Study On Tunable Chiroptical Effects Based On Phase Change Materials VO₂ And GST

Chirality refers to an object cannot be superimposed on their mirror images by translation or rotation operations.Chiroptical responses,such as circular dichroism（CD）,asymmetric transmission,and optical activity,observed in natural materials,indicate the corresponding structural symmetry inherent at the microscopic scale.However,due to the fundamental mismatch between the size of molecules and the wavelength of the incident light,the chiroptical response in natural materials is usually too weak to be detected.By engineering the artificial metamaterials composed of meta-atoms lacking mirror symmetry,the chiroptical responses can be significantly enhanced.However,the function of the chiral plasmonic metamaterials constructed by meta-atoms is usually fixed once being fabricated.Therefore,it is vital to design dynamically controllable chiral metamaterials to satisfy the versatile requirement of integrated optoelectronic devices.In this paper,we present three designs of chiral metamaterial devices for achieving tunable circular dichroism based on the tunable properties of the phase change materials vanadium dioxide（VO₂）and GST,and systematically investigate the origin of their chiral optical effects.The main research contents are as follows:A chiral metasurface integrated withη-shaped VO₂to achieve dynamic tuning of CD in the mid-infrared region is presented in this paper.The proposed chiral absorber can achieve near-perfect absorption due to the excitation of strong magnetic dipole（MD）resonance under the illumination of the right-handed circularly polarized（RCP）light,while the absorption is comparably low in the case of the left-handed circularly polarized（LCP）light as the electric dipole（ED）mode associated with the LCP light is weak,thus broadband CD effect with maximum CD of 0.63 can be realized due to the remarkable chiral-selective absorption performances.In addition,the CD value and the absorption peak of the chiral metasurface can be dynamically tuned by changing the volume fraction of the metallic state of VO₂,and good CD effect can be maintained even if the structural parameters are significantly altered.A complementary z-shaped VO₂chiral metamaterial to achieve dual-band CD responses is presented in this paper,and the two resonant CD peaks can be maintained in both metallic and dielectric states of VO₂.The mechanisms of the novel robust CD responses are associated with the excitations of hybrid multipole resonances according to the far-field multipole decomposition and the near-field distribution of the device,and their peak values as well as their resonance locations can be dynamically tuned in the mid-infrared region by changing the volume fraction.In addition,the variations of the length and the width of the proposed device slightly alter the CD responses particular the resonance location,but the CD responses are almost immune to the variation of its height.A chiroptical metasurface based on an asymmetric array of identical rectangular GST rods to achieve an ultra-high CD response is designed and simulated in this paper.A maximum CD of 0.93 is obtained by breaking all mirror symmetries and inversion symmetries to introduce chiral quasi-BIC.the ultra-high CD response originates from the ED-MD mode of excitation correlation.By employing the coupled-mode theory,we demonstrate that the chiral metasurface can transmit RCP light approximately perfectly and on the contrary block the transmission of LCP light.Furthermore,the CD values and absorption peaks of the chiral metasurface can be dynamically adjusted by changing the volume fraction of c-GST.