Focusing Controllable Multifocal Meta-lens Based On Terahertz-Metasurface

In the past two decades,terahertz technology has experienced considerable development,especially in the integration of terahertz functional devices.Regardless of the national "13th Five-Year" plan or the upcoming national "14th Five-Year" plan,terahertz-related fields have received strong support from the state and achieved great results.Because the terahertz band has the characteristics of higher frequency,larger bandwidth,better directivity,low photon energy,and fingerprint characteristics of biological macromolecules,terahertz is very useful in astronomical detection,national defense communications,security imaging,and biomedical testing.Potential applications.Although terahertz technology has achieved phased results,the applications of terahertz technology are in urgent need of development.As a novel and simple mean of controlling electromagnetic waves,metasurfaces can control the terahertz waves by modifying the structural characteristics of the metasurfaces,which is an excellent way to solve the problem of terahertz control.Based on the metasurface,this paper has carried out research on the control of terahertz waves,including the control of the phase and amplitude of the terahertz waves,and then conducted an in-depth study on the control of the wave front.The response of the metallic C-ring unit structure to terahertz waves is studied,and the Huygens cylindrical lens and the meta-lens that can actively switch the focus are designed according to the different phase gradient metasurfaces.The control of terahertz waves is realized by the proposed meta-lens and the performance of the lens is analyzed according to the focusing effect.The specific research work is as follows:A metasurface with abnormal refraction based on metallic C-ring structure is designed.Based on the theory of phase gradient,we systematically analyze the influence of metasurface structure on terahertz waves.Based on the theory of phase gradient,the influence of the metasurface structure on the terahertz wave was systematically analyzed.By changing the structural parameters of the unit cell,the amplitude and phase modulation of the electromagnetic wave are realized.We derive basic 8 uint cells of C-ring structure with consistent amplitude,uniform phase shift π/4(covering 0～2π range).The refraction angles of the metasurface at 0.65 THz,0.8THz,and 1.05 THz are simulated and verified.A terahertz metasurface Huygens cylindrical lens based on a metallic C-ring is designed,which improves the focus strength by 117% compared with a single-layer metasurface lens.The method for the arrangement of the cylindrical lens array is given,and the enhancement of the focusing efficiency is analyzed and researched in combination with Huygens principle.We design a metallic C-ring based metalens that can actively control and switch the focus.Laser excites the semiconductor to change its electrical conductivity,so that the silicon layer in different areas is changed from dielectric to conductor,and the double foci,single long focus and single short focus formed by the metalens are realized.and the three states can be switched by the patterned laser pump,the maximum focusing efficiency reaches 13.94%,and the diffraction limit and numerical aperture can reach 0.59λ and 0.90,respectively.The lens achieves complete separation and independent switching of multifocals,which will help the development of terahertz multi-functional applications,especially in microscope imaging and tomography.