Research On The Generation And Regulation Of Optical Orbital Angular Momentum Based On Metasurfaces

Optical vortex beam is a peculiar type of beam carrying orbital angular momentum which has doughnut-like intensity distribution and helically structured wavefront carrying orbital angular momentum.Optical vortex beams have great application potential in optical communication,photonic integrated circuit,optical capture devices,and so on.However,generating arbitrary orders of adjustable optical vortices remains to be solved.In the past,traditional devices such as space light modulators,vortex shifters,and spiral phase plates were used to generate and control vortex beams.But these methods often require complex optical systems and large optical components,which seriously hinder the development of integration and miniaturization.Metasurface is a kind of micro-nano optical element composed of multiple structural units.The size of these structures can reach hundreds of nanometers or even smaller,which makes the volume of metasurface smaller than traditional optical elements,so metasurfaces can realize the miniaturization and integration of devices.By designing the structural units of the metasurfaces,we can regulate the amplitude,phase,polarization,and transmission of optical field at the subwavelength scale to adapt to the needs of different optical applications.Therefore,using metasurfaces to generate vortex beams has become a research hotspot in recent years.In this paper,firstly,we propose a single-layer metal nanopore metasurface for generating vortex beams in infrared band.Since vortex beams have helical phase wavefront,based on geometric phase control principle,we design the azimuth angle and rotation angle of each structural unit of the metasurface to form helical phase gradient,so as to realize the generation of vortex beams.Vortex beams with topological charges of l=1,2,3,4 can be generated when the circularly polarized light is vertically incident on the designed metasurface.The vortex beams have good quality and can be widely used in optical communication and microscopic imaging.Then,we futher investigate the interaction between multichannel vortex beams of the same order.By using the aforesaid method of generating second-order vortex beams,we construct a metasurface that can generate four-channel second-order vortex beams.When the circularly polarized light incident on the metasurface,first,the four-channel second-order vortex beams will be generated,and then the four-channel vortex beams will interact with each other to generate a new inverted second-order vortex beam at the geometric center of the outgoing light.Energy conversion occurs during this process,and this phenomenon conforms to the conservation of angular momentum,which has certain application prospects in photonic integrated circuits and optical capture devices.