Study On The Generation And Focusing Properties Of Structured Beams Based On Polarization-insensitive Metasurfaces

In recent years,structured beams have received more and more attention due to some important characteristics that are different from traditional sources.However,previous methods of generation,focusing,and modulation of structured beams always require complex,expensive,and large optical systems.In order to find possible solutions to above problems,this research made some attempts on the combination of metasurfaces and structured beams.The polarization insensitive metasurfaces can provide flexible phase control under various polarization incidences.And the ultra-thin nature of metasurfaces makes it easy to be integrated into complex systems.By proper design of different metasurfaces,we generated or modulated a variety of structured beams in simulations.We made some analysis on the intensity,polarization of modulated beams and made some optimizations accordingly.Our polarization insensitive metasurfaces provide a high-performance,high-efficiency,and planar solution for structured beam generation and focusing.This research is mainly based on the finite time domain difference method(FDTD).By changing the size of the spatially isotropic nanopillars,the local effective refractive index is changed to achieve polarization-insensitive phase control.The generation of different kinds of optical vortices,the tight focusing of radially polarized beams and the generation of optical needles based on azimuthally polarized vortex beam are realized in this research.Firstly,we realized the generation of normal vortex beams,vector vortex beams,focused vortex beams and Bessel vortex beams based on polarization-insensitive metasurfaces.We obtain the relationships between the radii of nanopillars and the changes of phase and transmission efficiency by scanning the radius of the nanopillars in the unit structure,and then arrange the nanopillars according to the phase profile to construct the whole metasurface.We verified the polarization insensitivity of the metasurface and analyzed the electric field,phase,polarization and other characteristics of the generated optical vortex,and compared them with theoretical distributions.Secondly,we studied the tight focusing of radially polarized light based on polarizationinsensitive metalens.We verified the polarization insensitivity of the metalens,and analyzed the electric field components of focused radially polarized beams,and studied the influence of the numerical aperture and the annular aperture on the size of the focal spots.After the analysis and evaluation of the focused results,diffraction-limited focusing was achieved.Finally,we studied the generation of optical needles based on the focusing of azimuthally polarized vortex beams.We modified the focal phase profile for an extended focal depth,and produced an optical needle with sub-diffraction-limited width and long focal depth by introducing an annular aperture,and studied the polarization characteristics of the optical needle.This work combines the generation and focusing of structured light beams with metasurfaces.The flatness and flexibility of metasurfaces makes the optical system simpler.The comprehensive modulation of metasurfaces ensures that structured beams can be modulated with high quality and efficiency.This research may contribute to optical communications,high-resolution imaging and particle manipulation.