Broadband Achromatic Liquid Crystals Metalens And Imaging

Modern advancements in optical devices have focused on miniaturization and integration.Micro-nano optical devices,including metasurfaces and diffractive optics,have gained significant attention due to their ability to manipulate the light field within a compact size.Achieving high-contrast,colorful,and high-resolution optical imaging using micro-nano devices is an important research topic.However,conventional micro-nano optical devices such as metalens and diffractive lenses are limited to operate at single wavelength or narrow wavelength ranges due to severe chromatic aberration and require complex preparation processes.Recently,liquid crystal metasurfaces,a burgeoning branch of micro-nano optical devices,offer an attractive solution due to the inherent anisotropy and excellent photoelectric response of liquid crystals.These properties enable the precise control of liquid crystal molecule orientations and facilitate the development of various liquid crystal optical devices without complicated preparation procedures.Nevertheless,dispersion effect in liquid crystal hinders the widespread application and development of liquid crystal metalens for optical imaging,limiting their applications to single or narrow-band wavelengths only.In this thesis,the working mechanism,preparation technology,and imaging capabilities of a monolithic broadband achromatic liquid crystal metalens are investigated.The main content and results of the thesis are as follows:(1)A monolithic RGB three primary colors achromatic liquid crystal metalens(TPC-ALC-ML)based on wavelength multiplexing coding is proposed and experimentally demonstrated.To address the issue of chromatic aberration in traditional liquid crystal metalens,a metalens phase distribution that operates at three individual wavelengths corresponding to RGB colors is introduced with wavelength multiplexing coding.The contribution weights of each wavelength at the focal plane are also optimized in order to achieve balanced intensities of the three primary colors.The optimized phase distribution is then encoded into the orientation arrangement of liquid crystal molecules based on geometric phase,resulting in a monolithic three primary colors achromatic liquid crystal metalens.A wavelength multiplexing coded liquid crystal metalens with a 6 mm aperture and 200 mm focal length is fabricated using single projection exposure of a spatial light modulator,and comparative imaging experiments are conducted between TPC-ALC-ML and a conventional liquid crystal metalens(C-LC-ML)with identical structural parameters.The experimental results demonstrate that TPC-ALC-ML achieves clear images approaching the diffraction limit at the three RGB wavelengths of 450 nm,526 nm,and 630 nm.(2)A monolithic continuous broadband achromatic liquid crystal metalens based on cubic phase coding(cubic phase coded achromatic liquid crystal metalens,CPC-ALC-ML)is proposed and experimentally demonstrated.On the basis of the previous work,the imaging capability of the liquid crystal metalens at discrete wavelengths is further expanded to continuous broadband achromatic imaging.A cubic phase coding terms is added to the conventional parabolic phase distribution.The introduction of the cubic phase ensures that the point spread functions of different wavelengths within a wide spectral range at the focal plane remain unchanged.Exploiting this characteristic,the point spread function can be employed as a filtering function to perform inverse deconvolution with the intermediate blurred image obtained from the cubic phase encoded lens,and clear imaging within the wide spectral range can thus be achieved.The proposed CPC-ALC-ML with a 4 mm aperture,100 mm focal length,and coding coefficient of 50π is experimentally fabricated utilizing a spatial light modulator for single projection exposure.Comparisons of imaging performance between CPC-ALC-ML and a C-LC-ML with identical parameters are conducted.The experimental results agree well with theoretical simulations.The cubic phase coded liquid crystal metalens expands the achromatic bandwidth to 100 nm,enabling continuous broadband achromatic imaging from 500 nm to 600 nm.Moreover,the imaging resolution within the wide spectral range reaches the diffraction limit.(3)An achromatic liquid crystal metalens across the entire visible spectrum range from 400 nm-750 nm is proposed and demonstrated with an inverse design approach(entire visible spectrum achromatic liquid crystal metalens,EVS-ALC-ML).An inverse design method that utilizes neural network computation and optimization to determine the lens phase distribution that achieves a common focal plane for different wavelengths throughout the entire visible spectrum(400 nm-750 nm)and thus achromatic imaging is proposed and demonstrated.A continuous achromatic liquid crystal metalens with a 2 mm aperture and 20 mm focal length in the wavelength range of 400 nm to 750 nm is computationally designed and optimized.To minimize the impact of sidelobes on imaging quality,constraints are applied during the optimization process,and the energy efficiency is also enhanced.The inversely designed EVS-ALC-ML is fabricated using laser writing technology,and imaging evaluation are conducted with the fabricated metalens.The experimental results demonstrate achromatic imaging across the entire visible spectrum with the focusing approaching the diffraction limit.