Study On Polarization Selectivity In Chiral Metasurfaces

Chiral metasurface,a class of composite materials constructed from chiral structures arranged in a periodic way or achiral structures arranged in a chiral way,exhibits many fascinating electromagnetic responses,such as circular dichroism(CD),optical activity(OA),asymmetric transmission(AT),and so on.The polarization selectivity in chiral metasurface has important potential applications in polarization control,near-field imaging,biosensing and other fields.Therefore,how to improve the efficiency,bandwidth,and dynamic range of polarization selective response of chiral metasurfaces have become a hot topic in the research field of optics.In this thesis,we mainly study the polarization selectivity of chiral metasurface,explore the formation mechanism of chiral-optical response,design the simple and efficiency optical devices,and explore the applications of chiral metasurface in near-field imaging,sensing,electromagnetically induced transparency,etc.The specific content of this thesis are as follows:According to the symmetry-breaking condition of the perfect CD meta-mirror,a reflection-type chiral patterned graphene metasurface is proposed to realize high-efficiency and tunable CD effect.By exciting the polarization-dependent local surface plasmon resonances(LSPRs)in chiral patterned graphene and combing with the Fabry-Perot(F-P)resonance supported by graphene layer and Au substrate,perfect absorption of right-handed circularly polarized(RCP)waves and high reflection of left-handed circularly polarized(LCP)waves at 2.78 THz are achieved,with the CD value of 0.87(E_F=1 e V).The distributions of electromagnetic field and surface current at resonance frequency indicate that the LSPR modes excited by RCP waves are electric dipole resonance and magnetic dipole resonance.The effects of dielectric layer thickness and the twisted angleθon the CD signal were studied.Besides,the variation of the intensity and frequency of CD signal with the graphene’s Fermi level is also analyzed.Based on the giant CD effect,the application of this metasurface in polarization recognition and filtering is discussed.Furthermore,the performance of linear dichroism and linear polarization filtering of the metasurface is investigated.A zigzag metasurface composed of Z-shaped metallic strips(ZMSs)and L-shaped graphene strips(LGSs)for dual-band tunable and strong CD effect is proposed.At 0.86 THz,the proposed metasurface can absorb most of the LCP waves while reflecting most of the RCP waves,resulting in a negative CD of-0.88.In contrast,at 1.23 THz,most of the LCP waves are reflected by the metasurface while most of the RCP waves are absorbed,leading to a positive CD of 0.88.By analyzing the distribution of the surface current vector at the resonance frequency,it is revealed that the dual-frequency CD signal originates from the constructive interference between LSPRs and F-P resonance excited by LCP at 0.86 THz and RCP at 1.23 THz,respectively.Furtherly,the variation of CD signals at dual resonant frequencies with the thickness of the dielectric layer is discussed.Moreover,the effect of electrically controlled graphene’s Fermi level on the operating frequency and intensity of dual-frequency CD signals is studied.Considering the strong field localization of LSPRs,the biosensing property of zigzag metasurface is investigated.To analyze the linear polarization selectivity of the meta-graphene hybrid zigzag metasurface,the transmission spectra of x-and y-polarized incident waves are calculated,and the linear dichroism(LD)and linearly cross-polarization conversion at E_F=0.2 e V are demonstrated.The switchable effects of circular dichroism of transmission(CDT)and circular dichroism of reflection(CDR)are investigated using photosensitive silicon-based zigzag metasurface.At 3.42 THz,the metasurface without optical pumping possesses CDT/CDR of 0.89/-0.83 for forward-incident circularly polarized light(CPL)and-0.86/0.69 for backward-incident CPL.The relationship between the polarization selectivity and the propagation direction of incident wave is analyzed theoretically using Jones matrix,and the cross-polarization conversion in the process of circular polarization selectivity is discussed.The results of multipole decomposition and E-field distributions reveal that the circular polarization selectivity originates from the excitations of polarization-dependent Mie multipolar resonance.The variations of broadband CDT/CDR responses with the conductivity of photosensitive silicon are investigated.To verify the actual control effect of polarization selectivity,a dynamic near-field imaging utilizing the proposed zigzag structure and its enantiomer is designed.Polarization-controlled electromagnetically induced transparency(EIT)analogue in Z-L shape metasurface is investigated by introducing the L-shaped structures in the Z-shaped array.By exciting the destructive interference between magnetic dipole(MD),toroidal dipole(TD)and electric quadrupole(EQ)resonances,the metasurface exhibits a narrowband EIT window at 1.858 THz with transmission of 0.96 and Q-factor of 640.52 for y-polarization incidence.Based on the coupled Lorentz oscillator model,the formation mechanism of EIT window could be interpreted by the destructive interference between"bright mode"and"dark mode",where the MD and TD are serves as"bright mode"and EQR serves as"dark mode".In addition,the variations of EIT spectra and group delay with the polarization angle of incident waves are investigated,and the application of the metasurface in refractive index sensing is discussed.Finally,in order to solve the problem of single function of polarization metasurface,a double-layer chiral silicon column array was designed to achieve linear-to-circular conversion for forward-incident waves and polarization rotation for backward-incident waves.