Anisotropic Plasmonic Metasurfaces with Linear Polarization-Dependent Focusings

As a burgeoning approach to modulate electromagnetic waves, metasurfaces fascinate many researchers. However, it is a persistent problem to realize independent wavefront tailoring in different polarization states. In this work, anisotropic plasmonic metasurfaces are presented with linear polarization-dependent focusings. The designed plasmonic meta-atoms consist of cross-shaped gold, silica spacer, and gold substrate. Phase modulation in different polarization channels is independently achieved by adjusting the dimensions of cross-shaped gold in x- and y-directions. Three metasurfaces are presented to verify linear polarization-dependent focusings at 200 THz. First, a focusing metasurface is designed with focal lengths of 5.2 µm under x-polarized incidence and 6.5 µm under y-polarized incidence. Second, by applying convolution operation, the second metasurface exhibits different focusings with different deflection angles at opposite directions under orthogonally polarized incidences. Finally, a multi-focal metasurface is demonstrated, which switches between dual- and quad-focal points depending on polarization state. The work may provide a novel platform for near-infrared integrated photonics.     

基于电磁超表面的透镜成像技术研究进展

电磁超表面属于超材料的一种,是由许多亚波长纳米结构单元组成的二维功能性平面结构.根据惠更斯原理,超表面阵列可以任意调控光波的相位、振幅和偏振.与传统器件相比,基于这种超材料设计的光学功能器件最大的优势是其具有极薄的厚度.本文首先介绍了广义斯涅耳定律以及纳米单元结构调控相位的基本原理,重点归纳了电磁超表面在透镜成像技术方面的研究进展,包括等离子体超表面、全介质超表面以及金属/介质混合式超表面在成像方面的应用,最后指出了超表面在成像方面尚未解决的前沿问题以及与实际应用接轨的重要问题,希望能为以后的深入研究提供一定的参考和借鉴.     

Electro-optic lithium niobate metasurfaces

Many applications of metasurfaces require an ability to dynamically change their properties in the time domain. Electrical tuning techniques are of particular interest, since they pave a way to on-chip integration of metasurfaces with optoelectronic devices.In this work, we propose and experimentally demonstrate an electro-optic lithium niobate(EO-LN) metasurface that shows dynamic modulations to phase retardation of transmitted light. Quasi-bound states in the continuum(QBIC) are observed from this metasurface. By applying external electric voltages, the refractive index of lithium niobate(LN) is changed by Pockels EO nonlinearity, leading to efficient phase modulations to the transmitted light around the QBIC wavelength. The EO-LN metasurface developed in this study opens up new routes for potential applications in the field of displaying, pulse shaping, and spatial light modulating.     

Manipulating Spin-Dependent Wavefronts of Vortex Beams via Plasmonic Metasurfaces

Conventional metasurfaces based on geometric phase are constrained to spin-locked phase profile, resulting in mirrored functionalities for different spins. A single flat device that enables independent manipulation of wavefronts in two orthogonal circularly polarized channels is of paramount importance in wireless and optical communications. In this work, by tuning the dimension and rotation angle of H-shaped meta-atoms to synthesize propagating phase and geometric phase, spin-dependent plasmonic metasurfaces are presented to manipulate circularly polarized waves in the visible band. To verify the capability of spin-dependent wavefront manipulation, three metasurfaces are implemented. The first metasurface generates vortex beams with orbital angular momentum (OAM) l = 1 under left-handed circularly polarized (LCP) incidence and l = 2 under right-handed circularly polarized (RCP) incidence. By introducing convolution operation, the second metasurface is capable of producing vortex beams with different OAMs and different directions for two spins. The third metasurface produces dual-beam and quad-beam with different OAMs for different circular polarizations. This scheme can provide a new pathway in ultracompact nanophotonic devices and systems.     

Dielectric-Metasurface-Enhanced Kerr Self-Modulation for Passively Q-Switching

Dielectric metasurfaces are highly sought-after since they have found a variety of linear and nonlinear applications. Here, a novel transmission modulation is presented for pulsed laser generation by directly exploiting the enhanced Kerr effect from a periodic array of silicon nanodisks. The Mie resonance of the unit cells enhances the Kerr effect of the silicon material and is redshifted by the latter, so that the transmission of the metasurface at the working wavelength is strongly modulated (the observed modulation depth is above 3% under moderate exciting power). By inserting the fabricated metasurface as a self-modulator into a ring cavity fiber laser, stable Q-switched pulse laser output is achieved with a duration of 1.4 µs and a repetition rate of 60 kHz. This work opens a door for ultra-thin low-loss dielectric metasurfaces to efficiently shape the operation of pulsed lasing based on their enhanced nonlinear effects and flexible propagation manipulation.     

