Numerical Study On Circular Dichroism Of Chiral Metasurface

Chirality is closely related to the symmetry of objects.Generally,chirality is understood as the lack of mirror symmetry,while non chirality means the existence of mirror symmetry.Chiral media widely exist in various macro and micro systems in nature.Circular dichroism(CD)usually refers to the difference in the absorbance of chiral media to left circularly polarized light(LCP)and right circularly polarized light(RCP).Detecting the chiral optical response of micro materials is an important method to determine the structure of micro materials.However,the optical response of chiral media in nature is generally weak,and most of them are in the ultraviolet band,which greatly limits the detection sensitivity and various practical applications.Under the excitation of incident circularly polarized light,metamaterials will oscillate surface electrons,so as to localize the energy in a very small range and greatly enhance the circular dichroism response of matamaterials.Based on the previous research,this paper further designed two kinds of chiral metasurfaces: H-shaped metal chiral metasurface and catenary-shaped chiral metasurface,and studied their circular dichroism effect excited by circularly polarized light and the physical mechanism of generating circular dichroic signal.The main research work of this paper is as follows:A double twisted H-shaped metal sheets(DTH)model is designed.The symmetry of the structure is destroyed by twisting the upper H-shaped metal at a certain angle to make it chiral.The transmission spectra and CD spectrum of DTH model excited by RCP and LCP are calculated by using the numerical simulation software CST,and the surface current distribution at the resonant frequencies of RCP and LCP is studied.The results show that at the resonance frequency of RCP and LCP,the current distribution excited by RCP and LCP on the upper and lower metal surfaces of DTH model is different,and the coupling mode of equivalent electric dipoles on the upper and lower metal surfaces is also different,resulting in a large difference in the transmittance of RCP and LCP at the resonance frequency of circularly polarized light,resulting in a relatively large CD signal.In addition,the influence of structural parameters on CD signal is studied.By optimizing structural parameters,the position and size of CD signal can be adjusted.Under the excitation of RCP and LCP,DTH model can produce CD signal up to 0.6 with appropriate structural parameters.A catenary-shaped chiral metasurface(CCMS)was designed.By rotating the upper catenary-shaped metal surface,the symmetry of the structure is broken,making the structure chiral.Similarly,the transmittance and circular dichroism signal of the structure in microwave section are calculated by numerical simulation software CST.To study the physical mechanism of CD signal,the current distribution on the upper and lower surface of the model at the resonant frequency of circularly polarized lights were simulated.The results show that under the excitation of RCP,the equivalent electric dipole moments of the catenary-shaped metal surface of the upper and lower layers is coupled to form a "bonding" mode at the resonance frequency.Under the excitation of LCP,the equivalent electric dipole moments of the catenary-shaped metal surface of the upper and lower layers is coupled to form an "antibonding" mode.This shows that the mechanism of circular dichroism signal generated by this structure conforms to Born-Kuhn model.In addition,the influences of structural parameters,such as the torsion angle ? of the catenary-shaped metal surfaces,the thickness of the medium layer d,and the period p of the model on CD signal were also studied.By optimizing the structural parameters,the CCMS chiral metasurface can generate CD signals up to0.68.