Talbot effect of a high-density grating under femtosecond laser illumination

The Talbot effect of a high-density grating under femtosecond laser illumination is analyzed with rigorous electromagnetic theory which is based on the Fourier decomposition and the rigorous coupled-wave analysis (RCWA). Numerical simulations show that the contrast of the Talbot images steadily decreases as the transmitted femtosecond laser pulses propagate forward and with wider spectrum width of the femtosecond laser pulses. The Talbot images of high-density gratings have much higher sensitivity of the spectrum widths of the incident laser pulses than those of the traditional low-density gratings. In experiments, the spectrums and the pulse widths of the incident pulses are measured with a frequency-resolved optical grating (FROG) apparatus. The Talbot images are detected by using a Talbot scanning near-field optical microscopy (Talbot-SNOM) technique, which are in coincidence with the numerical simulations. This effect should be useful for developing new femtosecond laser techniques and devices.     

多层介质膜光栅用高反射镜的严格耦合波分析

基于严格耦合波和增强透射矩阵的方法,提出了一种数值稳定的求解多层介质膜光学特性的机理模型.利用该模型计算了多层介质膜光栅用高反射镜的优化设计膜系.使用电子束热蒸发方式制备的多层介质膜光谱特性和理论设计的结果符合得很好,该严格耦合波模型是分析介质膜光学特性有效的稳定的数值求解方法.     

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.     

利用保角变换实现环形光栅的Talbot效应

Talbot效应是一种近场自成像效应,通常只有周期光栅可以产生Talbot效应,而环形光栅无法产生.本文通过引入保角变换,发现可以在环形光栅外部设计适当的折射率渐变层介质,使得其中也能够产生严格的Talbot效应,并计算了对应的自成像半径表达式.本文利用FDTD软件分别将一个环形光栅放置在真空中以及人工设计的折射率渐变层中进行了模拟,并对二者的结果进行了比较分析,发现这种折射率渐变层介质确实对点光源入射的环形光栅的自成像情况有着很好的改善.希望这一工作能够推广Talbot效应的应用范围.     

The influence of the polarization effect in Talbot self-imaging of high-density gratings

The Talbot self-imaging of high-density gratings with different period at half Talbot distance for different polarization is analyzed with the finite-difference time-domain (FDTD) method. The numerical results indicate that the Talbot self-imaging of high-density gratings is obviously different for different polarization when the period d of the grating between 2λ and 3λ, which is verified through experimental results with the scanning near-field optical microscopy (SNOM) technique. Furthermore, the Fourier spectrum (far field) generated by the gratings is also given with the rigorous coupled-wave method, which is in agreement with the near field.     

Multiple Beam Scanning Optics for Laser Printer: Application of Optical Fiber Array Method

We have developed laser printer optics using various kinds of multiple beam generation devices, such as a polarization prism, grating, diode laser array, and optical fiber array. Recently we successfully demonstrated laser-print experiments using five beams from an optical fiber array. In the array, fibers are arranged with a spacing of 0.15 mm and emit circular Gaussian beams that have a diameter of 5 μm and a wavelength of 635 nm. The wavelength of diode lasers is determined from sensitivity of the photoconductor drum of As₂Se₃ material. The arrangement error vertical to the array is mostly suppressed below 0.4 μm. In the optics, curve of scan lines and print position errors in the scan direction are suppressed to a negligible size due to minimization of the magnification variation in the sub-scan direction along the print width and achromatized design, respectively.This paper was originally presented at the 2nd International Conference on Optical Design and Fabrication, ODF2000 which was held on November 15-17, 2000 at the International Conference Center, Tokyo, Waseda University, Japan.     

提高光栅衍射计算收敛性的途径

从数学原理、几何分层,以及数值计算三方面,分析了耦合波法在计算光栅衍射时的收敛性问题,提出了改善计算收敛性的途径.并明确指出对于不同的光栅问题,影响计算收敛性的主要因素是各不相同的,计算时应该针对具体的问题采用相应的改善方法. 关键词： 耦合波法 校正傅里叶展开 多层近似 收敛     

Metal-mirror-based grating for high-efficiency element in dense wavelength division multiplexing

It is desirable to obtain high efficiency with polarization-independence and wideband properties for incident wavelength. A metal-mirror-based grating is presented to diffract the incident wave into reflection orders with high efficiency for TE and TM polarization. The modal method and rigorous coupled-wave analysis (RCWA) are used together to optimize a metal-mirror-based grating effectively. From the analysis of the modal method, it is feasible to realize such a grating with the prescribed grating duty cycle and period. Accurate parameters of the grating depth and thickness of the connecting layer are optimized using RCWA. Compared with the reported binary simple grating, high efficiency can be improved greatly for the incident wavelength of 1550 nm in dense wavelength division multiplexing. The diffraction investigation indicates that a wideband property for incident wavelength can be obtained for such a novel metal-mirror-based grating.     

