Ultrafast Microscopy Studies Of Femtosecond-Laser-Driven Shock Waves' Characteristic

Shock waves induced by ultrafast laser pulses have already been an important tool in dynamic high-pressure physics. Since the transit time for shock waves induced by ultrafast laser pulse through films is about 1 ns at the most, it is really a difficult problem to measure the specific parameters (such as shock pressure P and shock temperature T) of shock waves generated by ultrafast laser pulses experimentally.In the related studies in engineering and materials science, such as the phase transformation of shocked materials, it is of great significance to determine the shock pressure P and shock temperature T in materials shocked by ultrafast laser pulses simultaneously. The mechanism of interactions between materials and femtosecond laser pulses is not clear at present, so it is of especially importance to set up the experimental research platform on femtosecond-laser-driven shock waves and carry out the experimental studies on the shock parameters.Based on the present situation and the existed problems, the generation and propagation of shock waves induced by ultrafast laser pulses have been described theoretically in this thesis. By using Quasi-balance states theory for continuum matters, the reflection and transmission of shock waves in two different kinds of materials are described in detail and the calculation method for shock pressure and shock temperature generated by laser shock are also described.On this basis, the shock pressure profiles induced by pulse duration in femtosecond, picosecond and nanosecond regime are numerical simulated respectively. The principle of shock pressure varied with time and position provided the important guidance for femtosecond-laser-driven shock wave experiments.Ultrafast time-resolved microscopy experimental platform for studying femtosecond-laser-driven shock waves in films was built. By adopting femtosecond laser pulses with various delay time of shock generation pulse and probe pulse, precise time synchronization of the generation of shock waves in films and its probe was obtained and the sub-picosecond magnitude time resolution was achieved. Hundreds of times of experiments were carried out on each thickness of aluminum films, so the accuracy of the experimental results was improved. Besides, the expensive streak camera was replaced by external triggered CMOS camera, which is more practicable for ordinary labratories to study the shock waves induced by ultrafast laser pulses in opaque thin films. With laser pulse duration of 130 fs as light source, shock waves generated by single femtosecond laser pulse (Power density I=7.84×10¹³ W/cm²) in various thickness of aluminum films (thickness range from 3μm to 10μm) have been studied experimentally with sub-picosecond time-resolved microscopy technique for the first time. The transit time t for femtosecond-laser-driven shock waves through different thickness aluminum films have been measured. The measured sub-picosecond time-resolved micrographs of shocked aluminum surface showed that Gaussian femtosecond laser pulse can drive planar, stationary, and clean one-dimensional shock waves in 3¹0μm thick aluminum films. From the best linear fit of transit time versus the thickness of aluminum films, the shock velocity in aluminium films was measured to be D=9.0±0.4 km/s. With the known Grüneisen equations of aluminium film andα-quartz, shock pressure P and temperature T in aluminium films were calculated to be P=69±5 GPa and T=1852±400 K respectively, and shock pressure P and temperature T inα-quartz were calculated theoretically to be P≈55 GPa and T≈4344 K.These results are very important for the following studies on shock phase transformation ofα-quartz induced by ultrafast laser pulses. Experimental results in good agreement with previous theoretical and experimental work indicate that time-resolved ultrafast microscopy technique is fairly accurate and feasible for studying fs-laser-driven shock waves in opaque thin films.This research work is supported by the National Nature Science Foundation of China (Grant No.: 10374022,60478015,20573028,10674034) .