offers chemical stability and inertness, and biocompatibility to be exploited as new microfluidic systems for chromatography and electro-osmotic pumps.     

基于直接胶体晶体刻蚀技术的高度有序纳米硅阵列的尺寸及形貌控制

以自组装单层胶体小球阵列为掩模，采用直接胶体晶体刻蚀技术在硅表面制备二维有序尺寸可控的纳米结构.在样品制备过程中，首先通过自组装法在硅表面制备了直径200nm的单层聚苯乙烯(PS)胶体小球的二维有序阵列；然后对样品直接进行反应离子刻蚀(RIE)，以氧气为气源，利用氧等离子体对聚苯乙烯小球和对硅的选择性刻蚀作用，通过改变刻蚀时间，制备出不同尺寸的PS胶体小球的有序单层阵列；接着以此二维PS胶体单层膜为掩模，以四氟化碳为气源对样品进行刻蚀；最后去除胶体球后得到二维有序的硅柱阵列.SEM和AFM的测量结果表明：改变氧等离子体对胶体球的刻蚀时间和四氟化碳对硅的刻蚀时间，可以控制硅柱的尺寸以及形貌，而硅柱阵列的周期取决于原始胶体球的直径. 关键词： 胶体晶体刻蚀 纳米硅柱阵列     

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.     

Subwavelength structures for broadband antireflection application

The subwavelength structures are designed and fabricated for broadband antireflection application. Under target of zero reflectivity, the parameters of periodic 2-D continuous conical structures are analyzed by the finite-difference time-domain (FDTD) method. The corresponding conical structures are obtained with spatial period of 350 nm and structure height of 300 nm, respectively. The 2-D continuous conical structured surface is fabricated by micro-replication process combining with the originated structure fabrication realized by interference lithography, Ni mold electroplation and replication by using UV imprinting into plastics. The average reflectances of the simulation and replicated polymer prototype are about 0.50% and 0.54% within the spectral ranges of 400-650 nm, respectively. In a word, the subwavelength structured surface with low reflection is developed and proved to be highly consistent with the simulation results.     

晶体硅太阳电池表面纳米线阵列减反射特性研究

为增强晶体硅太阳电池的光利用率, 提高光电转换效率, 研究了硅纳米线阵列的光学散射性质. 运用严格耦合波理论对硅纳米线阵列在310—1127 nm波段的反射率进行了模拟计算, 用田口方法对硅纳米线阵列的表面传输效率进行了优化. 结果表明, 当硅纳米线阵列的周期为50 nm, 占空比为0.6, 高度约1000 nm时减反射效果最佳; 该结构在上述波段的平均反射率约为2%, 且在较大入射角度范围保持不变. 采用金属催化化学腐蚀法, 于室温、室压条件下在单晶硅表面制备周期为60 nm,占空比为0.53, 高度为500 nm的硅纳米线阵列结构, 其反射率的实验测试结果与计算模拟值相符, 在上述波段的平均反射率为4%—5%, 相对于单晶硅35%左右的反射率, 减反射效果明显. 这种减反射微结构能够在降低太阳电池成本的同时有效减小单晶硅表面的光反射损失, 提高光电转换效率.     

Investigation of geometrical effects of antireflective subwavelength grating structures for optical device applications

We calculated diffraction efficiencies of subwavelength grating (SWG) structures for various optical device applications by using a rigorous coupled-wave analysis method. The geometrical effects, such as grating shape, height, and period, were investigated in order to obtain better antireflection performance. Cone shaped SWG structures with a taller height provide lower reflectance over a broadband wavelength range compared to that of flat surface and nanorod. It was found that the low reflection regions are quite related with the grating period and the refractive index of substrate materials. From the comparison between external and internal reflection of SWG structures, we also showed that the internal reflection requires shorter grating period than the case of external reflection to acquire broadband antireflection properties.     

亚波长消反射光栅

为了降低消反射光栅的偏振敏感性，将等效介质理论推广到二维亚波长结构，对一种特定的矩形柱状结构进行了分析。构造了一种分析方法，然后利用光栅结构的有关表达式得到这种二维结构的近似等效系数。并设计了适用于10．6μm红外波段的二维亚波长消反射光栅，用二元光学的制作工艺在si衬底上进行了实验制备。测试结果表明：对中心波长为10．6μm的红外光，这种光栅象单层消反射膜一样，具有很好的增透效果。     

Fabrication of large-scale periodic silicon nanopillar arrays for 2D nanomold using modified nanosphere lithography

We present a fabrication procedure that can form large-scale periodic silicon nanopillar arrays for 2D nanomold which determines the feature size of nanoimprint lithography, using modified nanosphere lithography. The size of silicon nanopillars can be easily controlled by an etching and oxidation process. The period and density of nanopillar arrays are determined by the initial diameter of polystyrene (PS) spheres. In our experiment, the smallest nanopillar has a full width half maximum (FWHM) of approximately 50 nm, and the density of silicon pillar is ∼10⁹/cm². Using this approach, it is possible to fabricate 2D nanoimprint lithography mask with 50 nm resolution.     

