Silicon suboxide (SiOx): laser processing and applications

Two types of periodic surface structures including micro-ripples and nano-ripples were observed on magnesium after femtosecond laser irradiation, and the color effect was reported by scanning a single laser beam over a large area at near damage threshold fluence. Optical reflectance measurement revealed that the color effect was mainly attributed to the nano-ripples as diffraction gratings, and intensity of the color was affected by morphology of the micro-ripples. AFM and SEM images showed that orientation of the micro-ripples was parallel to laser polarization and the period of the micro-ripples increased with scanning numbers, while the orientation of the nano-ripples was perpendicular to laser polarization and the period of the nano-ripples kept constant as laser sub-wavelength with increasing scanning numbers. These results suggested that formation of the micro-ripples was due to the combined effect of initial surface roughness and near-field interference, while formation of the nano-ripples was caused by the near-field interference between incident light and surface plasmons being excited in the air and metal interface. The potential application of such an effect is further proposed.     

飞秒激光烧蚀硅材料表面形成周期波纹形貌研究

基于Sipe-Drude模型和表面等离子体激元(SPP)的干涉理论分别对单脉冲飞秒激光诱导硅表面形成低频率周期性波纹进行分析研究.探究了波长800 nm、脉宽150 fs的单个飞秒激光烧蚀硅造成不同激发水平下波纹形貌的变化,考虑到材料的光学性质变化(由Drude模型得到的介电常数变化),引入包含双温方程的电子数密度模型.计算结果表明,Sipe-Drude和SPP理论都适用于分析和解释高激发态下周期性波纹,但Sipe-Drude理论更适合分析更为广泛的周期性波纹结构.同时,波纹延伸方向总是垂直于入射激光偏振方向,其空间周期略小于激光波长,并受到入射激光通量的影响.在激光通量为0.38 J/cm~2时,波纹周期达到最小值.另外,还得到了不同入射角度的波纹周期变化情况,并在不同偏振态下随入射角度增大时波纹周期呈现相反的变化趋势.该研究对于理解飞秒激光造成硅表面形成周期结构及其在加工硅材料领域具有重要参考意义.     

1.3μm InGaAsP／InP脊形波导激光器偏振输出的测量

本文对发射波长为１．３μｍ，具有不同脊形波导宽度ＩｎＧａＡｓＰ激光器的ＴＥ，ＴＭ模偏振输出特性进行了测量，实验观察到脊形波导比较窄的器件具有ＴＥ，ＴＭ模竞争现象，而脊形波导比较宽的器件在阈值电流以上，只有ＴＥ模激射，没有模式竞争。利用偏振度概念计算了阈值电流以上和阈值电流以下的偏振度，并对其进行了讨论。本文指出，只要严格选择激光器的参数，并控制环境温度，合理使用器件，就可获得稳定的偏振输出，而不必     

Phase interpretation for polarization-dependent near-field images of high-density gratings

It has been described that the near-field images of a high-density grating at the half self-imaging distance could be different for TE and TM polarization states. We propose that the phases of the diffraction orders play an important role in such polarization dependence. The view is verified through the coincidence of the numerical result of finite-difference time-domain method and the reconstructed results from the rigorous coupled-wave analysis. Field distributions of TE and TM polarizations are given numerically for a grating with period d = 2.3λ, which are verified through experiments with the scanning near-field optical microscopy technique. The concept of phase interpretation not only explains the polarization dependence at the half self-imaging distance of gratings with a physical view, but also, it could be widely used to describe the near-field diffraction of a variety of periodic diffractive optical elements whose feature size comparable to the wavelength.     

Plasmonic laser nanoablation of silicon by the scattering of femtosecond pulses near gold nanospheres

We present the fabrication of nanostructures ablated on silicon(100) by the plasmonic scattering of 780 nm, 220 fs laser pulses in the near-field of gold nanospheres. We take advantage of the enhanced plasmonic scattering of ultrashort laser light in the particle near-field to ablate well-defined nanocraters. Gold nanospheres of 150 nm diameter are deposited onto a silicon surface and irradiated with a single laser pulse. We studied the effect of laser polarization on the morphology of ablated nanostructures and estimated the minimum fluence for plasmonic nanoablation. When the polarization of the incident radiation is directed at a 45° angle into the substrate surface, a near-field enhancement of 23.1±7.6 is measured, reducing the required silicon ablation fluence from 191±14 mJ/cm² to 8.2±2.9 mJ/cm². Enhancements are also measured for laser polarizations parallel to the substrate surface when the substrate is angled 0° and 45° to the incident irradiation, giving enhancements of 6.9±0.6 and 4.1±1.3, respectively. Generated nanocrater morphologies show a direct imprint of the particle dipolar scattering region, as predicted in our theoretical calculations. The measured near-field enhancement values agree well with the maximum field enhancements obtained in our calculations. The agreement between theory and measurements supports that the nanocraters are indeed formed by the enhanced plasmonic scattering in the near-field of the nanoparticles. PACS 42.62.-b; 52.38.Mf; 81.65.Cf; 81.16.-c; 78.67.Bf     