非线性光学超构表面

在线性光学范畴内,人们已经通过亚波长尺度的超薄超构表面成功实现了对光的众多新颖特性的调控功能.其主要理念是通过对具有亚波长尺度且空间方向变化的超构功能基元进行特定的排列,从而实现对光的偏振、相位和振幅的有效控制.近来,超构表面上的非线性光学特性也引起了大家的广泛关注.在本综述中,我们对非线性光学超构表面的设计、超构功能单元的材料和对称性选择、非线性手性光学、非线性贝里几何相位和非线性波前整形等内容进行了总结;最后对非线性光学超构表面在调控光与物质的相互作用中面临的挑战和前景进行了展望.     

Transmission Reflection Integrated Programmable Metasurface for Real-Time Beam Control and High Efficiency Transmission Polarization Conversion

The functional versatility and flexibility of multifunctional metasurfaces are of great significance for the application of electromagnetic (EM) metasurfaces in military stealth, holographic imaging, base station antennas, and other fields. Nevertheless, the functions of active programmable coding multifunctional metasurfaces are mostly reflective or transmissive, and the research on active multifunctional metasurfaces with transmission and reflection integration is still in its infancy. Here, a transmission reflection integrated active programmable coding metasurface is proposed to manipulate the transmission and reflection of different polarized electromagnetic waves independently in real time. As a proof of concept, a metasurface that can implement diverse programmable functionalities is designed. Numerical and experimental results indicate that the strategy has excellent performance in beam control and polarization conversion, which provides tremendous potential for extending highly integrated multifunctional devices.     

主动式液晶超表面研究进展北大核心CSCD

超表面具有亚波长尺度下精密高效的光波操控能力,但在其单独实现主动式调控方面,目前仍有诸多技术困难亟待克服。液晶与超表面的结合有望发挥各自的长处,实现一种分辨率高、衍射角大、超紧凑的新型主动式光调控器件。以液晶与超表面两部分功能设计的独立与否作为分类依据,回顾了近年来主动式液晶超表面的研究进展,具体包括液晶波片与偏振敏感超表面结合、液晶环境与共振型超表面结合、液晶与超表面光学性质互补等。最后对主动式液晶超表面所面临的挑战以及发展前景进行了讨论和展望。     

Analysis and applications of nanocavity structures used as tunable filters and sensors

This paper reports on a novel design for a tunable filter and plasmonic sensor based on the metal–insulator–metal waveguide with a nanocavity resonator. Simulation results show that as a one-channel filter, the resonance wavelengths show a linear red-shift with an increase in nanocavity length with a slope of 1742 nm/μm and a nonlinear blue-shift with an increase in nanocavity width, respectively. A two-channel filter can be realized using two nanocavities and the arrangement of the two nanocavities with respect to the waveguide and the value of the distance between the nanocavities has only a marginal effect on the filter notch wavelength. Finally, both in-slit and out-slit refractive index plasmonic sensors are investigated with a sensitivity of 710 nm/RIU and 250 nm/RIU, respectively.     

Free control of far-field scattering angle of transmission terahertz wave using multilayer split-ring resonators’ metasurfaces

To enhance transmission efficiency of Pancharatnam–Berry (PB) phase metasurfaces, multilayer splitring resonators were proposed to develop encoding sequences. As per the generalized Snell’s law, the deflection angle of the PB phase encoding metasurfaces depends on the metasurface period’s size. Therefore, it is impossible to design an infinitesimal metasurface unit; consequently, the continuous transmission scattering angle cannot be obtained. In digital signal processing, this study introduces the Fourier convolution principle on encoding metasurface sequences to freely control the transmitted scattering angles. Both addition and subtraction operations between two different encoding sequences were then performed to achieve the continuous variation of the scattering angle. Furthermore, we established that the Fourier convolution principle can be applied to the checkerboard coded metasurfaces.     

