Research On Electromagnetic Focusing And Special Beam Generation Based On Dielectric Metasurface

Metasurface is an array structure composed of subwavelength units(meta-atoms)of different shapes and sizes.In the field of electromagnetic waves,metasurface is mainly used to control the amplitude,phase,and polarization of electromagnetic waves.Unlike traditional optical devices,metasurface has the characteristics of ultra-thin,easy to process and high integration.Since it was proposed,it has attracted the attention of many researchers.So far,metasurface has shown strong performance and great application potential in the fields of beam fabrication and control,quantum communication,information encryption and holographic imaging display.With the continuous development of science and technology,metasurface will appear in all aspects of life,such as medical devices,intelligent driving,optical communications and aerospace.Among them,using metasurface to focus electromagnetic wave and generate special beam has important application value.In this paper,a focused beam,a focused optical vortex,and an Airy beam are generated by using a metasurface.The specific research contents are as follows:1.Based on the principle of geometric phase,a meta-atom with multiple geometric dimensions is constructed by using particle swarm optimization algorithm in electromagnetic simulation software.Using this meta-atom,two kinds of metasurfaces have been designed,one is a metasurface lens that focuses the light beam,called a metalens,and the other is a metasurface that produces a focused optical vortex.These two metasurfaces are constructed by titanium dioxide nanocolumns,which has low loss properties in the optical band.The numerical aperture of the metalens is set to 0.45.Due to the assurance of the rotational symmetry of the phase,the entire metalens array arrangement has rotational symmetry,and its focusing efficiency is about 60%.Due to the particularity of geometric phase,we study the focusing of the metalens under blue,green and red light,and the results show that the metalens has the focusing ability in a wide band range.Furthermore,by combining the orbital angular momentum with the metalens,the metasurface is capable of producing a focused optical vortex.We design and construct a metamaterial that can generate focused optical vortices with different topological charges,and the beam generation efficiency is about 55%.This study enriches the design methods of metasurface and provides research ideas for developing more new functions of metasurface.2.Based on the principle of transmission phase,square nanorods are selected to construct the metasurface to generate Airy beams.Airy beam is a non-diffracting,self-bending,self-healing special beam.We have designed a metasurface that generates a one-dimensional,polarization-insensitive Airy beam.At the same time,we verified the non-diffraction,self-healing,and self-bending characteristics of the resulting Airy beam.Further,the self-focusing property of the airy beam was verified by arranging two identical hypersurfaces to form a new hypersurface.This study effectively avoids the ohmic loss caused by the interaction between light and metal and improves the efficiency of producing Airy beams on the metasurface.At the same time,the hypersurface is insensitive to light polarization and has great application potential.In this paper,the three different types of metasurface have the characteristics of small volume,light weight and ultra-thin,which can flexibly adjust the light field and produce specific beam,providing research ideas for the miniaturization and functionalization of optical equipment in the future.