Microchannels Fabrication In Transparent Dielectric Using Temporally And Spatially Shaped Femtosecond Laser Based On Electrons Dynamics Control

Microfluidics is a focused research topic of biology, chemical and fabrication fields in these years. Microchannels, especially high-aspect-ratio microchannels, are key components of micro total analysis systems(μTAS) and other microfluidic systems and devices. Transparent dielectrics, such as silica glasses and Acrylic(PMMA) are ideal substrates to establish microfluidic systems, due to their excellent optical, thermal and chemical properties. The advances of ultrafast lasers, especially femtosecond lasers make them novel and powerful tools for microchannels structures fabrication in dielectrics. The energy density, impact space and impact time of femtosecond all tend to limits, so we can achieve processing precision below the optical diffraction limit, and realize “cool machining” and three-dimensional machining with extremely small heat-affected-zone. To date, there are two major methods to fabricate microchannels using femtosecond laser: femtosecond laser irradiation followed by chemical etching, and liquid assisted femtosecond laser ablation. Besides that, for one-dimensional microchannels(microholes), femtosecond laser drilling, such as single-pulse drilling, percussion drilling, trepanning drilling and helical drilling, is fabrication method of mainstream. Microchannels fabrication in dielectrics using femtosecond lasers has the advantages of absence of mask, absence of ultra-clean environment, direct fabrication inside, absence of tackling, instinctive round cross section and three-dimensional fabrication capability.However, there exist many challenges that microchannels fabrication by femtosecond laser is facing:(1) The fabrication precision, aspect-ratio and surface roughness of microchannels are pushed to higher and higher level;(2) The integrated fabrication of microchannels and other microfluidic components are needed;(3) The fabrication efficiency of these methods are still in the level of laboratory, which cannot be maintained when improving the precision.Aimed to these challenges, we proposed a novel fabrication method by temporally and spatially shaping the femtosecond laser based on electrons dynamics control. Taking advantages of the ultrafast duration and ultrahigh power density of femtosecond lasers, we temporally or spatially shape the femtosecond laser individually or simultaneously, adjust the temporal and spatial distribution of laser energy, control the successive interaction processes of photon-electron-phonon, and then improve the phase change process and the ultimate fabrication results, to meet the increasing manufacturing demands.According to the aforementioned scientific thoughts, we choose silica glass and PMMA as representative materials of transparent dielectrics, and carry out the research on microchannels fabrication in dielectrics by temporally and spatially shaped femtosecond laser based on electron dynamics control. The major research work consists of:(1) Establishing the fabrication system based on temporally and spatially shaping to fabricate one and two dimensional mcirochannels;(2) Investigating the influence of various processing parameters(pulse number, focusing depth etc.) on the diameter, depth and aspect ratio of microchannels drilled by femtosecond laser;(3) Varying the processing environment and studying the laser drilling in vacuum;(4) Temporally shaping the femtosecond laser to generate pulse trains to study their influences on processing precision and efficiency;(5) Spatially shaping the femtosecond laser to generate Bessel beams to study their influences on processing precision and quality;(6) Temporally and spatially shaping the femtosecond laser simultaneously to generate Bessel beams pulse trains to study their influences on processing efficiency.The main innovations of this dissertation are as followed:1. Investigated the influence of every conventional parameter(laser energy, focus condition, etc.) on the Gaussian laser drilling. Fabricated microchannels by double pulse trains with different pulse delays with self-established system and found that the depth of the microchannels are largest when the pulse delay is zero and it declined with pulse delay. This indicated that temporal pulse shaping in the energy regime of tens of ablation threshold has no positive effects. However, in the method of femtosecond laser irradiation followed by chemical etching, with energy around the ablation threshold, the introduction of double pulse trains extremely enhances the processing efficiency, the limit depth and limit aspect ratio, and shows advantages in three-dimensional fabrication.2. Drilled microchannels by Bessel beams with self-established system with axicon. Simulated the spatial energy distribution of Bessel beams using MATLAB and conducted experiments with the same parameters, whose results meet well with the simulation. The spatially shaped femtosecond laser fabrication increased the the aspect ratio and quality of the microchannels extremely. The novel fabrication method based on electrons dynamics control has been applied in the processing of the fill channel in the target pellet for the NIF of China, supporting the practical processing.3. Temporally and spatially shaping the femtosecond laser simultaneously. Generating double pulse trains based on the concept of Michelson Interferometer and generating Bessel beams with axicon. During the process of femtosecond laser irradiation followed by chemical etching, the etching rate and aspect ratio of microchannels are highly enhanced by the shaped double Bessel pulse trains. Also the processing efficiency is increased due to absence of moving of the sample during fabrication.This dissertation is based on the research projects supported by the National Basic Research Program of China(973 Program)(Grant No. 2011CB013000), National Natural Science Foundation of China(NSFC)(Grant No. 90923039) and National Key Special Projects(Grant No. GFZX02040203.5). The novel fabrication method based on electrons dynamics control has been selected as the processing method of the fill channel in the target pellet for the NIF of China. High-aspect-ratio and high-quality microchannels are realized, meeting the various size demand of the contrat and supporting the practical processing. The main innovations of the dissertation were all published in the influential applied physical and optical journals.