Broadband dispersion characteristics of diffractive microlenses based on the finite-difference time-domain method

The broadband dispersion characteristics of diffractive microlenses are studied using the finite-difference time-domain (FDTD) method. The distributions of the diffracted electric fields obtained through a diffractive microlens are presented for illumination wavelengths ranging from 0.35 to 1.30 μm, along with the corresponding broadband dispersion curves. It is shown that both the principal focal length and the diffracted order of the principal focal point are dependent on the illumination wavelengths. The broadband dispersion characteristics of a diffractive microlens may be used to optimise light coupling for a broadband optical fibre source.     

旋转体时域有限差分法的另一推导方法和单向波方程吸收边界条件

提出了旋转体时域有限差分法的另一推导方法.基于这一方法,推导了旋转体电磁波的波动方程,进一步得出了旋转体时域有限差分法的单向波方程吸收边界条件.数值实验证实了该吸收边界条件的正确性.     

Infrared radiative properties of two-dimensional square optical black holes

Optical black hole (OBH) is a special optical structure, in which the dielectric function or refractive index of media becomes gradually larger from the outside to the core. The circular optical black holes (COBH) have been proved remarkably useful for broadband omnidirectional light absorption. The main goal of this paper is to propose an alternative square structure of OBH due to fabrication consideration. The infrared radiative properties of two-dimensional SOBHs are numerically studied with geometric optics approximation (GOA) and the finite-difference time-domain (FDTD) method. A critical wavelength is found in this paper: when the wavelength is smaller than half of the inner core side length, the Poynting vectors obtained by the FDTD method agree well with the ray trajectories calculated by the GOA, and the absorptance is nearly 100% with a proper refractive index gradient. While the wavelength is increasing, the net energy flows do not agree with the ray trajectories and the absorptance will be decreased. The reason is attributed to the diffraction effect, which is discussed in detail in this paper.     

平面光波导结构的FDTD分析

将各向异性理想匹配层(APML)吸收边界条件用于平面光波导结构的时域有限差分法(FDTD)分析中,导出了APML媒质中适用于角域和边缘的通用差分方程,并对平行介质带定向耦合器进行了数值模拟和验证,所得数值解与解析解非常一致.该方法可用于任意复杂结构的平面光波导的计算机辅助设计与分析.     

Time-Resolved Reflectance of an Optical Pulse in Adult Heads Based on the Finite Difference Time Domain (FDTD) Analysis

Time-resolved reflectance of an optical pulse in adult head models including non-scattering cerebrospinal fluid (CSF) has been analyzed by the finite difference time domain (FDTD) analysis formulated by the authors. Averaged light intensity and mean time of flight dependences on the source-detector separations calculated by the FDTD analysis are in good agreement with previous experiments, hybrid finite element method (FEM) and Monte Carlo calculations, which justify the FDTD analysis. Based on the analysis, time-resolved reflectance sensitivities to detect optical property changes in brain have been analyzed. As a result, it has been become clear that the sensitivities to detect absorption changes of brain are enhanced in time-resolved reflectance compared to the sensitivities in averaged light intensity, whereas the sensitivities to detect scattering property changes of brain are almost the same in time-resolved reflectance and in averaged light intensity. © 2005 The Optical Society of Japan     

All-optical broadband variable optical attenuators and switches in plasmonic teeth waveguides

All-optical broadband variable optical attenuators (VOAs) and switches based on plasmonic teeth shaped waveguides have been proposed and numerically investigated. The Finite Difference Time Domain (FDTD) simulation results reveal that VOAs based on three teeth structure filled with CdSe-doped PMMA can continuously attenuate the power of a light beam from 3.7 dB to 39 dB within a broad forbidden band by an external control light. For wavelengths within the forbidden band, the structure can also operate as a switch with presence or absence of pumping light corresponding to on or off state. For a modulation depth of 27 dB, a relatively small variation in the absorption coefficient of CdSe QD-PMMA in side cavity (from κ = 0 to κ = 0.15) is enough.     

Investigation of stimulated Brillouin scattering for broadband KrF laser

Stimulated Brillouin scattering (SBS) numerical mode for broadband multimode KrF laser pump with optical breakdown has been built up in this paper. The basic premises of the model are that KrF spectrum is composed of an umber of lines and SBS arises from these lines with coupling to some extent. The broadband SBS threshold strongly depends on pump bandwidth. SBS and optical breakdown occur nearly simultaneously when broadband SBS and breakdown have the same threshold. The decrease of saturation reflectivity for broadband pump radiation is explained with this model. Experimental results agree well with the model.     

