摘要
手性超材料圆二色性(CD)的动态调控在调谐传感器、动态成像和信息加密等领域具有广阔的应用前景。本团队设计了一种以Z字形二氧化钒(VO2)为谐振单元的手性超材料,其在近红外波段具有高品质因子(Q>100)的CD响应。利用相变材料VO2在相变过程中电导率的可调谐性,实现了CD响应的信号强度在-0.5~+0.5范围内的动态调控。在此基础上,本团队在原有手性结构中引入另一组几何参数相同、具有相反手性特征的Z字形VO2谐振单元,通过分别控制两种手性结构中VO2谐振单元的相变状态,实现了CD响应信号开关以及等强度极性反转功能。
Objective In recent years,chiral metamaterials composed of subwavelengthscale nanostructures with symmetrybreaking characteristics have attracted significant attention from researchers owing to their great potential for enhancing circular dichroism(CD)effects.The structural parameters and unit arrangements of most conventional chiral metamaterials(CMTMs)are fixed,leading to a critical limitation in which their modal resonance responses lack dynamic tunability.These materials typically generate CD signals with fixed intensities and polarities within specific wavelength bands,which restricts their application in multifunctional optical devices.The introduction of dynamic modulation capabilities to achieve the realtime control of CD signals in chiral metamaterials has substantial scientific and practical significance.This advancement could lead to new application prospects in fields such as adaptive optics,tunable sensors,dynamic imaging,and information encryption.Although there have been numerous studies on tunable chiral optical responses based on VO2 phasechange materials,current designs of tunable chiral metasurface devices predominantly adopt a hybrid configuration that combines phasechange materials with metallic structures.The inherent Ohmic losses in metals result in broad linewidths of circular dichroism(CD)responses,with quality factors mostly confined to the range of 10‒20.Furthermore,most tunable chiral metasurface devices operate predominantly in the terahertz band,whereas relatively few devices function effectively in the nearinfrared wavelength range.Methods In this work,we designed a chiral metamaterial structure with zigzagshaped VO2 as resonant units and discussed its optical absorption properties and circular dichroism(CD)in the nearinfrared band in detail.To study the optical chiral response of the tunable chiral metasurface,we conducted simulations using the finiteelement software COMSOL Multiphysics.In the calculations,the refractive index of the PMMA background material was set to 1.47.The dielectric constant of the phasechange material VO2 was described using the Drude model.Periodic boundary conditions were applied in the x-and ydirections of the Zshaped VO2 resonant unit structure to simulate the metasurface structure of an infinite array.Periodic port boundary conditions and a perfect matched layer(PML)were used in the positive and negative directions of the zaxis to prevent the reflected waves from interfering with the simulation results.A plane wave with left-or righthanded circular polarization state was vertically incident on a chiral metasurface along the zdirection,and its absorption rate can be obtained by A=1-T-R,where T and R are the transmittance and reflectance,respectively.The CD signal of a chiral metasurface was defined as the difference in absorption between right-and lefthanded circular polarization incident on the metasurface;that is,ξCD=ARCP-ALCP.Results and discussions Numerical simulations reveal significant differences in the absorption spectra of the chiral metasurface under LCP and RCP incidence in the nearinfrared range of 900‒1200 nm.When the incident light is LCP,the chiral metasurface exhibits almost no absorption at other wavelengths,except for a strong peak with an absorption rate of approximately 0.5,at a wavelength of 967 nm,and a weak absorption peak at 1011 nm(peak of approximately 0.026).For RCP incidence,the chiral metasurface only exhibits an absorption peak of approximately 0.48 at a wavelength of 1011 nm,while there is almost no absorption in other wavelength ranges.When VO2 is in a metallic state,the designed Zshaped VO2 chiral metasurface exhibits selective absorption properties for left-and righthanded circular polarization,owing to symmetry breaking.By using the formula for calculating circular dichroism,it can be concluded that the CD signal at a wavelength of 967 nm has a negative maximum value of approximately-0.48,while at a wavelength of 1011 nm,the CD signal has a positive maximum value of approximately+0.45.Furthermore,it is found that the designed metasurface has very narrow linewidths for the absorption peaks of LCP at a wavelength of 967 nm and RCP at a wavelength of 1011 nm,with full width at half maximum(FWHM)of 7.3 and 12.2 nm,respectively.The corresponding quality factors are Q≈132 and Q≈83,respectively.In order to more clearly demonstrate the trend of the CD signal of the metasurface with the variation of VO2 conductivity,we researched the CD spectrum of the metasurface when the VO2 conductivity increased fromσ=200 to 2000 S/m.It can be seen that each CD spectrum has two opposite polarity characteristic peaks,which are determined by the fact that the metasurface absorbs LCP more than RCP at the short wave characteristic peak and LCP less than RCP at the long wave characteristic peak.By utilizing the change in conductivity during the VO2 phase transition process,the peak values of the two CD characteristic peaks on the chiral metasurface can be dynamically controlled within the ranges of[-0.02,-0.48]and[+0.02,+0.45],respectively.On this basis,we introduced another set of Zshaped VO2 resonant units with the same geometric parameters and opposite chiral characteristics to those of the original chiral structure.When the conductivities of the left-and righthanded chiral VO2 bands are the same,the unit structure exhibits mirror symmetry with respect to the xz plane.Therefore,the CD values generated by the left-and righthanded chiral structures are always different,resulting in no chiral response on the metasurface,that is,a state of closed chirality.When the Zshaped VO2 bands of left-and righthanded chirality are in different states,that is,when their conductivities are different,the mirror symmetry of the unit structure is broken,and the metasurface produces a CD response corresponding to the chiral open state.By switching the states of the left-and righthanded chiral VO2 bands,a dynamic polarity reversal function of the CD signals with the same intensity can be achieved.Conclusions Dynamic control of CD in chiral metamaterials has broad application prospects in fields such as tunable sensors,dynamic imaging,and information encryption.A chiral metamaterial with Zshaped vanadium dioxide as the resonant unit was designed to obtain a CD response with high quality factor(Q>100)in the nearinfrared band.By utilizing the tunability of the conductivity of phase change material VO2 during the phase transition process,dynamic control of the CD response signal intensity from-0.5 to+0.5 has been achieved.In addition,we designed a structure comprising Zshaped VO2 resonant units with identical geometric parameters and opposite chiral characteristics,and controlled the phase transition states of the VO2 resonant units in both the chiral structures,thereby achieving CD response signal switching and an equalintensity polarity reversal function.
作者
陈笈
陈怡静
董雯
Chen Ji;Chen Yijing;Dong Wen(School of Physical Science and Technology,Soochow University,Suzhou 215006,Jiangsu,China)
出处
《中国激光》
北大核心
2025年第13期124-132,共9页
Chinese Journal of Lasers
基金
国家自然科学基金(12274312)。
关键词
超材料
相变
手性
圆二色性
metamaterial
phase change
chiral
circular dichroism
