Research On Laser Fabrication Of Micro-nano Structures On Phase-change Materials And Their Applications In Metasurface Absorber

Metasurface is an ultra-thin,two-dimensional planar material composed of artificial subwavelength structures that can flexibly control the amplitude,phase,or polarization of electromagnetic waves.The metasurface absorber,a type of amplitude-controlled metasurface,can achieve perfect or near-perfect absorption of a single wavelength,multiple wavelengths or a wide band of incident light and has important applications in the fields of solar energy utilization,photoelectric detection,camouflage,spectroscopy,etc.In most studies of metasurfaces,their ultimate functions are determined and are not suitable for applications that require dynamic modulation of the response to light.The phase-change material Ge₂Sb₂Te₅（GST）possesses properties of fast modulation speed,easy integration,and non-volatility.It is an ideal material for realizing tunable and reconfigurable metasurfaces.Femtosecond laser offers several benefits in material processing,including high processing precision and minimal thermal effect.By inducing phase change combined with etching methods or direct ablation,this technology can create micro-nano structures on the surface of phase-change materials.Moreover,it shows certain potential in preparing metasurfaces of phase-change materials with high efficiency and large area.To make this processing technique feasible for fabricating large-area functional devices,systematic process optimization and detailed design solutions are needed to apply it to the design and fabrication of high-performance metasurface devices.This dissertation aims to investigate the design and large-area fabrication of multi-band and broadband metasurface absorbers based on GST,utilizing femtosecond laser-induced phase change and ablation combined with the etching property of material,and also the laser-induced periodic surface structures.The main research contents of this dissertation are as follows:Firstly,experiments of phase-change property and optimal experiment of etching process were carried out on the laser-induced phase change process and the etching process of phase-change materials,respectively.The influence of laser parameters on the phase change of GST was studied,and the improvement methods that can increase the aspect ratio of the structure during the phase change and etching process were analyzed,which offers a technological foundation for the preparation of metasurfaces of phase-change materials in subsequent research.The basic optical constants of GST thin films in various bands were measured to provide theoretical data for the subsequent simulation design of phase-change material metasurface absorbers.Secondly,the different mechanisms of the femtosecond laser and the phase-change material GST during the laser-induced phase change and ablation process were explored.By exploring the relationship between GST phase change/ablation and the corresponding threshold under the Gaussian beam irradiation,the relationship between the local irradiated laser fluence and the formed structure was obtained.Based on the proven laser-induced phase change/ablation mechanism of GST materials,sub-diffraction-limited processing was realized.Combined with wet etching methods,size-controllable ring or disk structures can be directly fabricated in a single irradiation.Thirdly,a multi-band tunable metasurface absorber in the mid-infrared band of 2-6μm was realized by using the fabricated sub-diffraction-limit ring structure as the metasurface antenna structure,combined with different material layers.The phase change of the GST film from the amorphous state to the crystalline state can further regulate the controllable shift of the metasurface absorption peak.At the same time,the processing efficiency of the single-pulse irradiation processing method used to fabricate the ring structure can be greatly improved by laser pulse separation,so as to ensure the efficient and low-cost preparation of large-area metasurface absorber devices.Finally,by introducing a linear light source to further increase the single-irradiation processing area,and based on the ultrafast laser excitation of the material surface plasmon wave and the laser-induced GST thermal phase change mechanism,the rapid fabrication of large-area short-range disordered laser-induced periodic surface structures on the GST surface was realized.Due to the broken symmetry,the short-range disordered subwavelength metagrating is characterized by polarization-insensitive absorption.By combining the design of the interface matching layer,the constructed metagrating can achieve high broadband absorption in the visible and near-infrared band of 400-1100 nm.This dissertation mainly focuses on the phase-change and ablation characteristics of phase-change materials under laser irradiation,proposing an efficient and low-cost processing method for large-area metasurface absorbers with sub-diffraction-limit nanostructure features.The designed phase-change material metasurface absorbers in this paper are unique in structure and function,including ring and grating structures in structure,and multi-band absorption and broadband absorption in function.The spectral ranges are 2-6μm in the mid-infrared band and 400-1100 nm in the visible and near-infrared band,respectively.The proposed manufacturing method can achieve efficient large-area nanofabrication,and the designed and large-area fabricated devices have broad development potential in related fields such as solar energy absorption and sensing.Furthermore,the relevant structures and results in this dissertation have the potential to be extended to the design,preparation and application of other wavebands and even other functional metasurfaces,and also provide valuable reference for related research.