Research On Imaging Metalens With Wide Bandwidth And Large Viewing Field

Metamaterial,an artificially structured material that exhibits extraordinary electromagnetic properties not available or not easily obtainable in nature.However,three-dimensional metamaterials are often difficult to fabricate.Therefore,two-dimensional metamaterials based on the principle of metamaterials becoming more and more popular with researchers.Metasurface has excellent ability to regulate the amplitude,phase,polarization and other characteristics of electromagnetic wave,and it has the potential to replace the traditional large and heavy lens set.At present,the research of metalens mainly focuses on two aspects:one is how to achieve wide band achromatic,the other is how to achieve large field of view（FOV）imaging.In practical application,both of them are very important,so it is of great scientific significance and economic value to research the metalens which can simultaneously realize the imaging of large FOV and wide waveband.In this work,based on the phase modulation principle,the transmission phase principle applied to double-layer metasurface is proposed to realize the imaging of metalens with large FOV and wide waveband,and the imaging metalenses with large FOV and wide waveband working in visible and mid-infrared are designed.The work can be divided into the following parts:（1）In the visible light,a double-layer metalens to achieve achromatic imaging over a FOV of 60°in the range of 470 nm to 650 nm is designed and realized.First,the phase profiles of the two layers are optimized by ray tracing method.Then,the height of the layers and the distribution of the Ti O₂ nanorod in the whole metasurface are optimized,the phase and amplitude of multiple wavelengths are controlled by Ti O₂ nanorod with different radii.The simulation results show that in the RGB wavelengths,the focal spots of all the FOVs are almost located in the designed focal plane.MTF curves are very close to the diffraction limit.At the normal incident,the focusing efficiency of red light reaches81.6%,the efficiency of green light is 66.7%,and the efficiency of blue light is 42%.For the whole design waveband,the achromatic and large FOV imaging is achieved,and the average focusing efficiency at normal incidence is over 60%.（2）Based on the principle of deep learning,a feedback forward neural network model is designed to predict the radius of the nanorod by the required phase.First,the input and output are determined according to the requirements,and then the configuration of the hidden layer is determined according to the experience and experimental results.The trained neural network model is used to predict the unit of the metasurface,and the metalens is modelled for simulation.The results show that the linear regression coefficients of the first layer and the second layer are as high as 99.99%and 99.45%respectively.At RGB walengths,the metalens can focus in the same focal plane in the range of 0-±30°FOV,and the focusing efficiencies of RGB wavelengths at normal incidence are 77.2%,44.9%and 45.2%,respectively.（3）In the mid-infrared range of 10-11μm,the achromatic 0-±30°FOV imaging metalens is designed by using the principle of transmission phase.The metalens is fabricated and characterized by lithography and ion etching.The results show that the metalens has good appearance and the preparation process is reliable.The focusing effect can be achieved in the designed wide waveband,and the FOV is 60°.The average focusing efficiency is 20.62%at normal incident and 9.1%at 30°incident.