Polarization-independent and high-diffraction-efficiency Fresnel lenses based on blue phase liquid crystals

A polarization-independent and high-diffraction-efficiency Fresnel lens is developed based on blue phase liquid crystals (BPLCs). The optically isotropic characteristic of BPLCs is used to produce a polarization-independent Fresnel lens. The small optical phase shift of BPLCs that is induced by the Kerr effect is sufficient for the BPLC Fresnel lens to have high theoretical and experimental diffraction efficiencies of 41% and ～34%, respectively. An electrically erasable memory effect in the focusing diffraction at an electric field E>4.44?V/μm is observed. The electro-optical properties of the BPLC Fresnel lens are analyzed and discussed.     

Terahertz wave tomographic imaging with a Fresnel lens

We demonstrate three-dimensional tomographic imaging using a Fresnel lens with broadband terahertzpulses.Objects at various locations along the beam propagation path are uniquely imaged on the sameimaging plane using a Fresnel lens with different frequencies of the imaging beam.This procedure allowsthe reconstruction of an object's tomographic contrast image by assembling the frequency-dependentimages.     

软X射线Bragg-Fresnel二维聚焦镜的设计理论和方法

Ｘ射线Ｂｒａｇｇ－Ｆｒｅｓｎｅｌ元件是近十年发展起来的一种用于Ｘ射线波段的新型光学元件。本文介绍一种具有二维聚焦效能的Ｂｒａｇｇ－Ｆｒｅｓｎｅｌ元件的设计理论和方法。设计中采用衍射理论并用Ｃ语言编制计算机程序系统对衍射图形进行设计。对于使用波长１３．８ｎｍ，入射角５°，使用焦距２００ｍｍ的使用条件，设计结果为３５Ｍｏ／Ｓｉ双层多层膜，周期厚度７．１４ｎｍ，理论反射率约为６０％；衍射图形共４００个环带，最内环半宽度５２．５μｍ，最外环宽度１．３μｍ。     

Si_3N_4波导上的费涅尔透镜

本文提出一种新型的费涅尔波导透镜.它不仅具有衍射受限的聚焦特性,而且有与理想费涅尔波导透镜几乎一样的效率,并使工艺简化.在氮化硅波导上进行的工艺实验表明,实验结果与理论分析具有较好的一致性.     

Theoretical calculations of a square multilayer Bragg–Fresnel lens by quantum theory

A theoretical method based on the quantum scattering theory is presented to evaluate the performances of a two-dimensional (2-D) focusing square multilayer Bragg–Fresnel lens. The numerical application results of the square multilayer Bragg–Fresnel lens working at 0.7 nm wavelength (W/Si 25 periods with a double layer thickness of 5.38 nm, the size of the diffraction pattern is about 291×291 μm, the size of the center square in the diffraction pattern is 21.4×21.4 μm, and the size of the smallest square in the diffraction pattern is 0.39×0.39 μm) are given. Our theoretical results are compared with the experimental results of the linear Bragg–Fresnel lens reported by other researchers; an analysis and a discussion are carried out regarding the advantages of an optical system based on the 2-D focusing square multilayer Bragg–Fresnel lens, in contrast to a Kirkpatrick–Baez optical system on the basis of a two-linear Bragg–Fresnel lens.     

Narrowing behavior of chirped pulses with small Fresnel number in focusing systems

Based on the diffraction integral and Fourier transform, the fields of chirped pulses passing through a thin lens are derived. Influences of Fresnel number and chirp parameter on the transverse intensity distributions of the chirped pulses at focal plane are illustrated with numerical results. Also, effect of pulse duration on the intensities is investigated. It is found that the width of transverse intensity distribution of the chirped pulses decreases with increasing the chirp parameter and the narrowing behavior presents more obvious with smaller Fresnel number. A physical explanation for the narrowing behavior is given on the basis of a simplified form of the field expression.     