基于RCWA的太阳能电池抗反射膜分析研究

利用严格耦合波分析法（RCWA）分别计算了五种不同形貌的亚波长浮雕结构的太阳能电池抗反射膜性能,分别从占空比、光栅高度和周期等结构参数以及入射光角度进行模拟。研究结果表明：周期为200rim,高度为400nm,占空比为1的金字塔型浮雕结构的抗反射效果令人满意,且平均反射率低于1％;光波的入射角对光栅的反射率影响较大,光波在光栅法线的角度控制在40°以下合适。通过对亚波长浮雕结构的反射特性模拟和分析,为太阳能电池抗反射膜设计与制作提供理论支持。     

Unveiling Talbot Effect under Fresnel Diffraction at a Fork-Shaped Grating

The near-field effect of diffraction image self-reproduction or self-imaging of a periodic grating illuminated by quasi-monochromatic wave is well-known as the Talbot effect. Introducing a dislocation to a periodic structure provides a fork-shaped modulation of the phase/amplitude, which produces discrete diffraction pattern in a far-field consisting of optical vortices. In this paper, Fresnel diffraction at amplitude fork-shaped grating is theoretically and experimentally studied. The coexistence of spatial ordering and local violation of translational symmetry of the structure manifests itself in a strict diffraction pattern consisting of optical vortices in the far-field, which is shown to be accompanied by formation of a spatially ordered intensity distribution in the near-field, reminiscent the Talbot carpets for periodic structures. These results demonstrate the first evidence of Talbot effect occurred under light diffraction at fork-shaped gratings, being promising for deep understanding of near-field singular optics phenomena.     

Unified design of wavelength-independent deep-etched fused-silica gratings

We present a unified design of wavelength-independent deep-etched fused-silica gratings as polarizing beam splitters and polarization-independent two-port beam splitters by using the simplified modal method. By defining unified grating parameters as the ratio of incident wavelength to grating period and the ratio of groove depth to grating period, unified grating structures are found to be approximately wavelength-independent, which is based on the modal view of the accumulated phase difference of two excited propagating grating modes. Diffraction efficiencies given by the rigorous coupled-wave analysis (RCWA) verified this unified design at the wavelength of 1064 nm. It should be noted that this unified design of wavelength-independent gratings is an analytic result, which is impossible to be derived with the well-known numerical RCWA. Modal method is powerful and presents a clear physical picture for us to obtain this unified design. Therefore, this unified design can be used as a general guideline for designing deep-etched fused-silica gratings as beam splitters for practical applications from ultraviolet to near infrared.     

Modelling gratings on either side of the substrate for light extraction in light-emitting diodes

We present a three-dimensional model based on rigorous coupled wave analysis (RCWA) which allows us to study the influence of periodic structures on light extraction in light-emitting diodes. The gratings can be placed both at the interface between the emitting layers and the substrate, where they coherently interact with the dipole field, or at the interface between the substrate and air, where multiple incoherent reflections in the thick substrate contribute to the overall extraction. Even in the case of a grating at the interface between the substrate and air, these multiple incoherent reflections in the substrate still contribute to the overall extraction for large devices, an effect which has been mostly ignored in literature FDTD simulations.     

Blur Reduction in the Focal Plane of Optical Receivers by Exploiting the Negative Refractive Behavior of Photonic Crystals

We present a passive method for reducing the size of a focal spot blurred by wavefront distortions in an optical system. The key to this reduction lies in the rather unique behavior of photonic crystals to exhibit negative refraction. First, we review the design of such a photonic crystal structure and analyze its dispersion property to find the proper condition where negative refraction occurs. The predictions derived from theoretical infinite structures are confirmed by rigorous electromagnetic analysis using both the finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA) methods in combination with more realistic finite structures. Second, we discuss an unconventional approach to exploit the negative refractive behavior in reducing the focal spot area spread caused by a blurred wavefront. A proof-of-principle calculation of this approach is performed with the ray trace method to visualize the reduction of the illumination area while preserving the propagation angle of the incoming beam. Finally, we investigate the crystal’s performance when its two-dimensional structure is exposed to conical illumination.     

