Research On Micro/Nano Structure Devices Based On The Manipulation Of Light Field In Optical Band

Traditional optical devices rely on the propagation distance of light waves far exceeding the wavelength in the medium to accumulate phase and reshape the wavefront,resulting in large size and complex structure of the device,which is not conducive to the miniaturization and integration of optical systems.With the rapid development of large-scale and large-capacity optical information transmission,processing,and storage technologies,the need for effective regulation of optical fields at the micro-nano scale is becoming more and more urgent.Artificial micro-nano structure is a micro-nano structure that uses micro-nano processing technology to prepare materials into a specific design shape,size and arrangement mode.It has a structure size equivalent to wavelength or sub-wavelength,and can effectively regulate the amplitude,phase and polarization of electromagnetic waves.Optical devices based on artificial micro-nano structures can realize functions that are difficult to accomplish with traditional optical devices.In recent years,the graded-index photonic crystal（GPC）and metasurfaces based on artificial micro-nano structures have excellent optical field manipulation capabilities,as well as ultra-thin and ultra-light two-dimensional planar structures,which provide a new solution for optical field regulation at the micro-nano scale.This provides great potential for the miniaturization and integration of optical devices,and also creates positive conditions for the development of photonic intergraed circuits in the future.At present,many research results have been achieved in the research of GPC and metasurfaces in the optical band,but further researches are still needed in the theoretical design and experimental preparation of GPC,the polarization conversion of metasurface waveplates,and the multifunctional metalens.In this paper,based on the artificial design of the phase response of the micro-nano structural unit,the light field regulation including focusing and polarization conversion of GPC and metasurfaces is studied.The main research contents and achievements are as follows:（1）Since the gradual modulation of the GPC structural parameters increases the complexity and difficulty of the fabrication process,the traditional photonic crystal（PC）fabrication method is no longer applicable.In this paper,a new method for designing and fabricating GPC structures and lenses is proposed by using a two-parameter top-cut hexagonal prism to construct a double cone interference model.Firstly,a multi-beam double cone interference model is constructed based on the self-designed dual-parameter hexagonal prism,and the GPC structure is designed by adjusting the parameters of the double cone interference model.Then,based on the GPC structure,the size of the dielectric column is designed to gradually change from center to edge,so the effective refractive index（phase adjustment）of the lens is gradually adjusted.The focusing performance of the lens is studied,and a long focal depth focusing of 15.8 times the wavelength and a transmittance of over 86%in the visible light band are achieved.When the incremental angle of the double cone interference model is a fixed value,the main angle is the main factor that determines the focusing performance of the GPC lens.Finally,the double cone interference device based on the dual-parameter hexagonal prism is built in the laboratory,which realizes the rapid preparation of submicron-sized lattice periodic GPC structures.（2）Aiming at the problem of low polarization conversion efficiency of metal metasurface waveplates in the visible light band,two types of transmissive and all-dielectric metasurface waveplates are designed using low-loss TiO₂ materials to realize the polarization control function of the light field.Based on the structure of TiO₂ elliptical column unit and birefringence theory,by optimizing the size of the major and minor axes of the ellipse and adjusting the polarization conversion characteristics of the structure,a quarter-wave plate and a half-wave plate are designed.The polarization conversion efficiency,electromagnetic resonance characteristics and incident angle sensitivity are studied.The polarization conversion efficiencies of the designed waveplates are all over 90%at the wavelength of 532 nm,and they have a certain broadband working ability,among which the bandwidth of the quarter waveplate reaches 77 nm.At the same time,when the oblique incidence angle of the quarter-wave plate changes within 30°,and the oblique incidence angle of the half-wave plate changes within 28°,the polarization conversion performance of the wave plates remain stable.（3）In view of the limitation that the single-layer geometric phase metalens is sensitive to the polarization of incident light,based on the transmission phase theory and the unit structure of the square TiO₂ dielectric column,the cross-sectional size and height of the dielectric column are optimized to achieve 0～2pphase mutation.The metalenses are designed that are insensitive to left-handed circular polarization（LCP）and right-handed circular polarization（RCP）incident light,and achieve subwavelength focusing performance.At the same time,based on this lens,an amplitude control layer is added,and a theoretical model of the double-layer transmission phase is proposed.The total phase of the superunit structure can be controlled by the upper subunit structure,and the total amplitude can be controlled by the lower subunit structure,so that the two parameters of amplitude and phase can be controlled simultaneously.The unit structure size of the amplitude control layer is optimized to realize the adjustment of transmittance from 0 to 1.The multi-focal lens array with dual-parameter compound control of amplitude and phase are designed,and multi-point focusing characteristics with different beam splitting ratios are achieved through double-layer compound control.（4）Aiming at the problems of redundant diffraction orders and performance degradation of single-layer spatially multiplexed metasurfaces,a theoretical model of double-layer geometric phase metasurfaces is proposed.The theoretical model is established on the basis of geometric phase theory and joint Jones matrix theory by optimizing the structural dimensions of upper and lower TiO₂ nanopillars and designing their rotation angles.Using this model,the lenses structure with two operating wavelengths（532 nm and 632 nm）and two polarization modes（LCP and RCP）are designed and their focusing characteristics are studied.The multi-dimensional control of dual wavelength and dual polarization states is realized.At the same time,based on the combined advantages of geometric phase and transmission phase,a composite phase modulation principle is proposed.Through the joint design of the three parameters of length,width,and rotation angle of the nanopillars,the phase requirements of LCP and RCP light are met,and each unit structure can simultaneously control the separation and focusing of LCP and RCP light.So decoupling the regulation of LCP and RCP light is achieved using a monolayer metasurface.And three kinds of bifocal single-layer metalenses are designed,which can focus the LCP and RCP components in linearly polarized light at the symmetric,asymmetric and axial positions at the same time.The designed lenses have a polarization-sensitive control function,which can adjust the position and polarization state of the focal point according to the polarization state of the incident light.