Optical beam splitting and asymmetric transmission in bi-layer metagratings

In this work, inspired by advances in twisted two-dimensional materials, we design and study a new type of optical bi-layer metasurface system, which is based on subwavelength metal slit arrays with phase-gradient modulation, referred to as metagratings(MGs). It is shown that due to the found reversed diffraction law, the interlayer interaction that can be simply adjusted by the gap size can produce a transition from optical beam splitting to high-efficiency asymmetric transmission of incident light from two opposite directions. Our results provide new physics and some advantages for designing subwavelength optical devices to realize efficient wavefront manipulation and one-way propagation.     

Near-Zero-Sidelobe Optical Subwavelength Asymmetric Focusing Lens with Dual-Layer Metasurfaces

Existing metasurfaces with ultrathin volume for asymmetric transmission are often constructed from metal with low efficiency in optical frequencies, and cannot realize optical asymmetric transmission and focusing simultaneously. Although acoustic asymmetric focusing in asymmetric focusing lenses (AFL) has been realized, the obtained focal point is accompanied by a large sidelobe. To solve these problems, an AFL with dual-layer metasurfaces designed by using impedance matching materials can realize optical asymmetric transmission and focusing simultaneously in this paper. Furthermore, through optimizing the design theory of AFL, near-zero-sidelobe (NZS) focusing can be realized on the subwavelength scale. NZS asymmetric focusing of the optimized AFL is effective in broadband, which is expected to show tremendous potential in optical micro-nanoprocessing, quantum communication, optical micromanipulation, etc.     

Light transmission through metal films perforated with arrays of asymmetric cross-shaped hole

The transmission characteristics of metallic film perforated with an array of asymmetric cross-shaped hole are studied by using the three-dimensional finite difference time-domain method. We find that the wavelengths and intensities of transmission peaks depend strongly on the asymmetric parameters of the cross-shaped hole. The transmission peaks in the structure of asymmetric cross-shaped hole array originate from the splitting of the transmission peak in the corresponding one of symmetric cross-shaped hole array. Moreover, it is also found that the transmission spectra can be adjusted by changing other geometrical parameters of asymmetric cross-shaped hole due to the their effect on the distribution of the oscillating charges on metal surfaces.     

Valley splitting and anomalous Klein tunneling in borophane-based n-p and n-p-n junctions

We analytically investigate the transport properties of electron in borophane-based n-p and n-p-n junctions. When the electron beam in n region go through the n-p junction, the beam will be split into two valley-dependent beam in p region. This comes from the valley-dependent refraction angle induced by the anisotropic band structure of borophane. Interestingly, the behavior of valley splitting can be generated in a borophane-based n-p junction naturally without any external modulation methods. Generally, the Klein tunneling is described as the perfect transmission at a zero incident angle of electron regardless of the width and height of potential barrier. However, in a borophane-based n-p-n junction, the anomalous Klein tunneling, i.e., the perfect transmission exists at a nonzero incident angle, is found due to the anisotropic band structure of borophane. Our work designs an alternative valley splitter and provides an insight into the understanding of the Klein tunneling.     

可调分束角棱镜分束光强比研究

利用相位匹配条件,坐标转换和菲涅耳公式推导出了可调分束角棱镜的光强分束比的数学表达式,分析了光束的入射角、棱镜的结构角,以及入射波长对光强分束比的影响,给出了使光强分束比为1的条件,并进行了实验测试,测试结果与理论分析相一致,为设计出射光强相等或近似相等的可调分束角棱镜提供了理论依据。     

各向异性超常材料的偏振分离特性

提出了一种利用含有负介电常量和磁导率张量的各向异性超常材料实现偏振分离的方法。通过分析电磁波在无损耗各向异性超常材料中的传输性质以及透射率与入射角度的关系,分类比较了由不同符号的介电常量和磁导率张量组合而成具有不同波矢面的各向异性超常材料的偏振分离特性,比较结果表明波矢面为单叶双曲面和椭球双叶双曲面的各向异性超常材料的偏振分离特性强。前者可以实现入射波中的s分量和p分量中某一分量正折射,另一分量负折射,且在一定条件下可以同时全透射,从而实现大角度偏振分离。后者在一定条件下可实现入射波中的s分量和p分量某一分量全反射,另一分量全透射,从而实现偏振分离。最后,对从各向同性介质入射到这两种各向异性超常材料中的高斯光波的传输进行了模拟计算,结果表明这两种各向异性超常材料可以实现很好的偏振分离功能,有潜力成为新型的偏振分离元件。     

