Research On Beam Conversion Structure Based On Metalens

The metasurface is a two-dimensional artificial material composed of subwavelength scale unit structures.It can control the amplitude,phase and polarization of electromagnetic wave freely and efficiently.Compared with the traditional optical components,the metasurface has the outstanding characteristics of ultra-thin,small loss,easy machining,easy integration and common surface design.In addition,the metasurface can cause waves(electromagnetic waves)local phase mutation,much more than the traditional optical element degrees of freedom,the new regulation can produce some new and interesting optical phenomenon and has been showed great potential applications and value in such fields as super-resolution imaging,electromagnetic stealth,holographic display,medical monitoring,information encryption,and quantum communication,is a research hotspot in the field of metamaterials in recent years.With the continuous development of optical technology and its increasing application in communication,medicine,aerospace,military,industry and agriculture,many beam conversion problems need to be solved,such as collimation,focusing,coupling,synthesis/decomposition,shaping and so on.Beam expansion/contraction of light is a fundamental and important research topic.In order to give full play to the great advantages of the metasurface in light field regulation and expand its practical application,this paper proposes a planar lens structure based on the metasurface to realize the size conversion of the beam.The specific research contents are as follows:1?Two metasurface phase control mechanisms,high-contrast type and geometric phase type are studied.Based on the waveguide effect,using titanium dioxide(TiO2)material with almost no absorption loss in the visible light band,the simulation calculation is carried out by Lumerical's photonics simulation software FDTD Solution(Finite Difference Time Domain,FDTD),and the transmission and phase change of the circular nanopillars under different heights and different duty cycles are discussed.We found a group of nanopillars with the same lattice constant and height but different radius,whose transmission always remained above 91%,and could achieve complete 2? phase control.Based on the concept of geometric phase,using the anisotropy of the rectangular nanopillars,the phase of electromagnetic waves can be regulated by rotating them at different angles.When 532nm right-handed circularly polarized light is incident vertically,the polarization conversion efficiency of the unit structure under different sizes is studied,and it is as high as 95%.Combining with the phase conditions that need to be satisfied for lens focusing,we carry out focusing simulation for the above two different types of metasurfaces.At 532nm wavelength,the minimum focal spot of the high contrast type metalens with a numerical aperture of 0.65 is 0.8?,which realizes the focus close to the diffraction limit.And the deviation between the aperture Angle of the simulated lens and the theory is only 1.9%,which has a good coincidence degree.The effects of phase distribution and diffraction on the focusing efficiency of the metalens were analyzed and optimized.Finally,the maximum focusing efficiency reached 57.9%,the minimum numerical aperture of reached 0.79? after optimization.The minimum deviation of the simulated and preset focal length of the geometric phase type metalens with a numerical aperture of 0.65 is 0.75%.The simulation results show that the metalens designed has good focusing performance.2?A beam conversion structure based on the metalens is proposed.It works like a telescope and consists of a large diameter metalens at the top and a small diameter metalens at the bottom.After passing through this structure,the incident plane wave is well restricted to propagate forward in the form of an approximate plane wave within the size range of the same size as the bottom metalens,and the transmission within this size range reaches 55.9%.In addition,we also explore the application potential of this beam transform structure in realizing low noise detection,and obtain the beam propagation after the structure is integrated with Si detector.The simulation results of FDTD show that this structure has excellent beam transform performance.This provides theoretical guidance for the development of metasurface applications in weak light detection.3?We carried out wet etching experiments based on GaAs epitaxial structure.The etching results were analyzed by optical microscope and scanning electron microscope(SEM).The relationship between wet etching and crystal plane structure of GaAs was discussed in detail.We also compared the anisotropic and isotropic etching of the two corrosion solutions:CH3OH:H3PO4:H2O2=3:1:1 and H3PO4:H2O2:H2O=3:1:1 and obtained the etching rates of 0.589 ?m/min(0?)and 2.8218 ?m/min(room temperature),respectively.This provides a direction for high-throughput and low-cost experimental fabrication of beam conversion structures.