基于亚波长光栅的VCSEL偏振控制研究

为了确定亚波长光栅在微电机械系统(MEMS)波长可调谐VCSEL不同位置(上DBR上表面、上DBR下表面以及内腔)中实现TE和TM偏振控制的光栅参数范围以及光栅在哪个位置时实现偏振控制最稳定,通过MATLAB建立MEMS波长可调VCSEL的模型,然后计算光栅在3种位置时上反射镜(包括空气隙和光栅)随光栅参数变化的反射率,以此来确定它们实现TE/TM稳定偏振的光栅参数范围(即高反射范围内的参数)。将各自的高反射所对应反射率减去相同光栅参数范围内TM/TE低反射对应的反射率,通过反射率差值确定光栅在哪种位置时MEMS波长可调谐VCSEL实现偏振是最稳定的。最后得出的结论是光栅在上DBR下表面几乎无法控制TM偏振,而将光栅放置于内腔中,无论是在TE偏振控制上还是TM偏振上都是最稳定的。在实现TE偏振稳定的参数范围内,TE的阈值增益比TM最小少10 cm~(-1);而在实现TM偏振稳定时,在TE偏振稳定的参数范围内,TE的阈值增益比TM最小少5 cm~(-1)。     

A reflective surface-relief two-layer grating for splitting beams with the polarization-selective property

A polarization-selective beam splitter is presented based on a reflective surface-relief two-layer grating with a metal slab. The grating is composed of two dielectric layers and a metal slab on the substrate. For TE polarization, efficiencies of 97.58% can be diffracted into the reflective −1st order. For TM polarization, two-port output of 47.55% and 47.11% can be split into the 0th order and the −1st order, respectively. Such a polarization-selective beam splitter can be fulfilled by one grating in reflection. The grating depths of two layers are optimized by using rigorous coupled-wave analysis with the given duty cycle of 0.6 and period of 1100 nm. The performance for different incident conditions is investigated when the incident wavelength and angle deviate from the central wavelength and the Bragg angle, respectively. For TM polarization, good uniformity can still be achieved within the given incident wavelength and angle regions. For TE polarization, the reflective polarization-selective beam splitter can have merits of wideband for different incident wavelengths.     

Near-field distribution of broadband antireflective nanostructure arrays

Broadband antireflection (AR) nanostructure arrays patterned directly into a bulk substrate, behaving as the expected performance of high laser induced damage threshold (LIDT), is proposed for the application of laser-transmitting optical elements. These regular periodic arrays known as “moth-eye” structure exhibit particularly noteworthy AR property over a wide spectrum and over a large field of view in addition to overcome efficiently the disadvantages of multilayer AR thin-film. Furthermore, the near-field distribution in nanostructure arrays with respect to the period, the groove depth and incident angle, respectively, was numerically simulated using a rigorous Fourier modal method (FMM). It is interesting that the maximum value of the normalized electric field intensity is obviously distributed inside the air layer upon the nanostructure for the periodicity of 100 nm, which has significant potential to reach to ultra high LIDT for the application of short-pulse high-power laser system. Besides, the variation of the near-field distribution inside the nanostructure arrays exhibits critically dependent on the period of surface structure, but the effect caused by the discrepancy in groove depth and incident angle is unimportant.     

Connecting-layer-based polarizing beam splitter grating with high efficiency for both TE and TM polarizations

We describe a novel polarizing beam splitter (PBS) of transmission connecting-layer-based grating, which can show high efficiency for TE polarization in the ?1st order and TM polarization in the 0th order. According to the modal method, grating duty cycle and period are prescribed to obtain high efficiency for TM polarization. Using the rigorous coupled-wave analysis, grating depth and thickness of the connecting layer are optimized to achieve high efficiency for TE polarization. The presented PBS has advantages of wideband properties for incident wavelength and angle. Most importantly, the efficiency is improved greatly for TE polarization compared with the reported PBS based on fused-silica grating.     