五模声学超表面理论分析与定向反射声学仿真*

以广义斯奈尔定律为理论依据,对五模声学超表面定向反射的基本原理进行了解析推导和理论分析,获得了五模超表面的理想连续物性参数分布,并给出了五模超表面尺寸设计准则;然后将超表面离散,获得离散单胞的密度和体积模量,并以此为目标进行五模微结构设计,采用均匀化理论计算微结构的等效物性参数;最后,进行了水下声场的声波定向反射调控仿真实验,研究了入射波频率对超表面定向反射性能的影响,仿真结果展现了五模超表面宽频有效的声波调控能力以及调控的可靠性和准确性。本文的研究工作为五模声学超表面的设计和物理实现提供理论指导。     

电磁超表面在微波和太赫兹波段雷达散射截面缩减中的应用研究进展

电磁超表面由于其独特的电磁特性为调控电磁波提供了有力工具,合适地设计成编码、随机、相位不连续、完美吸收器等超表面,就能够控制电磁波的散射以及反射特性,实现雷达散射截面的缩减。本文综述了不同的电磁超表面利用漫反射或者吸收等特性实现在微波和太赫兹波段雷达散射截面缩减中的应用。分析表明,编码超表面由不同的数字单元组成,其反射相位差在很宽的频段范围内满足恒定的关系,设计特殊的单元序列使入射的电磁波产生非定向散射,更高bit编码超表面更容易灵活调控电磁波;随机超表面通过调节阵元的尺寸实现宽带移相从而将金属目标特征性强的反射峰打散成一个无规律、杂乱的波,产生漫反射;不连续超表面由于相位不连续可使电磁波发生漫反射或者异常反射;吸收器通过合理设计结构尺寸实现吸收电磁波能量来减小反射。因此电磁超表面在雷达隐身、宽带通讯、成像等方面具有重要的应用前景。最后对电磁超表面在雷达散射截面缩减中应用的发展趋势进行了初步探讨,未来将向着宽带、柔性、大角度等方面发展。     

基于开口椭圆环的高效超宽带极化旋转超表面

电磁波的极化态在信号传输和灵敏度测量中有非常重要的应用价值. 本文设计、仿真并实验验证了微波频段基于开口椭圆环谐振器的极化旋转超表面. 理论上, 将多阶表面等离子谐振和高阻抗表面相结合, 解释了多谐振点、高效率极化旋转的物理机理. 数值上, 通过对结构参数的仿真分析, 给出了运用开口椭圆环结构设计多频段、超宽带高效极化旋转超表面的方法. 所设计和制作的超表面能够在相对带宽104.5%的频率范围内实现大于85%的极化旋转效率. 这些工作将为极化操控超表面的设计和应用提供重要帮助.     

Metasurfaces enabling structured light manipulation:advances and perspectives [Invited]

Metasurfaces and structured light have rapidly advanced over the past few years, from being paradigms to forming functional devices and tailoring special light beams for wide emerging applications. Here, we focus on harnessing metasurfaces for structured light manipulation. We review recent advances in shaping structured light by metasurfaces on different platforms(metal, silica, silicon, and fiber). Structured light manipulation based on plasmonic metasurfaces, reflection-enhanced plasmonic metasurfaces, metasurfaces on fiber facets, dielectric metasurfaces, and sub-wavelength structures on silicon are presented, showing impressive performance.Future trends, challenges, perspectives, and opportunities are also discussed.     

A high-quality factor hybrid plasmonic nanocavity based on distributed Bragg reflectors

Herein,we propose a high-quality(Q) factor hybrid plasmonic nanocavity based on distributed Bragg reflectors(DBRs) with low propagation loss and extremely strong mode confinement.This hybrid plasmonic nanocavity is composed of a high-index cylindrical nanowire separated from a metal surface possessing shallow DBRs gratings by a sufficiently thin low-index dielectric layer.The hybrid plasmonic nanocavity possesses advantages such as a high Purcell factor(Fp) of up to nearly 20000 and a gain threshold approaching 266 cm~(-1)at 1550 nm,promising a greater potential in deep sub-wavelength lasing applications.     

