Graded photonic quasicrystals

We introduce graded photonic quasicrystals and investigate properties of such structures on the example of a Luneburg lens based on a dodecagonal photonic quasicrystal. It is shown that the graded photonic quasicrystal lens has better focusing properties as compared with the graded photonic crystal lens in a frequency range suitable for experimental realization. The proposed graded photonic quasicrystals can be used in optical systems where compact and powerful focusing elements are required.     

Kinoform单透镜的硬X射线聚焦性能

Kinoform单透镜可以高效聚焦硬X射线至纳米量级, 在X射线纳米显微学和纳米光谱学领域有着重要的应用前景. 基于衍射光学和傅里叶光学理论, 给出了X射线经由Kinoform单透镜聚焦的物理模型, 基于数值模拟, 研究了不同材料、光子能量、台阶数量和顶点曲率半径对Kinoform单透镜聚焦性能的影响. 结果表明, 孔径为1 mm的Kinoform单透镜对30 keV的X射线聚焦, 可以得到14 nm焦斑、62 μm焦深, 且可实现4个量级的光强增益和大于30%的光强透过率.     

High-resolution optical coherence tomography over a large depth range with an axicon lens

In optical coherence tomography, axial and lateral resolutions are determined by the source coherence length and the numerical aperture of the sampling lens, respectively. Whereas axial resolution can be improved by use of a broadband light source, there is a trade-off between lateral resolution and focusing depth when conventional optical elements are used. We report on the incorporation of an axicon lens into the sample arm of an interferometer to overcome this limitation. Using an axicon lens with a top angle of 160 degrees , we maintained 10-microm or better lateral resolution over a focusing depth of at least 6 mm. In addition to having high lateral resolution, the focusing spot has an intensity that is approximately constant over a greater depth range than when a conventional lens is used.     

Wavelength dependence of focusing properties of two-dimensional photonic quasicrystal flat lens

We investigated the wavelength dependence of the focusing properties of a germanium-cylinder-based two-dimensional (2D) decagonal Penrose-type photonic quasicrystal (PQC) flat lens for the first time, to the best of our knowledge. We found that near the second bandgap and in the high-frequency side (between the bandgap boundary and the first light intensity peak) of the pass band, the flat lens can exhibit a focusing effect for a point light source and that the focusing wavelengths can directly be drawn from the photonic band structure. For all the focusing wavelengths, the summation of the object distance and the image distance is less than the thickness of the flat lens when the object distance is half the thickness of the flat lens. As the wavelength increases, the image distance, the image quality, and the effective refractive index of the flat lens increase, whereas the image power of the point light source decreases. The effective refractive index of the flat lens is less than -1.     

Effect of the defect on the focusing in a two-dimensional photonic-crystal-based flat lens

We have investigated in detail the influence of defect on the focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens by using the finite-difference time-domain method. The result shows that many focusings can be observed at the symmetrical positions when a defect is introduced into the lens. Furthermore, the wave-guides in the lens can confine the transmission wave effectively and improve the quality of the focusing.     

基于近零折射率材料的声非对称聚焦透镜

研究基于近零折射率声子晶体的声非对称聚焦透镜。声非对称聚焦透镜的两侧分别为竖直平面与圆弧面结构,由于近零折射率材料的声波方向选择性,当声波从竖直平面一侧入射时,可以通过透镜,实现高性能声聚焦效应;而当声波从圆弧面一侧任意方向入射时,声波无法通过透镜,从而实现同时具有聚焦与非对称传输特性的声学透镜。在此基础上,改变透镜圆弧面的曲率半径,可以调控非对称透镜的正向聚焦焦点位置,且声非对称传输性能保持不变.此外,透镜内部的刚性散射体对声非对称聚焦性能的影响较小。研究结果表明所设计的声非对称聚焦透镜具有多功能、单一结构及高鲁棒性等优点,为设计新型近零折射率声学器件提供相应的理论方案与实验参考.      

Flat acoustic lens by acoustic grating with curled slits

We design a flat sub-wavelength lens that can focus acoustic wave. We analytically study the transmission through an acoustic grating with curled slits, which can serve as a material with tunable impedance and refractive index for acoustic waves. The effective parameters rely on the geometry of the slits and are independent of frequency. A flat acoustic focusing lens by such acoustic grating with gradient effective refractive index is designed. The focusing effect is clearly observed in simulations and well predicted by the theory. We demonstrate that despite the large impedance mismatch between the acoustic lens and the matrix, the intensity at the focal point is still high due to Fabry–Perot resonance.     

