基于拓扑优化的薄液晶灵活调控太赫兹波编码超表面

Flexible Modulation of Terahertz-Wave-Coded Metasurface in Thin Liquid Crystals with Topological Optimization Design

提出并研究了一种基于拓扑优化设计的太赫兹薄液晶(LC)编码超表面。基于遗传算法对金属谐振图案进行拓扑优化,设计了一种薄厚度反射型2 bit LC编码超表面单元,LC层厚度仅为14μm,克服了利用多层LC或LC谐振切换机制实现多比特编码外部馈电系统复杂的缺点,同时具有较快的响应速率。基于设计的LC 2 bit编码超表面单元,利用遗传算法对阵列编码排布进行逆向设计,增强波束调控的主动性和灵活性,仿真实现了0.394 THz处波束赋形和涡旋波束的功能。结果表明:对于波束赋形,该超表面单元不仅可以实现俯仰角在0°~30°范围内、方位角在0°~360°范围内任意角度的单波束和多波束的调控,还可以对多波束的各个目标波束进行独立调控,同时扫描和跟踪多个目标,实现对太赫兹波束更优性能的调控;对于涡旋波束,该超表面单元可以产生拓扑荷数l=±1、±2、±3且模式纯度大于70%的涡旋波束,同时可实现俯仰角在0°~30°范围内、方位角在0°~360°范围内任意角度的单涡旋和多涡旋波束的灵活调控。

Objective Encoding metasurfaces based on tunable materials can achieve dynamic control of terahertz beams with reconfigurability under the action of external control and are the main solutions to the design of encoding metasurfaces in terahertz bands.The liquid crystal(LC)is a more practical solution than other tunable materials because of its mature processing technology,low manufacturing cost,and simple driving scheme.However,most of the relevantly reported LCcoded metasurfaces employ one-bit encoding,inevitably producing symmetric beams and limiting the beam deflection efficiency to only 50%.Increasing the LC layer number and exploiting the resonance switching mechanism of LCs in different regions are two options to achieve multi-bit encoding,but will increase the complexity of the external voltage manipulation system.Meanwhile,since the corresponding rate of LC integrated devices is mainly related to the thickness of LCs,the design of thin LC multi-bit-coded metasurface cells with low voltage control based on simplifying the external voltage control has certain research significance and good application prospects.Additionally,for the encoding sequence design,under plane wave excitation,the traditional method is to adopt gradient phase encoding and complex encoding to independently regulate single and multiple beams.Meanwhile,there will be some limitations on the initiative,flexibility,and deflection accuracy of the design of the beam regulation encoding.Thus,we employ the genetic algorithm for the reverse design of the encoding sequence,which can overcome the shortcomings of traditional methods.Methods Compared to the optimization of the underlying shape and structural parameters,topological optimization by dividing the surface pattern into equal-sized pixel units is generally combined with optimization algorithms to increase the design freedom and yield better performance and has been widely applied to the design of various functional devices for metasurfaces.First,we aim at the achievable 2 bit encoding and thin LC for the reverse design of LC-coded metasurface cells based on topology optimization.The surface topological pattern and structural parameters are 2 bit encoded by adopting ABRR as the objective function,and they are optimized several times using a genetic algorithm.For the encoding sequence design,based on the far-field scattering principle of digital encoding and designed LC-coded metasurface unit,the different array encoding sequences obtained by reverse design according to the scattering principle of digital encoding,and the beam assignment and vortex beam functions are simulated in a full-wave simulation using the simulation software CST.Results and Discussions By employing the genetic algorithm to optimize the design several times,the designed singlelayer LC metasurface structure is shown in Fig.3.The LC thickness is only 14μm,the amplitude and phase response curves of the metasurface unit in reflection mode are shown in Fig.4,and theαABRR,min(n)of the encoding metasurface is 8.92 at 0.394 THz,which means that the designed metasurface unit can achieve 2 bit encoding.Then,based on the designed LC-coded metasurface units,the full-wave simulations of beam assignment and vortex beam functions are simulated with different array coding sequences obtained by reverse design.For beam shaping,flexible control of a single beam(Fig.6)and a specified number of multiple beams within a pitch angleθof 0°‒30°and an azimuth angleφof 0°‒360°is achieved.In particular,for multiple beams(Figs.7 and 8),independent control of the main flap of each target is realized.Compared to the method of reverse design by complex encoding and the addition law,it is advantageous to achieve independent multi-beam modulation of each target main flap under plane wave excitation with only 2 bit encoding rules,with improved design initiative.For vortex beams,single vortex beams with topological charges l=±1,±2,and±3 and mode purity above 70%are achieved at 0.394 THz(Figs.10 and 11).By conducting vortex phase convolution,double vortex beams to quintuple vortex beams with pitch anglesθwithin 30°and an azimuthal angleφwithin 360°are generated and flexibly regulated(Fig.12).Conclusions We design a thin-thickness reflective LC-coded metasurface unit based on topological optimization with a LC thickness of only 14μm,which simplifies the complexity of the LC multi-bit-coded external feed control system,with a fast response rate.The reverse design of the array encoding sequence using the genetic algorithm can realize the flexible regulation of beam assignment and vortex beams,which improves the design efficiency and the diversity of encoding functions.The results show that for beam assignment,not only the flexible control of a single beam at 0.394 THz with pitch angleθwithin 30°and azimuth angleφwithin 360°is achieved,but also the independent control of pitch angleθand azimuth angleφof a single beam from triple vortex beams to quintuple vortex beams is realized.For vortex beams,singlevortex beams with topological charges l=±1,±2,and±3 and mode purity above 70%are achieved at 0.394 THz.Meanwhile,by adopting vortex-phase convolution,double vortex beams to quintuple vortex beams are generated and flexibly tuned within a pitch angleθof 30°and an azimuthal angle ofφin the range of 360°are achieved.The proposed topology-optimized metasurface cell structure and reversely designed array encoding sequences have potential applications in terahertz beam manipulation devices.