多层介质膜脉冲宽度压缩光栅与超短脉冲作用时的性能分析

基于啁啾脉冲放大技术的超短脉冲激光系统是提供超快、超强激光的重要途径，具有良好输出波形和高损伤阈值的多层介质膜脉冲宽度压缩光栅是获得高峰值功率脉冲激光的关键. 基于傅里叶谱变换方法和严格模式理论，分析了多层介质膜光栅(MDG)在超短脉冲作用下的光学特性. 结果表明，当MDG的反射带宽小于具有高斯分布的入射脉冲的频谱宽度时，-1级反射脉冲呈非对称高斯分布，其前沿出现振荡，并且-1级反射脉冲能量开始剧烈下降. 讨论了MDG结构参数对其反射带宽的影响. 分析了MDG与超短脉冲作用时的近场光分布，对提高其抗激光损 关键词： 脉冲压缩光栅 傅里叶谱变换 模式理论 损伤阈值     

用数值方法分析微孔激光器出射光的近场特性

微孔激光器作为应用于近场光信息存储系统中的一种新型光源，它的出射光斑的近场特性对于近场光存储是十分重要的。针对纳米孔径运用角谱进行Fox-Li数值迭代，得到不同孔径微孔激光器的基模光强分布，然后运用二维非线性时域有限差分法分析微孔激光器出射端即微孔金属膜的近场光学性质，模拟计算了不同孔径和厚度的微孔金属膜的光强近场分布，从应用于近场光存储的角度，给出反映其近场光学特性的相关数据。发现由于TM模式下金属存在局域表面等离子增强效应，使得其出射强度比TE模式高一个数量级，从而更适于作为实际中近场光存储系统和原理试验的光学头。     

全内反射式衍射光栅近场光学特性

 利用傅里叶模式理论分析了具有高衍射效率的全内反射式衍射光栅在TE 和TM偏振态下的近场光分布特点，讨论了光栅结构参数以及入射角度对光栅内电场增强的影响。结果表明：全内反射光栅内部电场分布对偏振态较敏感，光栅槽深和占宽比对电场增强影响较小，光栅内的峰值电场随光栅周期增大而增大，并且峰值电场随着入射角度的增大而减小。在应用于高功率激光时，降低光栅内部的电场增强可以有效降低损伤风险。     

PMMA材料在线偏振和圆偏振光下的超连续光谱特性分析

飞秒激光在透明材料加工过程中会出现超连续光谱现象。在阐述超连续光谱产生的原理的基础上,为了分析PMMA材料在不同偏振光下产生的超连续光谱的阈值,设计了线偏光和圆偏光及不同能量加工PMMA材料的实验方案。利用光谱仪对产生的超连续光谱信号进行采集及处理,分析出不同能量下的线偏振(TE和TM)和圆偏振两种偏振态的超连续光谱的变化规律,并对比了相同能量下线偏振和圆偏振的超连续光谱的区别。实验中采用脉宽160 fs、中心波长为775 nm的飞秒激光,实验结果表明,同一偏振下能量越大,光谱谱宽越宽;通过对比不同能量下的光谱特性,观测出产生超连续光谱的阈值, 线偏振光的阈值为0.46 μJ,圆偏振光的阈值为0.586 μJ;对比相同能量下的线偏振和圆偏振光,线偏振的谱宽比圆偏振的宽。     

一维增透亚波长光栅的研究

将具有高透射性的亚波长光栅置于微机械波长可调谐垂直腔面发射激光器(VCSEL)的内腔当中可以提高波长的调谐范围,为了使波长调谐范围达到最优则必须优化高透射性的亚波长光栅使其透射率达到最大。利用严格耦合波法分析了亚波长光栅的占空比、周期、厚度和入射角对其透射率的影响并找出最优的光栅参数。通过计算分析可得,对于TE和TM偏振存在最佳的占空比使其透射率达到99.5%。在文中条件下,它们对应的占空比分别为0.23和0.80。而光栅厚度对于TE和TM偏振透射率的影响是周期性的,在一个周期内存在一个最佳值使其透射率达到最高。在文中条件下,TE偏振的厚度周期是150 nm,TM偏振的厚度周期是300 nm。当光栅参数不变时,无论是TE还是TM偏振光,它们的透射率只有在垂直入射光栅时(入射角为0°)才能达到最大。而通过等效介质原理可以得出,周期对透射率没有影响。最后计算了透射率在光栅厚度和占空比同时变化时的变化趋势,并从中得出最优的光栅参数。     

Si homojunction structured near-infrared laser based on a phonon-assisted process