Superhigh numerical aperture (NA > 1.5) micro gradient-index lens based on a dual-material approach

We describe a novel scheme for obtaining a superhigh numerical aperture gradient-index (SHNA GRIN) lens from multiple thin layers of two or more materials with large refractive-index contrast. Design procedures for the lens are described, including variation of the layer thickness to achieve focusing and of the thickness limit to reduce scattering loss. We use an exact numerical solution by the finite-difference time-domain method to evaluate the lens's performance. Specific examples of a SHNA GRIN lens with a SiO2-TiO2 material system designed for fiber coupling to a nanowaveguide are shown to have focusing FWHM spot sizes of 0.53-0.7 microm at lambda =1.55 microm (corresponding to a NA of approximately 1.6-1.1) with 2.7-2.4% more loss than an ideal continuous index profile GRIN lens. With this approach, a SHNA GRIN lens with a NA of > 1.5 and a length of <20 microm can be achieved with currently available thin-film deposition techniques.     

Applications of Liquid Crystals to Variable-Focusing Lenses

Liquid Crystal (LC) lens cells with variable focusing properties are fabricated using nematic LC materials and applicable to optical devices, and our recent work on these cells is described. First, the LC lens cells are prepared using lens-shaped substrates coated with transparent electrodes. Their focal length can be continuously varied between the values for an ordinary ray and an extraordinary ray by changing the voltage passing across them. Methods of improving properties and some applications of the lens-shaped LC lens are briefly described. The lens properties of these cells with plane-paralleled structure are then demonstrated, where the refractive index is graded to a quadratic distribution resulting from an axially symmetric non-uniform electric field. LC cells with axially distributed tilt angles are constructed using a pair of circular hole-patterned electrode substrates and very small LC lens (LC microlens) with variable focusing can be fabricated. Optimizing the electrode structure, device parameters, and material parameters of the LCs, excellent focusing properties can be obtained. The properties of the LC microlens are improved by using the polymer stabilization technique. The LC microlens with a divided electrode structure shows three-dimensional beam steering and focusing properties, and the astigmatic aberration caused by the molecular orientation effect can be compensated. Applications of the LC microlens to optical devices and systems are introduced.     

基于蜷曲空间结构的近零折射率声聚焦透镜

研究基于蜷曲空间结构的近零折射率声聚焦透镜.根据近零折射率材料的声波方向选择机理,采用蜷曲空间结构为基本单元进行排列,设计具有特定入射与出射界面的几何结构,对透射声波的出射方向进行调控,实现了平面声波与柱面声波的聚焦效应,并深入讨论了透镜内部刚性散射体对声聚焦性能的影响.在此基础上,改变近零折射率透镜的出射界面,可以精确调控声波阵面的形状与方向.该类型透镜具有单一的单元结构、高聚焦性能及高鲁棒性等优点.研究结果为设计新型近零折射率声聚焦透镜提供了理论指导与实验参考,同时也为研究声波阵面的调控提供了新思路.     

Thermoacoustic-reflected focusing lens based on acoustic Bessel-like beam with phase manipulation

We report the realization of broadband reflected acoustic focusing lenses based on thermoacoustic phased arrays of Bessel-like beams, in which the units of phase manipulation are composed of three rigid insulated boundaries and a thermal insulation film in air with different temperatures. Based on these units, we realize a reflected focusing lens which can focus reflected acoustic energy on a line, and its fractional bandwidth can reach about 0.29. In addition, we discuss the influences of the base angle of Bessel-like beam, the number of basic unit, and the variation of unit temperature on focusing performances in details. Furthermore, the reflected focusing lens for the cylindrical acoustic wave based on the Bessel-like beam is also demonstrated. The proposed focusing lens has the advantages of a broad working bandwidth, large focus size,and high robustness, which may provide possibilities for the design and application of acoustic lenses.     

Encircled energy measurement for focusing lens based on scanning Hartmann test

The encircled energy of a focusing lens is one of the parameters directly affecting the target efficiency in high-power laser facilities. The direct measurement method of the encircled energy for the focusing lens based on the scanning Hartmann test is proposed in this paper. With the scanning Hartmann test setup, the information in the whole aperture of the focusing lens can be achieved. The encircled energy can be obtained by analyzing the spot diagram on the focal plane of the focusing lens. In experiments, the encircled energy of an aspheric focusing lens is measured using this method. The measurement result is in good agreement with that derived from measurement data by an interferometer and the difference is 7.7%.     

Laser surface patterning using a Michelson interferometer and femtosecond laser radiation

We report on a simple method to obtain surface gratings using a Michelson interferometer and femtosecond laser radiation. In the optical setup used, two parallel laser beams are generated using a beam splitter and then focused using the same focusing lens. An interference pattern is created in the focal plane of the focusing lens, which can be used to pattern the surface of materials. The main advantage of this method is that the optical paths difference of the interfering beams is independent of the distance between the beams. As a result, the fringes period can be varied without a need for major realignment of the optical system and the time coincidence between the interfering beams can be easily monitored. The potential of the method was demonstrated by patterning surface gratings with different periods on titanium surfaces in air.     

