摘要
具有高品质因子(Q)和强圆二色性(CD)的手性超表面在光与物质相互作用中展现出显著的增强效应,这对于手性光学器件的性能提升具有重要意义。在本研究中,提出了一种基于连续域束缚态(BIC)的平面手性硅超表面设计,该设计在实现超高Q因子的同时,还获得了接近单位CD(CD为0.99)的手性响应。在不破坏结构面外对称性的前提下,通过打破结构的面内C2旋转对称性或改变光的入射角度,对基于BIC手性响应实现精确控制。此外,进一步探索了手性超表面在光学传感领域的应用,将所设计的手性超表面引入传感系统,实现了非凡的手性特性与传感性能(品质因数FOM=233 nm/RIU)的结合。这种做法不仅拓展了手性超表面的应用领域,也为光学手性传感提供了新的可能性和研究方向。
Objective This research aims to design and investigate a high-performance chiral metasurface based on bound states in the continuum(BICs),achieving both an ultra-high quality factor(Q)and near-unity circular dichroism(CD=0.99).The study focuses on precise control of chiral responses by breaking the in-plane C2 rotational symmetry or adjusting the angle of light incidence,while maintaining out-of-plane structural symmetry.In addition,we aim to expand the application of such chiral metasurfaces to optical sensing,specifically refractive index sensing,leveraging BIC properties to enhance sensitivity and figure of merit(FOM=233 nm/RIU).Methods To achieve these objectives,we design a periodic array of dielectric metasurfaces consisting of square nanodisks with double notches.As shown in Fig.1(b),the unit cell structure features a period P=850 nm,side length W 0=520 nm,notch width W 1=185 nm,and length L=234 nm.The nanodisks are made of high-refractive-index silicon(n=3.48)with a thickness of 350 nm,and the substrate is SiO2(n=1.46).Circularly polarized light propagates along the-z direction,perpendicular to the metasurface.Numerical simulations are conducted using COMSOL MULTIPHYSICS,with periodic boundary conditions in the x and y directions and perfectly matched layers(PMLs)in the z direction to ensure accuracy and reliability.We initially analyze the eigenmodes and Q factors of the metasurface in momentum space,as shown in Fig.1(c)and(d).The BICs are identified at theΓpoint,where the Q factor theoretically tends to infinity,and far-field polarization states are characterized,as shown in Fig.1(e).To study the chiral response,we introduce an in-plane asymmetry parameterδand investigate the CD spectra and transmittance components,as illustrated in Fig.2.We also explore the effects of oblique incidence on the chiral response(Fig.3)and analyze how the azimuthal angleφaffects CD(Fig.4).For the refractive index sensing application,we systematically analyze the influence of environmental refractive index changes on transmission components and CD response.Specifically,we examine shifts in cross-polarized transmission components T RL and T LR and the CD spectrum as the refractive index of the surrounding medium varies from 1.00 to 1.18,with a step size of 0.03[Figs.5(a)and(b)].The linear redshift in the CD spectrum and the correlation between the CD peak shift and refractive index are also studied[Figs.5(c)and(d)].Results and Discussions Key findings of our study include:1)High-Q factor and near-unity CD.By introducing an inplane asymmetry parameterδ,we achieve a near-unity CD(CD=0.99)alongside an ultra-high Q factor.While the Q factor decreases with increasingδ,CD remains close to unity,confirming that fine-tuningδoptimizes chiral q-BIC performance(Fig.2).2)Tunable chirality via oblique incidence.We find that chiral response can be precisely modulated by adjusting the incident angleθand azimuthal angleφ.Atθ=6°andφ=90°,the CD reaches a maximum value of 0.98,and the Q factor follows an inverse quadratic relationship with sinθ(Fig.3).The magnetic dipole(MD)plays a dominant role in the chiral qBIC mode,as evidenced by the electromagnetic field distribution[Fig.3(e)].3)Refractive index sensing performance.The chiral metasurface demonstrates high sensitivity(233 nm/RIU)and excellent FOM in refractive index sensing.As the refractive index increases,resonance wavelengths of cross-polarized transmission components T RL and T LR shift linearly to longer wavelengths,and the CD spectrum exhibits a corresponding linear redshift[Fig.5(c)].The linear relationship between CD peak shift and refractive index change[Fig.5(d)]confirms the sensor’s high sensitivity and reliability.4)Physical mechanisms of CD flipping.We observe that the CD sign flips with changes in azimuthal angleφ(Fig.4).At specific angles,CD exhibits a sign reversal,corresponding with the helicity flip in momentum space[Fig.1(f)].This behavior provides insights into the underlying physical mechanisms and offers new possibilities for designing chiral metasurfaces with custom CD properties.Conclusions In this study,we successfully design a high-performance chiral metasurface based on BICs,achieving both ultra-high Q factors and near-unity CD.We demonstrate precise control over the chiral response by breaking in-plane C2 rotational symmetry or adjusting light incidence angles.Furthermore,we extend the application of this chiral metasurface to refractive index sensing,achieving a sensitivity of 233 nm/RIU and a high FOM.Our results enhance the design principles and application scenarios for chiral metasurfaces,offering new directions for the development of optical sensors and photonic devices.The simultaneous achievement of strong CD,high Q factor,and tunable response opens up new possibilities for advanced optical and sensing applications.
作者
吕淑媛
胡飞
罗文峰
张婷婷
段多多
LüShuyuan;Hu Fei;Luo Wenfeng;Zhang Tingting;Duan Duoduo(School of Electronic Engineering,Xi'an University of Posts&Telecommunications,Xi’an 710121,Shaanxi,China)
出处
《光学学报》
北大核心
2025年第2期274-282,共9页
Acta Optica Sinica
关键词
连续域束缚态
手性
圆二色性
传感器
bound states in continuum
chirality
circular dichroism
sensor
