Fabrication Of Regular Subwavelength Gratings And Complicated Polygonal Structures By Femtosecond Laser

Micro/nanomanufacturing is of importance in many areas in the 21st century,including information science,aerospace,biomedicine,and engineering materials,etc.,due to the features of micro/nanodevices in light weight,small size,strong stability,and high integration.Up to now,traditional manufacturing techniques with a high precision mainly include electron beam lithography,projection lithography,and nanoimprinting.Their process procedures can be divided into two steps:one is to fabricate the fine patterns on the mask by direct wiring,exposure,or embossing;the other is to transfer the fine patterns from the mask to the substrate using dry/wet etching.Although ultra-fine and complicated surface structures can be achieved by these techniques,they still suffer from some significant defects,including expensive equipment,mask requirement,complicated process procedure,and low efficiency.Femtosecond laser micro/nanomachining has emerged in recent years,which enables precise structuring of all kinds of materials with a small heat-affected zone due to the characterizations of ultra-high peak intensity and ultra-short pulse duration.In comparison to traditional manufacturing techniques,this technique owning the single step and maskless features is capable of simplifying the processing procedure and reducing the manufacturing expenses.However,there are still some problems in femtosecond laser micro/nanomachining due to the complicated and unknown processes of laser-matter interaction,such as the controversial formation mechanism for femtosecond laser-induced periodic surface structures(LIPSS)and its poor quality with bending,breaking,and bifurcation.In addition,it is also difficult to fabricate complicated and fine three-dimensional micro/nanostructures on material surfaces using femtosecond laser direct writing technology.To solve the above problems,a femtosecond laser nanolithography based on dual-interface plasmons coupling has been proposed in this dissertation,where super-regular and deep-subwavelength grating structures can be achieved on the surfaces of thin Chromium(Cr)film and Silicon(Si)substrate;the influence of ambient air pressures on the formation of LIPSS on 50-nm thick Cr/Si<100>sample has been investigated,where the role of oxide film on the structure formation and their mutual transition has been pointed out;moreover,an etching approach with the aluminum sulfate crystal as the mask has been proposed,where pyramid microstructures and tetrahedron nanostructures can be fabricated controllably on the sapphire surface.In brief,the main content of this dissertation can be summarized as follows:1.The fabrication of super-regular and deep-subwavelength grating structures has been successfully achieved on the 25-nm thick Cr/Si<100>sample under irradiation of800 nm femtosecond pulses in the vacuum condition(P=2.0×10^-5 Pa).The influences of laser parameters,the thickness of the metallic Cr film,and the substrate material have been also investigated,and it is found that the thickness of the metallic Cr film plays an important role in the structural period and regularity.According to these phenomena,a theoretical model involving the excitation of dual-interface coupled plasmons in the thin metal layer has been proposed to explain the formation of nanogratings,and their intensity distributions in the thin Cr film greatly affect the structural regularity.2.Super-regular and deep-subwavelength grating structures have been experimentally demonstrated to imprint into the Si substrate via the Cr-layer mediated femtosecond laser machining.It is found that the Si nanograting structures exhibit a non-sinusoidal groove geometry and a large depth-to-width aspect ratio of 2.1.In addition,the grating structures can be also stitched perfectly in space with a proper line overlapping ratio,which provides a valuable study for large-scale fabrication.Moreover,the further deepening of periodic nanostructures into the Si substrate is also elucidated using the FDTD method,where the electric field enhancements arise at the bottom of shallow grooves under irradiation of subsequent laser pulses.In combination with a wet etching process,the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles,whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565-750 nm.3.The influence of ambient air pressure on the formation of LIPSS has been investigated through irradiation of 800 nm femtosecond laser pulses on 50-nm thick Cr/Si films.As the air pressure gradually decreases,the periodic surface structures undergo an evolvement from the anomalously oriented subwavelength to normally oriented deep-subwavelength LIPSSs.The Raman measurements and analyses reveal that such structure evolution is closely related to the amount of the oxide induced by the preceding pulses.The intriguing structure transition is explained in terms of the competitive excitation between the transverse-electric scattered surface wave and transverse-magnetic hybrid plasmon wave.4.The controllable fabrication of micro/nano polygonal structures was successfully achieved on the sapphire surface using the technique combining femtosecond laser irradiation and wet etching.It is found that pyramid microstructures and tetrahedral nanostructures can be selectively fabricated on the sapphire surface after processing with suitable laser energy and then etching in the sulfuric acid solution.Through the analysis and comparison of experiments,an etching model with the aluminum sulfate crystal as the mask is proposed to explain the formation of these complex polygonal structures.In addition,the large-scale fabrication of inverted-tetrahedron structures with a smooth surface can be also achieved on the sapphire surface after processing with a single femtosecond laser pulse and then etching in the mixed solution of sulfuric acid and phosphoric acid.