基于液晶空间光调制器的太赫兹波频谱调制

频谱可调制的太赫兹波具有广泛的应用价值。利用一台纯相位式的液晶空间光调制器对飞秒激光脉冲进行空间整形,通过改变飞秒激光脉冲的横向空间分布,实现太赫兹波频谱的调制。在实验中,利用光泵浦整流方式产生太赫兹波,并利用太赫兹时域光谱系统对太赫兹信号进行探测。通过GS算法在液晶空间光调制器上加载不同的相位图,获得了不同的飞秒激光脉冲横向空间分布。通过改变探测距离和飞秒脉冲的空间分布参数,实现了太赫兹波频谱的调制。还利用菲涅尔衍射算法对这一过程进行了理论模拟,理论模拟结果与实验结果吻合的较好,这充分说明了基于飞秒脉冲空间整形的太赫兹光谱调制技术的可行性。     

Fresnel light modulator with an array of channel waveguides

One-dimensional focusing and optical modulation have been achieved using a Fresnel light modulator (FLM) based on a Fresnel lens consisting of Ti-diffused channel waveguides on LiNbO₃. By this device, the height of diffraction pattern near the focal point is modulated with an applied voltage while the focusing effect is almost kept. The experimental results obtained from the FLM with 13 zones at 633 nm are as follows: the focal length is 12.4 cm, the full width of beam at half power 47 μm, the half-wave voltage 28 V, the extinction ratio 10.9 dB and the insertion loss 11.6 dB.     

高斯光束经过负球差透镜的聚焦

本文讨论高斯光束经过负球差透镜的聚焦，得到了聚焦光场轴上点的光强分布的表示式，并对其进行数值计算。数值计算结果表明，在负球差的情况下，得到光强的最大点位置（最佳聚焦点）相对于无球差情况，往透镜方向移动，并且，最佳聚焦点的光强较无球差时大，当Ｎｗ＝１，Ｎａ＝５，ｋＳ１＝－０．２时，得到的最佳聚焦点的光强是无球差时的１．３４倍。本文还讨论了透镜的菲涅尔数对聚焦光场分布的影响。结果表明，当透镜的菲涅尔数小于１时，球差对高斯光束的聚焦的影响可以忽略。     

Fresnel formulas for the forced electromagnetic pulses and their application for optical-to-terahertz conversion in nonlinear crystals

We show that the usual Fresnel formulas for a free-propagating pulse are not applicable for a forced terahertz electromagnetic pulse supported by an optical pulse at the end of a nonlinear crystal. The correct linear reflection and transmission coefficients that we derive show that such pulses can experience a gain or loss at the boundary. This energy change depends on linear dielectric constants only. We also predict a regime where a complete disappearance of the forced pulse under oblique incidence occurs, an effect that has no counterpart for free-propagating pulses.     

Proposal and analysis of artificial dielectric lens with metallic corrugated structures for terahertz wave band

Optical devices for the terahertz wave band are being developed now and require better designs. This paper proposes an artificial dielectric lens with metallic corrugated structures for the terahertz wave band. A periodic analysis model extracted from the full model by assuming periodicity confirms the phase delay, which produces the focusing effect. Full model analysis also confirms the focusing effect. The full model analysis also confirms that the focusing length is longer as the spacing of corrugated baffles is wider. The focusing length is longer the metallic groove width is wider. The focusing length is longer as the groove depth is shallower. The lens shape without grooves does not produce the focusing effect. The results of the full model analysis are qualitatively consistent with those of the periodic model ones. This implies that the design for an exact size lens is possible by using the periodic model.     

高功率激光通过有中心遮拦透镜的聚焦

用广义惠更斯－菲涅耳衍射积分对有振幅调制和位相畸变的高功率激光束通过有中心遮拦透镜的聚焦特性作了详细研究，并对焦面处光束的可聚焦性进行了讨论。数值计算结果表明，功率集中度与光阑的遮拦比，光束截断参数以及振幅调制和位要畸变参数等有关。     

Propagation of vectorial Gaussian beams behind a circular aperture

Based on the vectorial Rayleigh diffraction integral and the hard-edge aperture function expanded as the sum of finite-term complex Gaussian functions, an approximate analytical expression for the propagation equation of vectorial Gaussian beams diffracted at a circular aperture is derived and some special cases are discussed. By using the approximate analytical formula and diffraction integral formula, some numerical simulation comparisons are done, and some special cases are discussed. We find that a circular aperture can produce the focusing effect but the beam becomes the shape of ellipse in the Fresnel region. When the Fresnel number is equal to unity, the beam is circular and the focused spot reaches a minimum.     