A Simulation of Near Field Optics by Three Dimensional Volume Integral Equation of Classical Electromagnetic Theory

A numerical simulation code for three dimensional problems of near-field optics has been developed using the volume integral equation with the moment method. The object is assumed to be continuous and macroscopic dielectric and can be treated by macroscopic Maxwell#x0027;s equations. The code can treat the large-scale moment method matrix that is obtained by the discretization of the volume integral equation. The resultant matrix equation is solved by an iteration method called the generalized minimum residual method with reasonable computational cost for simple problems of near field optics. Simulation of a simplified model of a scanning near-field optical microscope has been performed and basic polarization characteristics of the system have been investigated in detail. The code is also applied to the collection-mode of a photon scanning tunneling microscope, where the incident wave is the evanescent wave, and basic relation between near-field and far field i.e., output image, is recognized.     

亚波长金属偏振分束光栅设计分析

结合有效介质理论和薄膜光学的抗反射设计方法,设计了基于0.65μm工作波长的亚波长金属偏振分束光栅,给出了光栅的优化设计参数,采用严格耦合波理论分析了光栅的偏振分束特性.结果表明,亚波长金属光栅对TE偏振表现为金属膜特性,具有高反射,对TM偏振表现为介质膜特性,具有高透射,在-30°<θ<30°的大入射角范围和0.47μm<λ<0.80μm的宽入射波谱内,该光栅的透射光和反射光均具有高偏振消光比和低插入损耗的特点. 关键词： 亚波长金属偏振分束光栅 有效介质理论 薄膜光学 严格耦合波理论     

滑动盖板对一维微尺度光栅红外辐射特性影响

电磁波入射到微尺度周期性结构表面时,一定条件下将在某一波段产生异常光吸收现象。基于该特性,本文提出了单边移动盖板光栅和双边移动盖板光栅两种光栅结构,并使用严格耦合波分析(RCWA)方法对TM波垂直入射的一维微尺度光栅表面辐射特性进行了分析,研究了滑动盖板的厚度和位置对光栅表面结构辐射特性的影响。结果表明,相同盖板厚度,双边移动盖板光栅形成的发射率峰较单边移动盖板光栅形成的发射率峰具有较宽的半高全宽(FWHM),但峰值较低;随盖板向关闭侧移动,高发射率峰对应波长向长波方向移动;随盖板厚度增加,发射率峰向长波方向移动,其中双边移动盖板光栅结构形成的发射率峰变宽,峰值变小。     

Modal analysis of deep-etched low-contrast two-port beam splitter grating

Modal analysis of a deep-etched low-contrast two-port beam splitter grating under Littrow mounting is presented. The guideline for the design of a subwavelength transmission fused-silica phase grating as high-efficiency grating, polarizing beam splitter (PBS), and two-port beam splitter, is summarized. As an example, a polarization-independent two-port beam splitter grating is designed at wavelength of 1064 nm. We firstly analyzed the physical essence of the grating by the simplified modal method. The guideline for the grating design and the approximate grating parameters are obtained. Then using the rigorous coupled-wave analysis (RCWA) with parameters varying around the approximate ones, optimum grating parameters can be determined. With the design guideline, the time for the rigorous calculation of the grating profile parameters can be reduced significantly.     

A Novel Fabrication Technique of Long Period Fiber Gratings Using a Holographic Optical Element

We develop a new flexible and precise ultraviolet exposure technique to fabricate a long period fiber grating using a computer-generated holographic optical element (HOE). The HOE can generate a striped beam pattern with about a 300 #x03BC;m period from a KrF excimer laser. This method offers (1) ease in varying the grating period and (2) grating period precision. We achieve a grating period within an accuracy of #x00B1;0.04 #x03BC;m. This means that the peak wavelength can be controlled within an accuracy of #x00B1;0.2nm with an operating wavelength of 1.55 #x03BC;m.This paper was originally presented at the 2nd International Conference on Optical Design and Fabrication, ODF2000 which was held on November 15-17, 2000 at the International Conference Center, Tokyo, Waseda University, Japan.     

A Practical See-Through Head Mounted Display Using a Holographic Optical Element

This paper describes a practical method for design and fabrication of holographic optical elements (HOEs) as off-axis imaging optics. The authors have designed and fabricated a compact glass-like and see-through display using HOE and a total internal reflection prism. Observers can see through the display because wavelength selectivity of the HOE provides high transparency. Wave front of the HOE is designed by commercial software, though the actual fabrication method is not provided. The authors propose a practical method to fabricate HOEs, which can be applied to a wide range of imaging optics.This paper was originally presented at the 2nd International Conference on Optical Design and Fabrication, ODF2000 which was held on November 15–17, 2000 at the International Conference Center, Tokyo, Waseda University, Japan.     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.