静/动态刻蚀对熔石英表面质量和激光损伤的影响

为了研究静/动态刻蚀过程中熔石英表面质量和抗激光损伤性能的演变规律,优化化学刻蚀工艺,使用HF酸缓冲液对熔石英分别进行了不同时间的静/动态刻蚀处理。实验表明,由于兆声场辅助搅拌作用,熔石英动态刻蚀的刻蚀速率快于静态刻蚀。动态刻蚀后熔石英表面均方根(RMS)粗糙度和反射面形分别为 < 1 nm和0.46λ,其3倍频透射率先小幅增加后保持稳定,相比初始表面增加约0.1%。而静态刻蚀使得表面RMS粗糙度和反射面形分别增加至~5 nm和0.82λ,其3倍频透射率先基本不变后下降,相比初始表面下降约0.4%。二者损伤阈值呈现明显不同变化规律:静态刻蚀使熔石英损伤阈值先小幅增加约30%后逐渐降低,动态刻蚀使熔石英损伤阈值增加近一倍后保持相对稳定。结果表明,动态刻蚀后熔石英光学元件性能明显优于静态刻蚀。     

THz波段亚波长半导体球形阵列光学特性的数值模拟

利用T-matrix方法对太赫兹波段亚波长半导体球形阵列进行了数值模拟并在数值模拟结果的基础上讨论了其光学特性。在太赫兹波段可以通过掺杂等手段调节半导体的表面等离子体特性。以半导体InSb为例并采用Drude模型,对单个亚波长球及两个或多个亚波长球组成的阵列进行了数值模拟,主要以归一化消光截面为参数,讨论了不同阵元半径、不同球形单元间距、不同单元数目及入射波不同极化方向对阵列特性的影响。     

金膜上亚波长小孔阵列表面等离激元颜色滤波器偏振性质

金属薄膜上制备的表面等离激元颜色滤波器具有很强的颜色可调性. 在200 nm厚的金膜上, 通过聚焦离子束刻蚀, 制备一系列周期逐渐变化的圆形、方形、矩形亚波长尺寸小孔方阵列表面等离激元颜色滤波器, 改变入射光的偏振方向, 观察其超透射滤波现象. 研究发现: 对于矩形小孔阵列, 其透射光颜色随入射光偏振方向的变化而改变; 而对于圆形、方形的小孔阵列, 其透射光颜色对入射光的偏振方向并不敏感. 分析表明, 对于金膜上刻蚀的小孔结构, 虽然结构的周期性导致的表面等离激元极化子会对透射光的颜色变化产生一定影响, 但是随小孔形状变化的局域表面等离激元共振才是影响透射光颜色的决定性因素. 如果入射光没有在小孔中激发出局域表面等离激元, 则表面等离激元极化子对透射光的影响也会消失. 根据不同形状小孔周期结构透射光颜色随入射光的偏振变化特点, 制备出了包含两种小孔形状的复合周期结构. 随着入射光偏振方向的改变, 该结构会显示出不同的颜色图案. 关键词： 表面等离激元极化子 局域表面等离激元 颜色滤波器 亚波长小孔阵列     

Antireflection subwavelength structures analyzed by using the finite difference time domain method

Antireflection subwavelength structures (ASSs) are analyzed by using the finite difference time domain (FDTD) method in the visible light spectrum. Low reflectance can be obtained by both the conical and pyramidal shapes over a broadband range. Comparing the reflectance of different structure shapes and aspect ratios by the FDTD method, it shows that the antireflection efficiencies of the pyramidal structures are better than that of the conical structures when the aspect ratio is up to 0.8. It is found that, for the conical structure surface, the average transmittance increases gradually with the aspect ratio and the average transmittance is about 99.6% with the aspect ratio of 2.0. However, for the pyramidal structure with the aspect ratio ranging from 1.0 to 2.0, the average transmittance is up to 99.7%.     