柱面球差透镜聚焦部分相干光束的焦移

通过定义聚焦光束的等效菲涅耳数,研究了部分相干光经柱面球差透镜聚焦后的轴上光强分布,得到了相对焦移的简单公式,即相对焦移反比于等效菲涅耳数的平方.结果表明,这种基于等效菲涅耳数的计算聚焦光学系统的焦移方法是计算部分相干光束经球差透镜聚焦相对焦移简单且有效方法.还讨论了一种特殊等效菲涅耳数为纯虚数情况.结果证明,在这种情况下,该相对焦移简单公式仍然有效.     

Analysis of lateral-spread photorefractive planar lenses

The photorefractive planar lens for converting a vertical incident plane wave to a lateral-spread spherical wave and vice versa, is suggested. Using the two-beam coupled-wave theory, the coupled wave equations are derived and their half-analytical solutions are also given in terms of an infinite series. The diffraction properties (beam profiles, diffraction efficiency) of the local volume grating in the lens are presented. And the focusing property of the lens is discussed and compared with that of an ideal convergent spherical wave. It is demonstrated that the suggested photorefractive planar lens shows a good focusing effect.     

Super-Enhancement Focusing of Teflon Spheres

A Teflon (polytetrafluoroethylene) sphere can be used as a focusing lens in the applications of imaging and sensing due to its low-loss property in the terahertz band. Herein, field intensities and focusing parameters are analytically calculated for Teflon spheres at different low-loss levels and then a super-enhancement focusing effect in the spheres with particular size parameters was discovered, which can stimulate about 4000 times stronger field intensity than that for incident radiation as well as the great potential of overcoming diffraction limit despite high sensitivity to the magnitude of Teflon loss. A subsequent analysis of scattering amplitudes proves that the strong scattering of a single-order mode in the internal electric or magnetic field is the main factor causing this phenomenon.     

Parallel plate lens with metal hole array for terahertz wave band

Optical devices for terahertz wave band from 0.1 to 10 THz are rapidly expanding and require better designs. This paper proposes and designs a parallel plate lens with metal hole array for the terahertz wave band. The fast wave effect is due to the parallel plate. For this lens, the parallel plate spacing and hole array dimensions control the phase velocity and the focusing effect. It is not necessary to control the phase through the lens shape, which is flat, itself. The periodic analysis model extracted from the full model confirms the phase control by the metal hole array dimensions. The periodic model can be used for efficient iterative design. The full wave analysis results are also obtained by ANSYS HFSS and the focusing effect is confirmed. Phase control using both the parallel plate and the hole array enhances the focusing effect over the focusing effect controlled only by the metal hole array dimensions.     

部分相干光被环形透镜聚焦而产生的焦移

 从理论上研究部分相干光被一环形透镜聚焦，在焦点附近的轴上点的光强分布。研究结果表明，当部分相干光被一环形透镜聚焦时，最大聚焦光强不在几何焦点，而是位于透镜与几何焦点之间，出现焦移现象。并且，焦移量不仅依赖于透镜外半径的菲涅耳数，还依赖于部分相干光的空间相干度和中心拦截比。透镜的菲涅耳数越小，焦移越大；部分相干光的空间相干度越低，焦移越大。当菲涅耳数一定时，环形透镜的中心拦截比越大，焦移越大；当空间相干度很小时，情况就变得比较复杂。     

Nanometer linear focusing of hard x rays by a multilayer Laue lens

We report on a type of linear zone plate for nanometer-scale focusing of hard x rays, a multilayer Laue lens (MLL), produced by sectioning a multilayer and illuminating it in Laue diffraction geometry. Because of its large optical depth, a MLL spans the diffraction regimes applicable to a thin Fresnel zone plate and a crystal. Coupled wave theory calculations indicate that focusing to 5 nm or smaller with high efficiency should be possible. Partial MLL structures with outermost zone widths as small as 10 nm have been fabricated and tested with 19.5 keV synchrotron radiation. Focal sizes as small as 30 nm with efficiencies up to 44% are measured.     

Study of terahertz pulses at an edge

The propagation behaviour of terahertz （THz） pulses at an edge is characterized. The phenomenon that the amplitude oscillates periodically in the frequency spectrum is similar to Young＇s interference, if the absorption effect is neglected. The oscillation cycle is shorter for a thicker sample. THz pulses at an edge are analyzed by the broadband Huygens-Fresnel diffraction integral. The experimental results are in agreement with the simulation results approximately. The simulation errors are also analyzed.