Study On Spatial Dispersion And Chiral Beam Splitting In Metasurfaces

Chirality is an important geometric attribute of nature that is related to the lacking of mirror symmetry or inversion symmetry in objects.Chiral properties of the structure can give rise to many novel phenomena,for example,the chirality of the drug molecules can affect the living organisms,electron transmission yield through chiral molecules depends on electron spin orientation.In the context of optics,chiroptical responses occur when chiral objects interact with circularly polarized light,including optical activity and polarization conversion dichroism,which play an important role in communication,imaging,sensing and so on.However,chiroptical effects in naturally occurring materials are often very weak due to the size mismatch between molecules and the wavelength of light,which creates difficulties In chiral optics studies.With the development of research,the appearance of metamaterial has provided great help for the research of chiral optics,which is a great candidate for achieving strong chiral response and controlling circularly polarized wave.As an artificial structural material,metamaterial can also show many extraordinary properties which not exist in natural materials,such as negative refraction and strong chiral response.Metasurface,the two-dimensional counterparts to bulk metamaterials,can also have some unusual characteristics.Compared with three-dimensional metamaterials,metasurface has smaller size,which is highly desirable for miniaturization and integration.Here,we focus on the chiral response of metasurfaces,study the new applications and design methods.The specific research contents are summarized as follows:1.First,we analyze the general properties of the metasurfaces based on scattering matrix theory.The conditions for the existence of optical activity and polarization conversion dichroism are then introduced and the polarization conversion limits of the metasurface are explained.Finally,we concisely summarize the scattering matrices of several common metasurfaces,which lays a foundation for our subsequent research.2.We investigate the spatially dispersive dichroism of bianisotropic metamirror,in which giant linear and circular dichroism can be achieved simultaneously.By covering the metallic mirror with an array of bianisotropic resonators,specific linearly and circularly polarized waves can be largely absorbed under normal and oblique incidences,respectively,which has been proved by simulation and experiment.Furthermore,dual-band and hybrid-chirality metamirrors for asymmetric spin reflection have been realized by adjusting the geometries and arrangement of the bianisotropic resonators.3.We transplant the concept of van der Waals heterostructure to metasurface,demonstrating that flip-stacked metasurface can give rise to circular spin-flipped splitting.In this structure,one kind of circularly polarized wave is reflected and the other kind is transmitted in their orthogonal states,which attributes to the special magneto-electric coupling between the two layers.In other words,incident electromagnetic waves with arbitrary linear polarization orientations can be efficiently separated into two orthogonal circularly polarized ones with different trajectories,leading to an ideal circular beam splitter.In addition,the structure we designed also has asymmetric transmission properties.Under opposite direction illumination,the chirality of the structure will be reversed.4.By a combination of the metasurface and the diodes,metasurfaces with controllable chirality are designed.Firstly,we design a chirality switchable metasurface,which can be switched from weak chiral response to strong by controlling the state of the PIN diode.Secondly,we design a chirality-adjustable metasurface,which can be adjusted in four states by combing two diodes,corresponding to left-polarized wave absorption,right-polarized wave absorption,both absorption and neither absorption.5.We have realized a chiral beam splitting flat metalens with a triple-layer metasurface depending on geometric phase manipulating principle.This metalens can transmit and focus one of circularly polarized wave,reflect and diverge the other circularly polarized wave.Moreover,this structure is asymmetry.We have simulated and experimentally verified the function of the metalens in the microwave regime.