Study on symmetry of output beam of corner-pumped solid-state fundamental-mode laser

Aimed at solving the problem of the asymmetry of the output beam of corner-pumped solid-state fundamental-mode lasers, a simple resonant cavity design is presented. Using this method, no optical element needs to be inserted into the cavity. The symmetry of the output beam can be improved effectively by changing cavity parameters. The validity of this method is demonstrated by a corner-pumped Nd:YAG/YAG composite slab Q-switched fundamental-mode laser. The asymmetry of the output beam can be corrected effectively by adjusting the short arm length of the linear cavity and a symmetric output beam is obtained. The resonant cavity design method is simple and practical, which also applies to other lasers with the asymmetric output beam resulting from asymmetric cooling or asymmetric pumping.     

Field-assisted spin-polarized electron transport through a single quantum well with spinben orbit coupling

We have investigated theoretically the field-driven electron transport through a single-quantum-well semiconductor heterostructure with spin-orbit coupling. The splitting of the asymmetric Fano-type resonance peaks due to the Dresselhaus spin-orbit coupling is found to be highly sensitive to the direction of the incident electron. The splitting of the Fano-type resonance induces the spin-polarization dependent electron current. The location and the line shape of the Fano-type resonance can be controlled by adjusting the energy and the direction of the incident electron, the oscillation frequency, and the amplitude of the external field. These interesting features may be used to devise tunable spin filters and realize pure spin transmission currents.     

Field-assisted spin-polarized electron transport through a single quantum well with spin-orbit coupling

We have investigated theoretically the field-driven electron transport through a single-quantum-well semiconductor heterostructure with spin-orbit coupling.The splitting of the asymmetric Fano-type resonance peaks due to the Dresselhaus spin-orbit coupling is found to be highly sensitive to the direction of the incident electron.The splitting of the Fano-type resonance induces the spin-polarization dependent electron current.The location and the line shape of the Fano-type resonance can be controlled by adjusting the energy and the direction of the incident electron,the oscillation frequency,and the amplitude of the external field.These interesting features may be used to devise tunable spin filters and realize pure spin transmission currents.     

Ultra-broadband asymmetric acoustic transmission with single transmitted beam

We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the asymmetric excitation of the leaky asymmetric zero-order Lamb mode in the brass plate induced by the incident angle of external bulk waves. The results show that the bandwidth of the asymmetric acoustic transmission could reach 2000 k Hz, and the positive transmitted wave is only a single acoustic beam. The device has the advantages of ultra-broadband, single transmitted beam,and simpler structure, which has great potential applications in ultrasonic devices.     

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene.In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance(TMR) effect.The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.     

Optical phase front control in a metallic grating with equally spaced alternately tapered slits

A technique capable of focusing and bending electromagnetic （EM） waves through plasmonic gratings with equally spaced alternately tapered slits has been introduced. Phase resonances are observed in the optical response of transmission gratings, and the EM wave passes through the tuning slits in the form of surface plasmon polaritons （SPPs） and obtains the required phase retardation to focus at the focal plane. The bending effect is achieved by constructing an asymmetric phase front which results from the tapered slits and gradient refractive index （GRIN） distribution of the dielectric material. Rigorous electromagnetic analysis by using the two-dimensional （2D） finite difference time domain （FDTD） method is employed to verify our proposed designs. When the EM waves are incident at an angle on the optical axis, the beam splitting effect can also be achieved. These index-modulated slits are demonstrated to have unique advantages in beam manipulation compared with the width-modulated ones. In combination with previous studies, it is expected that our results could lead to the realization of ootimum designs for plasmonic nanolenses.     

Positively and negatively large Goos–Hänchen lateral displacements from a symmetric gyrotropic slab

A detailed study on the lateral displacements of a transverse magnetic (TM) wave transmitted and reflected from a symmetric gyrotropic slab is presented. We give the analytic formulas for the transmission coefficient and the reflection coefficient, as well as the corresponding lateral displacements. It is found that due to the external magnetic field the displacement of a transmitted beam is different from that of reflected one, even for a lossless symmetric configuration. Furthermore, within the chosen frequency band, when the incident angle is near the Brewster angle, the shift of a reflected wave can be large with nonzero reflectance, and can be positive or negative depending on the direction of the applied magnetic field and the incident wave.     

