Design And Performance Of Achromatic Metalens For Ultraviolet And Visible Light

Dispersion is one of the important parameters to evaluate the quality of an optical system,but it is ubiquitous in almost all optical systems.Since Sir Isaac Newton discovered the dispersion effect of the prism in the 1770s,people have never stopped pursuing ways to control the dispersion.At present,the traditional optical devices of dispersion regulation method is glued a variety of different refractive index of glass,but because of the traditional refractive lenses realize wavefront shaping by processing the materials into curved surfaces,coupled with the types of natural materials and regulation range is very limited,these reasons lead to these traditional optical devices often bulky volume,what’s more,the material is not compatible with semiconductor devices.Therefore,in miniaturization,lightweight,wearable devices are gradually popular today,traditional optical devices began to meet people’s needs for miniaturization,integration,multi-function,so looking for new materials and design methods to create miniaturized lenses has become an important task of micro-nano optics.Metasurface is composed of a series of subwavelength scale artificial microstructure of ultra-thin surface,it has abnormal electromagnetic characteristics which natural materials do not have.By adjusting the parameters of artificial microstructure,metasurface can adjust the phase,amplitude,polarization and dispersion for arbitrary regulation of the incident light.This characteristic soon attracted the attention of scientists.Scientists hope that metalenses made of metasurface will replace traditional lenses and achieve ultra-thin optical devices with dispersion control.Based on the application requirements of ultraviolet band and visible band,by means of finite difference method of time domain,using generalized Sner law,waveguide theory and effective medium theory.The specific research content of this article is as follows:（1）A design method for a high-efficiency polarization-insensitive achromatic metalens in the UV band（300 nm-400 nm）is presented.With the nanopillar as the cell structure,the silicon nitride with high refractive index and transmittance in the ultraviolet band is selected as the material.By adjusting the period,height and thickness of the nanopillar,try to avoid the structure of resonance absorption and improve the transmittance of the unit cells.And by using higher nanopillars,the phase range of the structural library can reach 0-8π,while the phase coverage of the normal dispersion metalens is often only 0-2π.Due to the expanded scope of the phase library,each required outgoing phase on the metalens has multiple equivalent units available for a single wavelength,and these units have different dispersion responses at other wavelengths.Multiwavelength dispersion independent control is achieved by selecting the structure with the smallest phase error at multiple wavelengths.Except for the optimized wavelength,the phase error analysis found relatively high phase matches at all the other wavelengths in the bandwidth.The designed metalens can achieve a continuous achromatic difference in the 300 nm to 400 nm bands,with a focal shift rate of 6.3%.Moreover,the average focusing efficiency of this metalens is 50.4%.（2）The design method of achromatic metalens with high efficiency linear polarization in the visible light band is proposed.The design method of an achromatic metalens with high efficiency linear polarization in the visible light band（450-800 nm）is presented.The achromatic metalens is based on a series of titanium dioxide nanofins of varying length and width.For the low transmittance of partial structure at 450 nm,transmittance is also a negligible factor in the process of error analysis.The complex amplitude analysis is used here to calculate the phase error of the metalens as well as the amplitude error.By selecting the structure with the lowest complex amplitude error,we designed achromatic metalens for 450 nm,600 nm and750 nm.Through the complex amplitude error analysis,it was found that the complex amplitude matching degree at 800 nm is also high in addition to the optimized wavelength.When the incident light is an x-line polarized light of 450 nm-800 nm continuous broadband,the metalens produces a continuous stable focus of only 5.29%compared to the chromatic metalens（44.70%）.Due to the error analysis of the metalens using the complex amplitude profile and thus avoiding those cells with lower transmission rates,the average focusing efficiency of the entire achromatic metalens reaches 64%in the working bandwidth.（3）The design method for two dispersion control metalens in two visible bands is presented.First,a multifocal metalens based on the polarization selectivity of Ti O₂ nanofins of equal height is presented.When the incident light fraction is only x-ray polarized light or y-ray polarized light,a multifocal metalens based on the polarization selectivity of the equal height titanium dioxide nanofin will produce unique but different foci,indicating that the metalens is polarization selective.Different from the previous hybrid metalens,each part is independent working metalens,because of the global optimization method,each unit can provide high focus efficiency under the condition of arbitrary polarized light incidence,so the focus efficiency under the condition of x-ray polarized light and y-ray polarized light incidence can be reached respectively.Last,a design method for a polarization-insensitive large achromatic metalens is proposed.A polarization-insensitive large chromatic difference metalens is realized in visible light bands ranging from 432 nm-832 nm,using cylindrical polarization-insensitive structures.A dispersion distribution of 9.38 mm can be achieved by a 250μm metalens in the working band.