Investigations On Focus Tuning Design Andlarge-Aperture Fabrication Technology Of Metalens

With the continuous breakthrough of the precision of semiconductor industry,and the continuous decrease of the core chip size,the trend of optical instruments is revolutionizing from bulky volume to be miniaturized,compatible and integrated.For traditional optics,optical elements based on the refraction principle,such as lenses,prisms,etc.,are manufactured by milling,grinding,polishing and so on.Not only the manufacturing process is complicated which leads to the large size and the weight,but also limited by the design principle.Volume and weight cannot be further reduced by optimizing the design method.Planar elements based on diffraction optics(such as gratings,Fresnel lenses)and thin-film optics(such as dielectric filters,Bragg reflectors)have existed for decades,however,they have limited functionality and are difficult to integrate with the back-end electronics,which limits the further development.The appearance of meta-surface gives further options in the field of optoelectronics.Meta-surface is an artificial micro-structural array with densely spaced sub-wavelength individuals in format of nano-cell in planar space.Unlike traditional optical elements,the phase of a meta-surface element is almost independent of the cumulative path of light in the transmission medium,which reduces its length in the propagation direction to more than the ten of microns or less,and because its phase is related to the shape,size,location and orietation of the element,the above properties of light can be manipulated by tuning the parameters(phase,amplitude,polarization)to the arbitrary design of the wavefront.Because of its potential for vertical integration and significant design flexibility,meta-surfaces are considered capable of overcoming the difficulties in both traditional and binary optics.The meta-surface greatly expands the traditional optical design,enabling multifunctional components to effectively reduce thickness,size,and complexity.In recent years,advances in the preparation,theory and design of meta-surfaces have provided option for the new optical platform to replace or complete traditional components in many applications.However,there are still some issues need to be addressed in the following areas.For example,the meta-surface design theory is not completely developed yet,and there is no clear link between the unit and the bulk in the current meta-surface design theory,so the overall design results of the unit often need the help of electromagnetic calculation software to get an accurate result,in a larger aperture design or a complex design,the computing requirements of electromagnetic simulation will undoubtedly increase exponentially,and there is no doubt that better solutions need to be found in this case.In addition,the degree of freedom of meta-surface design mainly comes from its unit structure.Further design of the unit structure to achieve more complex composite functional meta-surface is also one of the hottest research topics.Based on the theory of phased array antenna in radar theory,this paper studies the far-field optical field distribution of meta-surface lens,and discusses the design method of near-infrared focus tuning meta-surface lens and the long-wavelength infrared large-aperture meta-surface processing technology.The main research contents and achievements of this work are as following:1.Based on the phased array theory of microwave radar theory,the light field control mode of meta-surface is analyzed.Theoretical calculation and simulation experiment comparison are used to prove that the meta-surface element can be equivalent to the phased array antenna line element to some extent.The relationship between the electric field intensity distribution formulas in the plane where the focal point of a certain focal length of a meta-surface lens is located and the phase parameter of the element is given by using two calculation methods of radiation field in phased array theory(the synthetic aperture method and the phase factor method),respectively.The focal spot radius and focal depth under this condition are described by using the formula.Through the finite-difference time-domain software,the meta-surface lens model corresponding to the calculation results is simulated,the comparison between the simulation results and the theoretical calculation results is given,the causes of the errors in the theoretical calculation are analyzed,and the reliability of the theoretical calculation is verified.2.In the conventional meta-surface lens structure design,a meta-surface structure unit with variable phase in near infrared is designed.Due to the plasma dispersion effect of semiconductor silicon materials,changing the voltage of PN electrode in the semiconductor silicon layer will change the carrier concentration in the semiconductor silicon structure and then the refractive index.In order to amplify the phase change caused by this kind of refractive index change,we introduce the Fabry-Perot resonator structure,and form the Fabry-Perot resonator by adding a multilayer distributed Bragg reflector on the top and bottom of the semiconductor silicon film.A cascaded Fabry-Perot resonator is built under the resonance phase metasurface element to make its electronic control phase meet the phase distribution of 0-2 ?,so as to construct a metasurface lens with the ability of electronic control phase.The simulation results of FDTD software show that it has the ability of continuous focus adjustment in the range of 1 / 4 focal length.3.The processing technology and optical parameters of long-wavelength infrared large-aperture meta-surface lens have been tested.The silicon base is coated with a chromium metal with a thickness of 200 nm and coated with a photo-resist.The maskless pattern is etched on the photo-resist by direct laser writing.After removing the exposure part,the chromium metal is wetly etched to form a metal protection layer.The silicon is then dry etched by ICP etcher.After the etching is finished,the remaining metal is removed by wet etching again to obtain a meta-surface lens sample.In this experiment,the meta-surface lenses with diameters of 20 mm and 50 mm were fabricated by this method,and their focal positions and focal shape were tested by using an optical platform.At the same time,for a 20 mm diameter meta-surface lens,infrared imaging experiments are completed by replacing the traditional lens of the infrared thermal imager.