Investigation On Chromatic Control Based On Polarization-dependent Metalens

Polarization is an important physical quantity to describe the properties of electromagnetic waves.The detailed characteristics of material structure and composition can be obtained by analyzing the polarization state of electromagnetic waves.The polarization measurement system can obtain the polarization degree of incident light and the Stokes vector by characterizing the intensity of each polarization component.Conventional polarization detection systems usually combine polarizationdependent devices with other photoelectric devices,which generally lead to the problems of complex structure and difficult integration.Metasurface is a twodimensional artificial electromagnetic material composed of subwavelength structures.On the one hand,it is expected to replace the traditional optical devices which can realize the miniaturization and lightweight of the system because of the characteristics of ultra-light,ultra-thin,and easy integration.On the other hand,the amplitude,phase,and polarization of electromagnetic wave parameters can be flexibly manipulated by varying the spatial rotation and size of the unit-cell.Hence metasurface devices are widely investigated in optical field modulation and measurement technology.Metasurface devices are considered as planar lenses for imaging and deflection,but they are usually difficult to realize color imaging and display due to the narrow working bandwidth and chromatic aberration.At the same time,they can also be used in the polarization measurement system,but they usually detect the polarization information of a specific wavelength or respond to a single polarization component.Thus,our paper focuses on the above issues and the main research contents include:1.Aiming at the chromatic aberration control problem of circular polarizationdependent metalens,a broadband achromatic metasurface focusing lens is designed in the visible region.The metalens is composed of titanium dioxide dielectric nanopillar arranged periodically.And the nanopillar possesses low loss and high refractive index which can be treated as a truncated waveguide to control the propagation phase in visible wavelengths,and the phase response database was constructed by using the CST simulation software for parameter scanning.The polarization conversion efficiency was over 40%.At the same time,we analyze the dispersion modulation mechanism which merges the geometric and propagation phases,and the particle swarm optimization algorithm is used to optimize the phase response database,which can accomplish the phase matching between the ideal and actual wavefronts to realize the designed function.The device can realize the broadband achromatic focusing within 400-660 nm,and the diffraction efficiency at the focal plane in the bandwidth range is 20-45%.We employed the design of simple rectangular nanopillar structures while ensuring the operating bandwidth and diffraction efficiency.The strategy reduces the difficulty of full-mode optimization design and the complexity of processing technology.2.In the polarization measurement system,a multi-wavelength achromatic full Stokes polarization measurement metasurface device is designed to solve the problem in which the polarization-dependent metasurface lens operate at a specific wavelength.The guide mode resonance effect and transmission phase which excited by the anisotropic amorphous silicon(?-Si)structure are used to realize the three-wavelength chromatic aberration control in the state of linear polarization.Similarly,the chromatic aberration control of circular polarization can be realized by combining the mechanism with the geometric phase control principle.Besides,six multi-wavelength achromatic metalenses were obtained by optimizing the arrangement of the nanopillar structure,which responded to 0?,90?,45?,135? linear polarization,left and right-hand circular polarization states independently.Finally,those metalenses can be combined into a metasurface polarization measuring device.The simulation adopts three discrete wavelengths and six kinds of polarized incident light.The metasurface device can realize the achromatic focusing function,and the ability to characterize full Stokes parameters is verified.Hence,the designed metasurface devices are expected to realize multi-wavelength achromatic and multi-polarization components imaging.