Asymmetrical photonic spin Hall effect based on dielectric metasurfaces

The photonic spin Hall effect has attracted considerable research interest due to its potential applications in spin-controlled nanophotonic devices. However, realization of the asymmetrical photonic spin Hall effect with a single optical element is still a challenge due to the conjugation of the Pancharatnam-Berry phase, which reduces the flexibility in various applications. Here, we demonstrate an asymmetrical spin-dependent beam splitter based on a single-layer dielectric metasurface exhibiting strong and controllable optical response. The metasurface consists of an array of dielectric nanofins, where both varying rotation angles and feature sizes of the unit cells are utilized to create high-efficiency dielectric metasurfaces, which enables to break the conjugated characteristic of phase gradient. Thanks to the superiority of the phase modulation ability, when the fabricated metasurface is under normal incidence with a wavelength of 1550 nm, the left-handed circular polarization (LCP) light exhibits an anomalous refraction angle of 28.9°, while the right-handed circular polarization (RCP) light transmits directly. The method we proposed can be used for the flexible manipulation of spin photons and has potentials in high efficiency metasurfaces with versatile functionalities, especially with metasurfaces in a compact space.     

基于石墨烯的太赫兹波散射可调谐超表面

设计了一个可调谐的太赫兹超表面,由在随机反射超表面基底中嵌入可偏置的双层石墨烯构成,可以实现对太赫兹波散射特性的动态调控.全波仿真试验结果证实了所预期的超表面散射可调性能.通过增大偏置电压提升石墨烯的费米能级,使得该超表面的太赫兹波散射样式从漫反射逐渐向镜面反射过渡,从而实现散射特性的连续调控,且该超表面具有对电磁波极化角度不敏感的特点.这些特性使得该超表面能很好地融合到变化的环境中,在太赫兹隐身方面具有潜在的应用价值.     

Characteristic Attributes of Multiple Cascaded Terahertz Metasurfaces with Magnetically Tunable Subwavelength Resonators

The characteristics of multiple cascaded metasurfaces comprising H‐shaped, magnetostatically controllable, subwavelength terahertz (THz) resonators made of InAs were systematically investigated, using a commercial solver based on the finite‐integration method, for the design of tunable filters. Three configurations of the biasing magnetostatic field were compared with each other as well as with the bias‐free configuration for filtering of normally incident linearly polarized plane waves. A close study of only one metasurface was found sufficient to broadly determine the sensitivity to the direction of the magnetostatic field and the bandwidth of a stopband. Furthermore, the effects of metasurface geometry and biasing field can be considered separately for initial design purposes. All features in the transmittance spectra for the bias‐free configuration that are related to the number of cascaded metasurfaces are also observed when the biasing magnetostatic field is applied. The coupling of adjacent metasurfaces in a cascade is strongly affected by the relative permittivity and the thickness of the spacer between the two metasurfaces. The spectral locations of stopbands scale with respect to the spacer's relative permittivity, the scaling rule being different from a classical one. The stopbands are redshifted when the spacer thickness is increased, with the redshift dependent on the polarization of the incident plane wave. Inter‐metasurface coupling and inter‐resonator coupling on the same metasurface affect the spectral location of a stopband in opposite ways. On‐off type switching can be obtained by changing the orientation of magnetostatic field. The elucidated characteristics are expected to be important for not only filters but also other tunable THz devices.     

Nonlinear thermotics: nonlinearity enhancement and harmonic generation in thermal metasurfaces

We propose and investigate a class of structural surfaces (metasurfaces). We develop the perturbation theory and the effective medium theory to study the thermal properties of the metasurface. We report that the coefficient of temperature-dependent (nonlinear) item in thermal conductivity can be enhanced under certain conditions. Furthermore, the existence of nonlinear item helps to generate high-order harmonic frequencies of heat flux in the presence of a heat source with periodic temperature. This work paves a different way to control and manipulate the transfer of heat, and it also makes it possible to develop nonlinear thermotics in the light of nonlinear optics.     

Multi-channel plasmonic waveguide filters with disk-shaped nanocavities

A kind of multi-channel plasmonic filters based on metal–insulator–metal waveguides with disk-shaped nanocavities is proposed and numerically investigated. By calculating the resonant mode of disk-shaped nanocavity, it is found that the radius and refractive index of the nanocavity effectively control the resonance wavelength, which is consistent with the results obtained by finite-difference time-domain method. The characteristics of resonance spectra are influenced by the gap width between the cavity and waveguide, as can be exactly analyzed by temporal coupled mode theory. In addition, multi-channel plasmonic filters are achieved by increasing the number of nanocavity.