Design Of Longwave Infrared Broadband Achromatic Metalense

With the development of society,infrared thermal imaging technology is widely used in life scenes such as rapid body temperature detection and fault detection.However,the traditional lens modules used in existing infrared thermal imaging cameras are limited by the working principle of curved dimming.The space utilization of the refractive lens is low,and it is difficult to adapt to the requirements of miniaturization.As two-dimensional planar arrays based on sub-wavelength dimension nanostructures,metasurfaces can flexibly adjust the physical parameters of electromagnetic waves such as amplitude,phase,and polarization through artificial design.Metasurface is considered to be the third generation optical elements after refractive optical elements and diffractive optical elements.The metalens developed from metasurface realizes the adjustment of the direction of beam deflection based on the phase gradient introduced by the microstructure in the generalized Fresnel's law.Metalens has a thickness of sub-wavelength size,which theoretically solves the problem that traditional lens curved dimming is not conducive to miniaturization.It provides an ultra-thin and ultra-light flat lens alternative for the design of infrared imaging systems.At the same time,since the fabrication of the metalens is compatible with the semiconductor process,while realizing onchip integration,it can also be fabricated on a large scale,which will greatly reduce the manufacturing cost.metalens naturally does not have spherical aberration,chromatic aberration is the main source of aberration that affects the imaging quality of a metalens.Therefore,how to eliminate the chromatic aberration of metalens has become an important subject in the field of metalens imaging research.The research of this paper focuses on the design of broadband achromatic metalens for long-wave infrared.The main research contents are as follows:(1)A set of design process of broadband achromatic metalens was built,including the simulation of nanostructure units based on the phase distribution function of the metalens at the micro level,the establishment and optimization of the nanostructure units' database,and the numerical simulation and performance characterization methods of the broadband achromatic metalens at the macro level.When the units are used to form the metalens and the numerical simulation of metalens is completed,the rationality of the metalens units' design is reversed according to the simulation results,and repeated optimization is performed until the optimal result of the overall performance is achieved.(2)A broadband achromatic method for metalens is proposed,which uses the dispersion of the nanostructure unit to continuously compensate the dispersion introduced by the wavelength dependence of the metalens' imaging formula.The phase distribution function of the broadband achromatic metalens is derived based on the principle of achromatic aberration.The selection of the aspect ratio of the microstructure is analyzed,and a simulation method of phase-dispersion parameters of the microstructure is proposed by first-order linear fitting.At the 8-12?m and 10-12?m long-wave infrared wavelength,a database of all-silicon microstructure units with a period of 3?m and 5?m for the design of achromatic metalens has been established.The design results of the database show that the nanostructured units in the database have high transmittance,which basically meets the "phase-dispersion" condition corresponding to broadband achromatic.(3)We have designed three broadband achromatic metalenses with different specifications.A high-efficiency long-wave infrared broadband achromatic metalens at the wavelength of 8-12?m,with a diameter of 200?m and a numerical aperture of 0.48 is designed.Its axial focal length drift is only 4%,the focus spot is near-diffraction-limit,and the average focusing efficiency reaches as high as 50%.We alse designed a broadband achromatic metalens with a low aspect ratio at the wavelength of 10-12?m with a diameter of 1mm unit and another near-infrared broadband achromatic metalens at the wavelength of 1.4-1.6?m.The focus spot of two metalenses are both near-diffraction-limit,the average focusing efficiency is 30% and 40% respectively.Taking the time-bandwidth product as a benchmark,comparing the three kinds of super lenses designed in this paper with the existing broadband achromatic super lenses,it verifies the superiority of the overall performance of our high-efficiency achromatic metalens designed in this paper with a diameter of 200?m.(4)We verified the process feasibility of our long-wave infrared broadband achromatic metalenses,and initially prepared an achromatic metalens working in the wavelength of 10-12?m and a diameter of 1mm,and made a brief analysis of its process errors.