Research On Focusing And Bessel Beam Based On Au/WS₂ Hybrid Nonlinear Metasurface

With the advancement of information technology,due to the limitation of the diffraction limit of traditional photonics devices and the poor quality of information transmission,people have begun to move towards the research of metasurfaces based on the generalized law of optical refraction and reflection.Because linear metasurfaces have lower second harmonic efficiency,non-linear metasurfaces not only have higher non-linear efficiency,but also have huge potential applications in non-linear imaging systems and optical processing.Therefore,I have designed two kinds of non-linear metasurfaces independently,namely,a metasurface capable of achieving multi-focus focusing function and a metasurface that generates a non-diffractive Bessel beam.In this work,the main contents of this thesis are as follows:(1)We propose a new multi-focal point focusing nonlinear metalens based on a gold-tungsten disulfide composite structure.For this composite nonlinear multifocal metalens,the gold nanohole array acts as a phase plate,and the tungsten disulfide single layer acts as an efficient second harmonic(SHG)transmitter and enhances the SHG signal of the structure.In this paper,two-dimensional material transfer method was used to prepare the composite metasurface,and the self-constructed confocal test microscope system was used to test the sample.Experiments have proved that a multi-focal point non-linear metalens can generate three kinds of focal points with focal lengths of 50?m,100 ?m and150 ?m,respectively.When excited by the left-handed circularly polarized(LCP)fundamental frequency light,the output right-handed circularly polarized(RCP)frequency-doubled light is focused at two different points.Under the excitation of RCP incident light,the generated LCP second harmonic light is focused on the third point.This multi-focus non-linear metasurface also shows the advantages of small focus,long focal depth,and low background noise.(2)We use the same principle to propose a non-linear metal-axicons to generate a second harmonic bessel beam.By designing different phase plates,the metasurface can generate zero-order and first-order non-diffractive Bessel beams with frequency doubling under the excitation of 810 nm femtosecond laser.In addition,the excellent performance of non-linear metal axicon lenses,such as enhanced SHG signal,small focus and low noise,long non-diffraction distance and full-width at half maximum(FWHM)experimental results and theoretical results are consistent with each other,so that the micro non-linear optics make it possible to generate non-linear Bessel beams.In addition,Bessel beams have potential applications in non-linear optical manipulation,imaging,and traction beams due to their non-diffraction and self-reconstruction characteristics.