High-throughput Microchannels Fabrication In Fused Silica By Temporally Shaped Femtosecond Laser Bessel Beam

Microholes / microchannels are very common structures in daily life,which have a wide range of applications in microfluidic devices,three-dimensional packaging technology,aerospace,microsensors and other fields.Glass,take advantage of i outstanding optical,chemical,and thermal properties,has also been widely used in various fields.Especially in the field of microfluidic devices,glass may be the best substrate material.But,the way fabricate microchannels in the glass remains a challenge.The conventional processing methods,such as electric discharge machining,electrochemical machining,focused electron beam and focus ion beam processing are not suitable for preparing microchannels on glass due to the requirements of material conductivity,inefficiency,complexity and high cost.Compared with these methods,femtosecond lasers offer the advantages of high precision,high quality,and three-dimensional fabrication of microchannels in fused silica because of their ultrashort pulse duration and ultrahigh intensity.However,the commonly used femtosecond laser microchannel processing methods have have some drawbacks.However,because of direct percussion drilling,the aspect ratio and quality of microchannels were often limited by the plume effects.To avoid these deleterious effects,two main approaches to microchannels fabrication in fused silica have been used: liquid-assisted femtosecond laser drilling and femtosecond laser irradiation followed by chemical etching(FLICE).However,both approaches use a Gaussian beam,which requires the scanning of the sample across the laser focus.Thus,the fabrication efficiency and microchannnels depth may be limited by the scanning velocity,and the side-writing method using unshaped Gaussian beam may result in an elliptical cross-section.Recently,by spatially shaping a Gaussian beam into a Bessel beam,which has the advantages of ultralong focal depth,an intense central core,and uniform intensity distribution,high-aspect-ratio microchannels were drilled without the movement of the sample or laser focus.However,because of the wide band gap of glass,very high single pulse energy(5–7 m J)was required in this case to induce sufficient modification in the material,and such high energy is not readily available.Therefore,the current difficulties and challenges in efficiently processing microchannels in glass are how to improve the energy deposition efficiency in Bessel beam processing,so that to realize direct ablation to form microchannels,or to improve the degree of material modification so as to improve the microchannel etching efficiency.Based on the basic idea of electronic dynamic control(EDC)proposed by our group,this paper proposes a microchannel processing method based on femtosecond laser temporalspatial shaping method.In this paper,the Gaussian beam is spatially transformed into a Bessel beam,which changes the laser field distribution and enhances the laser focal depth.The single pulse is temporally shaped into a double-pulse train to control the transient local electron dynamics during laser-material interaction.By combining the temporal shaping method with the spatial shaping method,combined with the chemical etching technique,the microchannel fabrication efficiency is greatly improved.The main innovations of this thesis are as follows:1.Propose a femtosecond laser spatiotemporal shaping method(double-pulse Bessel beam)in microchannel processing.In this thesis,an axicon is used to shape the Gaussian beam into a Bessel beam,which greatly increases the laser focus depth and realizes the in-situ machining in high aspect-ratio mcirocahnnels processing.Michelson interferometer setup was used to temporally shape the pulses into doublepulse trains(subpulse energy ratio,1:1)with variable pulse delays.And a femtosecond laser spatiotemporal shaping processing platform is constructed.2.Comparative analysis of the fabrication results of transparent materials(PMMA,fused silica,Corning glass)by using the double pulse Bessel beam direct drilling,Microholes of 2 μm in diameter,660 μm in depth,and 330:1 in aspect ratio are achieved in Corning glass.Flexible transitions between different types of machined structures are achieved in fused silica and Corning glass by adjusting laser energy and pulse delay.And microholes with high aspect ratio with a diameter of ~ 400 nm,a depth of ~ 45 μm and a aspect ratio of greater than 100: 1 is realized in Corning glass.This method uses single-pulse ablation to form microchannels,greatly improving the processing efficiency.3.By using a temporally shaped femtosecond laser Bessel beam assisted chemical etching method,the energy deposition efficiency was improved by adjusting the pulse delay to yield a stronger material modification,and thus a higher etching depth.The etching depth was enhanced by a factor of 13 using the temporally shaped Bessel beam.The mechanism of etching depth enhancement was elucidated by localized transient free electrons dynamics induced structural and morphological changes.4.Micro-Raman spectroscopy was conducted to verify the structural changes inside the material.It was found that modification changes(4-ring structure)fluctuation against the pulse delay was highly consistent with the experimental results for etching depth variation.It proves that there is a positive correlation between the material modification and the etching rate.Combined with the lifetime of self-trapped excitons(STEs),the relationship between laser energy deposition efficiency and the material modification efficiency was revealed.