Air-clad optical fibre filament for generating broadband radiation

A simple broadband optical source is demonstrated by using a high numerical aperture air-clad optical fibre to collect broadband emission of blackbody radiation from the fibre during exposure to high intensity electric arcs.     

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%.     

适用于色敏解调的光位移传感器宽带LED

为保证光位移传感器的工作性能,输入光源在500~800 nm波长范围内需具有较高的光谱能量,照明用LED波长在700 nm以上光谱能量陡降,限制了传感器的工作范围.针对照明用LED光谱能量不足的问题,研制出适用于色敏解调光位移传感器的宽带LED光源.首先利用单色仪对光位移传感器色敏元件WS7.56的性能进行了测试,依据色敏解调结果提出了光位移传感器正常工作所需的输入光源的光谱能量阈值.在照明用LED光谱特性基础上,配比掺杂氮氧化物红色荧光粉提升LED红光及近红外光光谱能量,得到了满足输入光源光谱能量阈值的宽带LED.最后,对该宽带LED光位移传感器进行了位移测量实验,实验结果较使用照明用LED光源有明显改善,位移解调线性度良好.本文研制的宽带LED光源体积小、效率高,是光位移传感器较为理想的宽带光源.     

Digital holographic tomography based on spectral interferometry

A digital holographic tomography system has been developed with the use of an inexpensive broadband light source and a fiber-based spectral interferometer. Multiple synthesized holograms (or object wave fields) of different wavelengths are obtained by transversely scanning a probe beam. The acquisition speed is improved compared with conventional wavelength-scanning digital holographic systems. The optical field of a volume around the object location is calculated by numerical diffraction from each synthesized hologram, and all such field volumes are numerically superposed to create the three-dimensional tomographic image. Experiments were performed to demonstrate the idea.     

椭圆柱形光学黑洞红外辐射特性分析

本文主要采用时域有限差分(FDTD)方法计算了二维椭圆柱形光学黑洞的红外辐射特性。并且采用几何光学近似(GOA)对椭圆柱形的光学黑洞的光线入射轨迹进行分析和比较。研究结果表明,FDTD计算的光学黑洞内的能流分布结果与GOA计算的光学黑洞内的光线入射轨迹结果相吻合,椭圆柱形光学黑洞的红外辐射特性主要受折射率梯度和入射波长的影响。     

IR Emission of Single-Crystal Silicon Excited by Broadband Light

Room-temperature mid-IR emission from single-crystal silicon exposed to broadband (320 to 700 nm) light with power up to 150 m W has been revealed by Fourier transform IR emission spectroscopy. It is shown that broadband optical excitation significantly enhances IR emission in comparison with thermal emission. Spectral lines of lattice vibrations caused by interaction between combinations of longitudinal and transverse optical or acoustic phonons are identified in experimental IR emission spectra.     

基于法布里-珀罗腔的纳米环滤光器

同轴纳米环结构由于具有特殊的光学特性, 近年来引起了科学界的广泛关注. 本文将重点研究在以纳米环形结构为基础的法布里-珀罗腔中所存 在的两种形式的表面等离子共振, 平面型和传输型. 通过使用固定圆环阵列的周期而只改变圆环孔径大小的方法来实现调 节传输型共振并达到滤波的效果. 同时, 控制圆环阵列的周期使其足够大, 从而使得平面型共振峰位于近红外波段, 以避免对处于可见光波段的传输型共振模式形成干扰, 最终实现滤光效果. 在实验中, 通过使用周期固定为1200 nm而孔径大小从10到180 nm (以10 nm递增)的同轴圆环结构, 实现了把一束宽带的白光源分成不同颜色的单色光. 实验结果表明, 该方法解决了天线凹槽和一维层堆光栅型滤光器都普遍存在的偏振敏感性问题, 使得类似滤光器件的应用范围更广, 更能适应非偏振的自然光. 通过有限时域差分法分析得到的理论计算结果和实验结果相匹配, 实验现象得到了很好的理论支持和解释. 关键词： 表面等离子体 同轴圆环纳米腔 透射型滤光器     

二维光子晶体中与电磁波偏振态无关的自准直

利用有限时域差分法(FDTD)进行数值模拟，在二维光子晶体中实现了电磁波两种偏振态的自准直——全光自准直(full-light-self-collimation).研究表明，通过对光子晶体的结构做适当的调整，可以在较宽频率范围内实现横电波(TE波)和横磁波(TM波)沿着相同方向传播，同时保持较强的能流强度.全光自准直可以显著提高光源利用率和光波导的传播效率，在高密度集成光路中有非常重要的用途. 关键词： 光子晶体 偏振 自准直     

