Research On Ultrafast Laser Fabrication Of Large Area Sapphire Micro-nano Structures

Sapphire crystals have a wide spectrum of high transmittance from ultraviolet to mid-infrared,ultra-hardness,good chemical and thermal stability,and are commonly used as window materials and substrates in essential fields such as military,aerospace and semiconductor.The performance of sapphire optoelectronic devices can be further modified by preparing micro and nano structures on their surfaces.However,the high stability of sapphire makes its micro and nano processing difficult by traditional processing techniques,which limits the further development of sapphire micro and nano optoelectronic devices.For example,it is difficult to prepare micro and nano-scale structures by mechanical processing,and the materials that can be processed are relatively limited;the planar etching process faces problems such as the inability to prepare structures with complex morphology,slow etching rate,and the inability to prepare micro and nano structures with high aspect ratio.Femtosecond laser processing is widely used in microelectronics,micro-optics and micro-sensing as an advanced micro-nano processing technology with high precision,high intensity and maskless.Femtosecond lasers have great potential for preparing sapphire micro and nano structures.However,the current work on femtosecond laser processing of sapphire is mainly focused on the study of the interaction mechanism between laser and sapphire crystal and the preparation of functional microdevices,and there is less research on the preparation of large-area sapphire micro and nano structures for application level.The main reason is that the conventional laser processing process influences the subsequent laser energy injection due to the damaged structure and debris accumulation during the processing,which leads to the inability to realize the efficient preparation of high aspect ratio and large area complete micro-nano structures,which seriously affects the development of functional micro-nano surfaces of sapphire.Based on the demand for the fabrication of large-area sapphire functional micro-nano surfaces,this paper proposes an aberration-based femtosecond laser self-modulation processing technique,which solves the problem of complex preparation of high aspect ratio and large-area complete micro-nano structures due to damaged structures and debris accumulation during conventional laser fabrication.Based on this,combined with the wet etching process,we significantly increased the depth of laser-induced surface periodic structures and prepared large-area high aspect ratio nanograting structures;increased the depth of laser direct writing microstructures and prepared large-area sapphire high subwavelength anti-reflection pyramidal array structures;realized patterned sapphire substrates and confirmed that high-quality Ga N films could be grown on PSS prepared by laser fabrication.The main research results are listed as follows:1.Femtosecond laser self-modulation processing based on aberration.To address the problem that it is difficult to achieve complete preparation of large-area structures and preparation of high-depth structures due to damaged structures and debris accumulation affecting laser energy injection during conventional laser fabrication on sapphire surfaces.In this paper,we propose an aberration-based femtosecond laser selfmodulation processing technique combined with a wet etching process to prepare various large-area depth-controlled micro-nano structures on sapphire surfaces successfully.This technology can give full play to the longitudinal length of the focused spot,effectively increasing the depth of the prepared structure.Compared with conventional processing methods,the depth of the microstructure prepared by selfmodulated processing technology can be increased by 8 times;this technology can avoid the influence of surface damage and debris on the subsequent laser energy during processing,ensuring the integrity of the large-area sapphire microstructure prepared by laser processing.In addition,the feasibility of laser self-modulated processing technology in preparing high numerical aperture microlenses was verified.This technique provides a new idea for laser processing of high aspect ratio micro-nano structures of transparent materials and technical support for the subsequent preparation of large-area functional sapphire micro-nano surfaces.2.High aspect ratio sapphire periodic nanostructures prepared by femtosecond laser-induced processing.To address the problem that the depth of laser-induced surface periodic nanostructures is generally shallow and difficult to be tuned,we achieved a sapphire periodic nanostripe structure with a maximum aspect ratio of 55 and a tunable aspect ratio using laser self-modulated induced processing combined with the wet etching technique.The self-modulated processing technique avoids the effect of debris accumulation during the processing of large-area structures,and sapphire periodic nanostripe structures with a diameter of 4 cm were successfully fabricated.As a proof of concept,nanostripe structures with high aspect ratios exhibit higher diffraction efficiency as gratings than gratings with shallower depths.We have used laser-induced processing to prepare high aspect ratio periodic nanostripe structures on sapphire surfaces,which solves the problem of shallow depth of laser-induced surface periodic nanostructures and contributes to the development of femtosecond laser fabrication of nanostructures for high-quality transparent materials.3.Femtosecond laser direct writing process to prepare sapphire high-transmission mid-infrared windows.We have used laser self-modulated direct writing processing technology combined with a wet etching process to solve the challenge of the subwavelength anti-reflection microstructure of sapphire,which is difficult to be produced efficiently and completely by laser fabrication.The processing efficiency can be increased by 100 times,and the sapphire subwavelength reflectance pyramidal arrays of centimeter size have been successfully fabricated.The transmittance of sapphire crystal with a single-sided reflectance structure can get 94% in the mid-infrared band.The transmittance of a double-sided reflectance structure can reach 98%.The excellent reflectance reduction effect can be maintained at a large angle(≤ 60°)incidence.Furthermore,the laser self-modulated direct-write processing technology can produce anti-reflective structures on curved surfaces.The sapphire anti-reflective microstructure has high structural strength,which overcomes the problem of damage or even peeling due to insufficient strength of anti-reflective films,which has significant application value in the field of infrared guidance.4.Femtosecond laser fabrication for the preparation of patterned sapphire substrates for Ga N film epitaxial growth.To address the problems faced by the fabrication of patterned sapphire substrates,such as complex processes and inflexible graphic design,we used femtosecond laser fabrication technology to prepare patterned sapphire substrates with technical advantages such as high precision,maskless and flexible structural design.The patterned sapphire substrates with periods in the nanometer and micron scales were prepared by the induced and direct writing processing modes,respectively,and the problem of high surface roughness of the patterned substrates prepared by laser fabrication was solved by post-treatment processes such as etching or mechanical polishing.Ga N crystalline films were successfully epitaxially grown on the surface of the prepared patterned substrates using a metal-organic chemical vapour deposition process.Their crystal quality was higher than that of Ga N films grown on flat sapphire substrates.The laser-processed patterned sapphire substrate has great potential for the growth of high-quality Ga N epitaxial films and further expands the application area of femtosecond laser fabrication of functional sapphire surfaces.In summary,this paper proposes a femtosecond laser self-modulation processing technique,which achieves a breakthrough in processing deep structures and complete large-area microstructures while ensuring processing accuracy and is of great significance for the efficient preparation of microstructures on the surface of transparent sapphire crystals.In addition,this paper is based on sapphire material and oriented to its applications,such as optical windows and epitaxial substrates.Various large-area functional sapphire surfaces,including high-transmission sapphire infrared windows and high-quality patterned sapphire substrates,are realized using laser fabrication.It provides new technical solutions and ideas for the practicalization of sapphire micronano structures prepared by femtosecond laser.