We fabricated several near-infrared Si laser devices (wavelength ～1300 nm) showing continuous-wave oscillation at room temperature by using a phonon-assisted process induced by dressed photons. Their optical resonators were formed of ridge waveguides with a width of 10 μm and a thickness of 2 μm, with two cleaved facets, and the resonator lengths were 250–1000 μm. The oscillation threshold currents of these Si lasers were 50–60 mA. From near-field and far-field images of the optical radiation pattern, we observed the high directivity which is characteristic of a laser beam. Typical values of the threshold current density for laser oscillation, the ratio of powers in the TE polarization and TM polarization during oscillation, the optical output power at a current of 60 mA, and the external differential quantum efficiency were 1.1–2.0 kA/cm², 8:1, 50 μW, and 1 %, respectively.     

Simple technique for measuring the Goos-Hänchen effect with polarization modulation and a position-sensitive detector

An original approach to directly measuring the Goos-H?nchen longitudinal shift between TE and TM polarization states during a total internal reflection is introduced. The technique is based on the modulation of the polarization state of a laser by an electro-optic modulator combined with a precise measurement of the resulting spatial displacement with a position-sensitive detector. This method presents many advantages over other techniques and allows measurements at different wavelengths over a broad range for the incident angle.     

提高多层介质膜脉宽压缩光栅阈值的清洗方法

采用HPM溶液（盐酸、双氧水和去离子水的混合液）结合氧等离子体对多层介质膜脉宽压缩光栅进行清洗研究。用X射线光电子能谱检测光栅表面的元素成分及其原子含量的变化。实验结果表明,氧等离子体处理能有效去除光栅表面残留光刻胶和碳氟化合物; 再经HPM溶液清洗,反应离子束刻蚀和氧等离子体处理过程产生的金属污染物被进一步去除。经过上述清洗工艺处理后,光栅一级衍射效率仍保持在95%以上,光栅表面激光诱导损伤阈值达到1.6 J/cm2 (1053 nm, 10 ps)。实验结果说明了氧等离子体和HPM溶液相结合能有效清洗多层介质膜脉宽压缩光栅,并显著提高光栅损伤阈值。     

Narrow-bandwidth tunable bandstop filters with circularly cylindrical self-suspended silicon gratings

A circularly cylindrical self-suspended silicon grating (SSG) is proposed, and its reflectance spectrum is investigated by the rigorous coupled-wave method (RCWM). For TM incident polarization, when the grating period is tuned, numerical results by the RCWM reveal that the circularly cylindrical SSG possesses a tunable narrow reflectance bandwidth property in the optical telecommunication wavelength region. It is also demonstrated that both TM incident polarization and the circular grating cross section are easier to bring narrow reflectance bandwidth, compared with TE incident polarization and square grating cross section. Via a micro-electromechanical systems actuator, the proposed circularly cylindrical SSG is expected to serve as a narrow-bandwidth tunable bandstop filter in micro-optical telecommunication systems.     

Adaptive ultrafast nano-optics in a tight focus

We theoretically demonstrate the control of electromagnetic field properties on a sub-diffraction length scale, by polarization shaping of tightly focused femtosecond laser pulses. The field distribution in a tight focus is represented as a superposition of plane waves. The near-field of a model nanostructure is then obtained as a sum of the near-field distributions induced by the planar waves components. A self-consistent solution of Maxwell’s equations in the frequency domain yields the near-field distributions for planar wave illumination. Adaptive optimization of the incident polarization pulse shape using an evolutionary algorithm allows controlling of a number of observables, such as local nonlinear flux, simultaneous spatial and temporal control of the intensity evolution, and control of the local spectrum. The tight focusing reduces the controllability of the flux distribution in comparison to plane wave illumination. However, it is still possible to control the spatial and temporal field evolution for particular locations in the vicinity of the nanostructure. PACS 42.65.Re; 78.47.+p; 78.67.-n; 82.53.-k     

多层介质膜脉冲压缩光栅近场光学特性分析

利用傅里叶模式理论分析了TE波自准直角入射的使用条件下，多层介质膜光栅的光栅区和多层膜区电场分布的特点.分别讨论了HfO₂和SiO₂为顶层光栅材料时，光栅结构参数对光栅脊峰值电场的影响，结果表明，对于不同膜厚的顶层材料，存在一个最佳膜厚度，使光栅脊峰值电场最小，并且当膜厚增大时，设计大高宽比的光栅可以降低该电场峰值.最后，在大角度条件下使用多层膜光栅也可以降低光栅脊处的峰值电场. 关键词： 衍射光学 多层介质膜光栅 模式理论 损伤阈值