Broadband scattering by tapered nanoantennas

We realize a new design of tapered Yagi–Uda nanoantennas and experimentally study their far‐field light scattering. We show that in comparison with untapered structures the tapered nanoantennas exhibit broadband resonances featuring a distinct line shape. Our observations are supported by numerical calculations, which further reveal the tapered nanoantenna's wavelength‐dependent optical near‐field confinement. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

一种新型的专用于光学相干层析系统的宽带光纤光源

报道了一种新型的专用于光学相干层析系统的输出光谱为准高斯型的宽带超荧光光纤光源.该光源采用掺饵光纤作为增益介质.其关键技术是在抽运源的输出端增加了光耦合器，并在光源输出端插入多级长周期光纤光栅对铒离子的自发光谱进行调制和整形；同时采用光控器和温控器来控制抽运源的输出以提高光源输出功率的稳定性.该光源的中心波长为1.57μm，输出光谱的3dB带宽大于75nm，输出功率为27mW.实验结果表明，该光源输出光谱的自相关函数的旁瓣峰被大大削弱，可以满足光学相干层析系统的应用. 关键词： 光学相干层析术 超荧光光纤光源 长周期光纤光栅 光耦合器     

Analysis of nano-grating-assisted light absorption enhancement in metal-semiconductor-metal photodetectors patterned using focused ion-beam lithography

We present finite-difference time-domain (FDTD) analysis results of light absorption enhancement factor dependence on the profile shape of nano-gratings etched into the surfaces of metal-semiconductor-metal photodetector (MSM-PD) structures. The MSM-PDs patterned by nano-gratings are optimized geometrically, improving the light absorption near the design wavelength through plasmon-assisted electric field concentration effects. FDTD simulation results show about 50 times light absorption enhancement prediction for 850 nm light due to improved optical signal propagation through the nano-gratings in comparison with the conventional MSM-PD designs employing only a subwavelength aperture. We also report on the nano-grating profile shapes obtained typically in our experiments using focused ion-beam lithography and discuss the dependency of light absorption enhancement on the geometric parameters of nano-gratings inscribed into MSM-PDs.     

光子晶体光纤非线性光谱特性的理论与实验研究

光子晶体光纤具有特殊的导光机制和结构可调性,可以产生奇异的色散特性及高非线性,为非线性光纤光学领域的研究提供了新的条件。受多种非线性光学效应的共同作用,在不同泵浦光脉冲参数条件下,不同结构参数及传输特性的光子晶体光纤能产生丰富的非线性光谱。利用分步傅里叶方法求解非线性薛定谔方程,模拟飞秒激光脉冲在光子晶体光纤中的传输过程,获得输出光谱与入射光脉冲参数(泵浦光峰值功率P、泵浦光波长λ、光脉冲形状、光脉冲宽度T_FWHM)、光纤结构参数(孔间距Λ、空气填充比d/Λ、光纤长度z)、传输特性(色散、非线性系数)的关系,分析拉曼孤子、色散波、自相位调制等非线性效应产生的光谱特性。利用光子晶体光纤包层节区进行非线性光学实验研究,获得了孤子波和色散波的宽带光谱输出。理论分析与实验测量的光谱中都包括了波长0.5 μm附近可见光波段的蓝移色散波、0.82 μm波段的剩余泵浦光、1.1 μm波段的孤子波、2 μm附近的红移宽带色散波。理论分析与实验测量结果一致,阐明光子晶体光纤中非线性光谱产生的物理原理,实现了对宽带光谱的可控输出,为高非线性光子晶体光纤的结构设计、制备及非线性光谱的应用研究奠定基础。     

大规模集成光波导时域有限元差分法仿真的子域合成法

用时域有限元差分的子域合成法(Synthetic subdomain method of FDTD)对大规模的光波导模拟,把仿真的光波导划分成两个或多个区域,在不影响仿真精度的条件下,删除对仿真对象影响不大的区域,并相应改变吸收边界条件进行时域有限元差分法数值模拟,考查光的传播和损耗状况,并与常规方法的运算结果相比较,结果一致且不影响计算精度。与常规方法相比,两段子域合成法所占内存约为前者的55%,时间约为前者的60%,三段子域合成法所占内存约为前者的31%,时间约为前者的28%,可见子域合成法比常规方法更有利于应用于大规模集成光波导的数值模拟仿真,对节省硬件及时间资源具有实际意义,提高工作效率。