Measurement And Control Of The Electron Dynamics During Femtosecond Laser Micro/Nano Fabrication

Microminiaturization is an important trend of the development of modern high-tech equipment.Micro/nano-manufacturing is the basis of micro/nano equipment and micro/nano technology,playing an important role in the development of national defence and science and technology.Femtosecond?fs?laser has great application potential in micro/nano processing,due to its advantages of minimized thermal effects,high processing precision,non-contact,no pollution,and no tool abrasion,etc.Recent years,femtosecond laser has been widely studied in fabricating microholes,mciro-fluids,micro-channel,optical fiber sensor,and waveguides etc.Even though,the mechanism of femtosecond laser-material interaction is very complex,and affected by many factors;there are still many unknown mechanisms needed to be explored.So femtosecond laser processing has the disadvantage of large influence from materials and poor repeatability.The mechanism of femtosecond laser processing must be further understood to overcome those weaknesses.Since the time duration of fs laser pulse is much shorter than the electron-lattice relaxation time(10^-1010^-12 s),on account of ultrashort time of femtosecond laser-material interaction,only the electrons absorb the laser energy in the femtosecond laser pulse duration;other subsequent ablation and processing effect is the result of the electron relaxation.So the key to control fs laser machining process is to control the localized transient electron dynamics in materials.Shaping femtosecond laser in temporal and spatial domains are two important methods to control the eletrons dynamics.In this thesis,we did a comprehensive research in various kinds of fs laser pulse spatial and temporal shaping methods,and several effective approaches to shaping fs laser pulses in space and time domain were proposed,which supplied a foundation for the further research on electronic state regulation tools.At the same time,in order to examine the results of electron dynamics control and further understand the ultrafast mechaniisms during fs-laser-material interaction,we also made a multi-timesacle observational study of fs laser fabrication processes.The main innovations of this dissertation are as followed:?1?We proposed a variety of methods and devices for generating pulse train based on geometrical optics principle,and experimentally verified them respectively.These methods/devices include simple forms of Michelson interferometer that can be extended easily for more sub-pulses and easy to be collimated,and monolithic planar pulse train generator relying on multiple bounces back and forth of light between multilayer semi-reflection films.These new methods/devices overcome the disadvantages of existing commercial pulse shaper,such as complex control system,high cost,and low output energy and so on,expanding the research scope and ability of electron dynamics control using temporal pulse shaping method.These results have been applied for several patents,and widely used in the experimental study in our laboratory.?2?When generating femtosecond laser Bessel beams with an axicon,the influence of pulse duration on Bessel focal spot diameter is studied.The relation of pulse duration and beam diameter is deduced to meet the condition where the pulse duration will not affect the criterion of Bessel focal spot diameter.By introducing the femtosecond laser double-pulse and pulse train into the Bessel beam shaping process,multilayer hollow cylindrical Bessel focal spot is obtained,which can be used for fabrication of complex three-dimensional structures forming at a time.We put forward the concept of mismatch of double pulse focus to improve length-to-diameter-ratio of the laser focus by combining temporal and spatial pulse shaping technology,which can be used in fabricating high-aspect-ratio holes.?3?A pump-probe shadowgraph imaging setup was developed to investigate the nonlinear propagation of high-intensity fs laser in air.The light-speed propagation of fs laser beam in air was recorded with a time resolution of 100 fs,which was accompanied by a dynamic competition of Kerr self-focusing and plasma defocusing process.As a result,a double-foci was fored.We also found that a competitive relationship exists between the external focusing effect of the lens and the nonlinear shaping effect of air due to laser-air nonlinear interaction.?4?The femtosecond-picosecond-nanosecond dynamics during fs laser Bessel beam drilling in PMMA was studied by pump-probe shadowgraph imaging technique.The superluminal propagation of fs laser Bessel beam in transparent material and it induced cylindrical shockwave expansion in material were first observed during drilling processes.The cylindrical expansion of shockwave indicates that the microhole was formed due to a homogeneous extension of material in the center towards the radial direction along the whole depth of the hole,instead of a material ejection mechanism along the depth dirction.This explains why fs laser Bessel beam pulse can drill such high aspect-ratio microholes in PMMA.?5?The ultrafast,ultrahigh-speed imaging technique was studied and a pulse train modulation-demodulation pump-probe idea was proposed to achieve such a goal.As the first step,a three-frame ultrafast,ultrahigh-speed pump-probe setup based on direction-division-multi-channel-parallel?DDMCP?was built and tested.As the result,three contionuous pictures with a frame rate of 10¹³ s^-1 and an exposure time of 10^-13s were obtained.