Research On Terahertz Chiral Metasurface Coupling Theory And Functional Devices

Chirality usually manifested by the inability of its geometry to coincide with its mirror image by translation or rotation,which is common in nature.It is of great research value in molecular biology,medicine and bioscience.However,natural chiral materials have extremely weak optical chirality,and therefore require very large size materials to enhance the chirality to make usable optical devices,which greatly limits the application and development of chiroptical devices.Metasurfaces are a kind of artificially designed periodic microstructure with sub-wavelength,which have special properties that natural materials do not possess.In recent years,chiral metasurfaces have been rapidly developed,as they have the advantages of smaller size and one or even several orders of magnitude higher chiroptical effect than natural materials.Also,it can achieve the response of other optical properties,such as negative refractive index.In addition,chiroptical effects can be realized by introducing asymmetric factors into the geometry,which greatly reduces the difficulty of producing chiroptical effect.However,planar chiral metasurfaces also have the limitation of weak chirality.Thus,in order to obtain a larger chiral response,researchers enhance chirality by optimizing the structure of the metasurface.Nevertheless,this paper reveals the relationship between coupling strength and chirality by studying planar chiral metasurfaces,so as to enhance the chirality response according to its principle.What’s more,based on the above study of chiral metamaterials,this paper designs a tunable bi-layer chiral metasurface through theoretical design and structural optimization,which significantly improves the performance of chiral devices.The specific research of this paper is as follows:1.Using a simple single-layer metasurface structure,combined with simulations and experiments,via moving the relative position of two resonators(the metal rod(wire)and the spilt resonant ring(SRR)),we observe the asymmetric transmission(AT)from the circular cross-polarization conversion spectrum and circular conversion dichroism(CCD),and the distinguished evolution of chirality is derived.Meanwhile,combined with coupled-mode theory(CMT),it is revealed that there is a high correlation between the chirality of the metasurface and the coupling between the intrinsic structures.And the strong correlation between the difference of the coupling coefficients of two resonators at different relative positions and CCD is further demonstrated that the coupling is the main factor affecting the chirality.2.A bilayer fourfold rotational symmetry(C4)J-type chiral metasurface structure based on the flexible material PDMS was designed.The metasurface made of flexible material is very ductile and can achieve dynamic modulation of the metasurface response by stretching and bending.In this paper,we design a J-type chiral metasurface and simulate the modulation of circular dichroism(CD)under different stretching conditions,and analyze and discuss the causes of the chiroptical effect.Moreover,we simulated the refractive index sensing capability of the structure,and it shows good sensing performance,which has an important potential application in the field of wearable sensors.The research in this paper provides a theoretical basis for studying the relationship between chirality and mode coupling,and a theoretical approach for designing chiral metasurfaces and enhanced CCD with potential applications in optical sensing,polarization imaging,and bio/chemical detection.The designed bilayer chiral metasurface can maintain high CD and good sensing characteristic within a certain stretching range,which has potential applications in the field of wearable sensors.