各向异性超常材料平板透镜的聚焦特性分析

运用矢量角谱理论研究各向异性超常材料平板透镜的聚焦特性,得到了光束在平板透镜内外各区域的矢量场分布,揭示了超常材料平板透镜的聚焦特性跟材料的各向异性参数之间的定量关系,发现聚焦场的偏振态因平板透镜的各向异性特性而发生改变.作为矢量理论的具体应用,分析了光轴方向磁导率为-1,其他各向异性参数均为1的超常材料平板透镜的聚焦特性,发现此类透镜对初始沿某一横向方向偏振的光束能实现部分聚焦,并发生偏振旋转现象.     

Focusing effect of a graded index photonic crystal lens

We present a new graded-index photonic crystal (GRIN PhC) structure which can be made by laser interference lithography (LIL). It can be applied as miniaturized focusing lens. Numerical simulations are carried out on the structures with different widths, lengths, and modulations. In the qualitative analysis of the focal length, focusing of a GRIN PhC was proved to originate from both the graded sizes of the rods and multimode interference. Multimode interference dominates the focusing effect when a GRIN PhC has a small length or a small difference in the rod sizes. Intensity distributions in different cases were given to support the conclusion.     

周期场透镜的成象特性

由三个或五个透镜元素组成的周期场,可以作为离子透镜使离子束成象。通常它具有比单个透镜元素强得多的聚焦能力(在相同的电压比下)。这种周期场透镜曾用于静电加速器离子源初聚系统中,但它的成象特性尚未有详细的分析。本文提出一种研究这类透镜特性的方法。将周期场看作由若干个透镜元素合成的组合透镜,然后以Ferraris特征行列式求它的基点表示式。作为普遍情况,以浸没透镜作为透镜元素,导得了三个和五个透镜元素的周期场透镜基点表示式。式中只包含透镜元素的基点参量和电极系统的几何参量。式子十分简洁,便于计算,并有足够的精确度。通过解析表示式中各个因子的分析,可以方便地掌握透镜的特性。由短磁透镜组合的周期磁场是上述普遍情况的一个特例。文中以等径小间隙双圆筒透镜为浸没透镜元素,对周期场透镜的特性用导得的基点表示式进行了计算和分析。     

Influence of line defects on focusing in a two-dimensional photonic-crystal flat lens

We investigate in detail the influence of line defects on focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens. Through simulations, we find that a focusing can always be observed when a line defect in the lens is introduced along the light transmission direction and the width of the line defect is less than \lambda /2. However, there appear two focusings when the width of the line defect is more than \lambda /2. When the line defect is introduced along the direction perpendicular to the transmission, there is always one focusing.     

Negative refractive behavior of a two-dimensional negative-index photonic crystals using a wave vector diagram method

We have theoretically studied the negative refractive behavior and the focusing effect in a two-dimensional square-lattice photonic crystal made of air rods in a dielectric background. Detailed explanations are given for the effect of the negative refraction, and the imaging of the plano-concave lens is shown by the use of a wave vector diagram formalism. The typical negative refractive behavior is demonstrated by considering the Bloch mode with the wave vector inside the first Brillouin zone, because only those wave vectors inside the first Brillouin zone of multiple Brillouin zones have a definite meaning. The single propagating beam is analyzed by the use of the wave vector diagram formalism following the folding of the wave vectors. Good-quality focusing of a plane wave can be realized by using a photonic crystal plano-concave lens, while a plane wave is formed by a point source placed at the focal point. Our results are in good agreement with the experimental ones shown for a negative-index plano-concave lens by Vodo et al. [Appl. Phys. Lett. 86 (2005) 201108]. Finally, we also have shown the focusing behavior of a plane wave and a Gaussian pulse by a plano-concave lens structure with high-index modulation instead of air in the concave region.     

Focusing surface plasmon polariton wave packets in space and time

The spatiotemporal focusing of surface plasmon polariton (SPP) wave packets (WPs) by planar plasmonic‐lens coupling structures is described using combined femtosecond interferometric time‐resolved photoemission electron microscopy (ITR‐PEEM) imaging and model simulations. The focusing properties of lens structures inscribed lithographically into Ag films depend on the angle of incidence of the excitation field. Severe aberrations are introduced by the phase delay in the interaction of obliquely incident plane waves with the commonly employed circular arc‐shaped lens structures. It is shown that the aberration can be corrected by accounting for propagation delays caused by the incidence angle‐dependent retardation of the optical field‐lens structure interaction. The focusing of SPP‐WPs in both space and time is demonstrated with aberration corrected lens structures.     

亚波长金属聚焦透镜结构参数的优化与分析

提出同时优化亚波长金属透镜结构凹槽宽度和深度,以改善亚波长金属透镜的聚焦特性.基于有限时域差分法,详细研究了凹槽宽度和深度变化对其焦斑的峰值半宽、焦斑强度、归一化聚焦效率以及焦距的影响.通过探讨金属透镜的聚焦规律及其物理机理,给出了金属透镜的优化设计方法,为利用金属透镜实现光波的纳米聚焦及灵活操控提供了理论基础.