Effect of etching parameters on antireflection properties of Si subwavelength grating structures for solar cell applications

We fabricate the aluminum-doped zinc oxide (AZO) subwavelength gratings (SWG) on Si and glass substrates by holographic lithography and sequent CH₄/H₂/Ar reactive ion etching process. The etch selectivity of AZO over photoresist mask as well as the nano-scale shape is optimized for better antireflection performance. To analyze the antireflective properties of AZO SWG surface, the optical reflectivity is measured and then calculated together with a rigorous coupled-wave analysis. The reflectance spectrum can be considerably changed by incorporating the SWG into AZO film. As the SWG height of AZO on Si substrate increases, the magnitude of interference oscillations in the reflectance spectrum tends to be reduced with the larger difference between its maxima. The use of optimized SWG can significantly reduce the surface reflection of AZO film at the desired wavelengths. The measured reflectance data of AZO SWG are reasonably consistent with the simulation results. No considerable change in transmission characteristics is observed for AZO SWG structures.     

Antireflective properties of AZO subwavelength gratings patterned by holographic lithography

We fabricate the aluminum-doped zinc oxide (AZO) subwavelength gratings (SWG) on Si and glass substrates by holographic lithography and sequent CH₄/H₂/Ar reactive ion etching process. The etch selectivity of AZO over photoresist mask as well as the nano-scale shape is optimized for better antireflection performance. To analyze the antireflective properties of AZO SWG surface, the optical reflectivity is measured and then calculated together with a rigorous coupled-wave analysis. The reflectance spectrum can be considerably changed by incorporating the SWG into AZO film. As the SWG height of AZO on Si substrate increases, the magnitude of interference oscillations in the reflectance spectrum tends to be reduced with the larger difference between its maxima. The use of optimized SWG can significantly reduce the surface reflection of AZO film at the desired wavelengths. The measured reflectance data of AZO SWG are reasonably consistent with the simulation results. No considerable change in transmission characteristics is observed for AZO SWG structures.     

锥形轮廓亚波长二维表面浮雕结构的矢量衍射特性

本文利用严格耦合波理论对多台阶逼近锥形轮廓的亚波长二维表面浮雕结构进行了计算分析.从本文的计算结果来看,采用多台阶来逐步逼近锥形轮廓的亚波长零级二维表面浮雕结构具有在宽波段、大视角上能够实现抗反射,并且对光波的偏振态下不敏感的特性.从而使得该结构为薄膜光学在材料的选择方面提供了极大的选择自由度.     

Fourier-domain low-coherence interferometry for light-scattering spectroscopy

We present a novel method for obtaining depth-resolved spectra for determining scatterer size through elastic-scattering properties. Depth resolution is achieved with a white-light source in a Michelson interferometer with the mixed signal and reference fields dispersed by a spectrograph. The spectrum is Fourier transformed to yield the axial spatial cross correlation between the signal and reference fields with near 1-microm depth resolution. Spectral information is obtained by windowing to yield the scattering amplitude as a function of wave number. The technique is demonstrated by determination of the size of polystyrene microspheres in a subsurface layer with subwavelength accuracy. Application of the technique to probing the size of cell nuclei in living epithelial tissues is discussed.     

Analysis of phase shift of surface plasmon polaritons at metallic subwavelength hole arrays

We present the transmission spectra of light transmitting a metallic thin film perforated with differently shaped sub- wavelength hole arrays, which are calculated by a plane-wave-based transfer matrix method. We analyze the transmission peak positions and the phase-shift angles of different surface plasmon polariton （SPP） modes by using the microscopic theoretical model proposed by Haitao Liu and Philippe Lalanne [Liu Haitao, and Lalanne Philippe 2008 Nature 452 728], in which the phase shift properties of the SPPs scattered by the subwavelength hole arrays are considered. The results show that the transmission peak position and the minus phase shift angle of the SPP increase as the hole size increases. On the other hand, the effective dielectric constant of the metallic film can be deduced by the microscopic theoretical model.     

光学元件亚表面缺陷的损伤性检测方法

在磨削、研磨和抛光加工过程中产生的微裂纹、划痕、残余应力等亚表面缺陷会导致熔石英元件抗激光损伤能力下降,如何快速、准确地检测亚表面损伤成为光学领域亟待解决的关键问题。采用HF酸蚀刻法、角度抛光法和磁流变斜面抛光法对熔石英元件在研磨加工中产生的亚表面缺陷形貌特征及损伤深度进行了检测和对比分析,结果表明,不同检测方法得到的亚表层损伤深度的检测结果存在一定差异,HF酸蚀刻法检测得到的亚表面损伤深度要比角度抛光法和磁流变斜面抛光法检测结果大一些。且采用的磨粒粒径越大,试件表面及亚表面的脆性断裂现象越严重,亚表面缺陷层深度越大。