新型光学长程吸收池的仿真设计研究

为了解决传统的长程光学吸收池设计中镜面利用率低导致反射次数少的问题,根据传输矩阵理论,开展了离散型Herriott池的结构参数设计研究。基于MATLAB数值计算模拟详细研究了吸收池内光线的传输次数与机械结构参数、光线的入射位置及入射角之间的依赖关系。仿真结果为实际应用中设计便携式长程光学吸收池提供良好的参数和模型借鉴作用。     

Transparent and Ultra-lightweight Design for Ultra-Broadband Asymmetric Transmission of Airborne Sound

Acoustic one-way manipulations have recently attracted significant attention due to the deep implications in many diverse fields such as biomedical imaging and treatment.However,the previous mechanisms of asymmetric manipulation of airborne sound need to use elaborate heavyweight structures and only work in certain frequency ranges.We propose a mechanism for designing an ultra-lightweight and optically transparent structure with asymmetric transmission property for normally incident plane waves.Instead of fabricating solids into complicated artificial structures with limited bandwidth and heavy weight,we simply use xenon to fill a spatial region of asymmetric shape which allows the incident plane wave to pass along one direction while reflecting the reversed wave regardless of frequency.We demonstrate both analytically and numerically its effectiveness of producing highly-asymmetric transmission within an ultra-broad band.Our design offers new possibility for the design of one-way devices and may have far-reaching impact on various scenarios such as noise control.     

基于一维金属-介质周期结构的偏振分束

分析了一维金属一介质周期结构的能带特性,根据一定频率范围内TM波(磁场方向与界面平行)在结构中的负折射以及TE波的正常折射,提出了一种偏振分束器件.利用传输矩阵法(TMM)模拟了该结构对入射高斯光束的偏振分束作用,讨论了不同入射角度下的偏振分束能力,并结合实际金属参量,分析了金属层吸收对结构特性的影响.结果表明该结构在55°附近入射时有最好的性能;在吸收作用下结构偏振分束能力有一定的减小,TM波透射比发生了较大变化,TE波效果较好;随着周期数增加,结构透射比下降,但分光能力显著提高;在工作波段上随着波长增大,金属层吸收对器件的影响减弱.该结构能实现宽波段、宽角度、较高透射比的偏振分束.     

新型偏振干涉成像光谱仪中Savart偏光镜透射率的研究

Savart偏光镜是自行研制的新型稳态偏振干涉成像光谱仪中的核心部件，阐述了该Savart偏光镜的分光机理.基于电磁场边值条件，分析了入射面与Savart偏光镜左板主截面重合及垂直时，光透过Savart偏光镜各界面的反射和折射，得出了各界面透射系数的表达式以及Savart偏光镜透射率的理论计算公式.采用计算机模拟，给出了Savart偏光镜透射率随视场角和波长的变化曲线，并与研制的Savart偏光镜实验测试结果进行了分析比较，两者变化规律相符. 表明该Savart偏光镜具有大视场、高通量的显著特点，适宜作为静态干涉仪和稳态偏振干涉成像光谱仪的高效横向剪切分束器. 这为新型偏振干涉成像光谱技术的研究以及仪器研制提供了重要的理论依据.     

Design of a 50/50 splitting ratio non-polarizing beam splitter based on the modal method with fused-silica transmission gratings

The optical design of a beam splitter that has a 50/50 splitting ratio regardless of the polarization is presented. The non-polarizing beam splitter (NPBS) is based on the fused-silica rectangular transmission gratings with high intensity tolerance. The modal method has been used to estimate the effective index of the modes excited in the grating region for TE and TM polarizations. If a phase difference equals an odd multiples of π/2 for the first two modes (i.e. modes 0 and 1), the incident light will be diffracted into the 0 and ?1 orders with about 50% and 50% diffraction efficiency for TM and TE polarizations, respectively.     

基于耦合波理论设计偏振分束光栅

亚波长周期结构光栅具有传统光栅所不具有的特殊性质。针对通信中常用的1.55μm光波,采用严格耦合波理论分析了亚波长光栅的偏振分光特性,设计了对应的偏振分束光栅。所设计的光栅在入射角为56°时,透射消光比和反射消光比都大于9000,且在1.27μm~1.67μm全光波段内,入射角在51°~57°之间时,透射消光比和反射消光比都大于100,达到了宽带宽、宽角度以及透射消光比和反射消光比都较高的要求。     

Optical properties of light filter designed on absorbing axicon

The optical properties of a light filter designed based on an absorbing axicon are studied. It is shown that if axes of the filter and a radially symmetric incident beam coincide, then the light transmission through the filter is minimal. This phenomenon can be used to find the beam center, radius and divergence, as well as to form properly collimated Gaussian beams. The filter allows the apodization and continuously tuning of the laser radiation intensity.