Study of Picosecond Laser-Induced Shock Waves and Phase Transformation in Solids

1.GENERATION OF SHOCK WAVES STUDIED BY A PIEZOELECTRIC TRANSDUCERGeneration and propagation characteristics of picosecond laser-generated acoustic or shock waves in Si were studied by a ZnO transducer.In the Plasma formation region，the propagation velocity of acoustic pulses exceeds the sound velocity。At early stage of propagation.The amplitude of shock waves is affects by the formation and expansion of laser-generated plasma at the silicon surface.2。INTENSE ACOUSTIC WAVES IN GLASS STUDIED BY AN OPTICAL METHODLongitudinal waves propagate at the direction of the normal to the surface plane，but shear waves propagate at about45 to the normal.At the early stage of propagation，the velocity of the wavefront exceeds the sound velocity in glass.Moreover，it is pointed out that the generation and propagation behavior of shear waves is very sensitive to the surface conditions.3。NEW PHOTOACOUSTIC SPECTROSCOPY USING A TRANSPARENT TRANSDUCERWe developed new photoacoustic spectroscopy system using a transparent transducer，and studied laser-induced structural changes at the surface of solids.The signal detected at the light-irradiated front surface has a large value and a good signal-to-noise ration compared with that the back surface.Laser-induced surface structural changes are clearly observed using this system.4。SURFACE TRANSFORMATIONS AFTER HIGH-POWER LASER IRRADIATIONCrack formation in glass surfaces and explosive crystallization in a-Si were observed after high-Zower laser irradiation.The damage in the irradiated region was caused by laser ablation and plasma formation，but cracks and liquid drops also appeared outside the irradiated region over a distance of many millimeters.Delayed crack formation is caused by chemical reactions of component of glass with atmospheric water vapor at microcracks in the surface induced by laser-driven shock waves.We also observed explosive crystallization of a-Si initiated by shock waves.

1. GENERATION OF SHOCK WAVES STUDIED BY A PIEZOELECTRIC TRANSDUCERGeneration and propagation characteristics of picosecond laser-generated acoustic or shock waves in Si were studied by a ZnO transducer. In the Plasma formation region, the propagation velocity of acoustic pulses exceeds the sound velocity. At early stage of propagation. The amplitude of shock waves is affects by the formation and expansion of laser-generated plasma at the silicon surface.2. INTENSE ACOUSTIC WAVES IN GLASS STUDIED BY AN OPTICAL METHODLongitudinal waves propagate at the direction of the normal to the surface plane, but shear waves propagate at about 45 to the normal. At the early stage of propagation, the velocity of the wavefront exceeds the sound velocity in glass. Moreover, it is pointed out that the generation and propagation behavior of shear waves is very sensitive to the surface conditions.3. NEW PHOTOACOUSTIC SPECTROSCOPY USING A TRANSPARENT TRANSDUCERWe developed new photoacoustic spectroscopy system using a transparent transducer, and studied laser-induced structural changes at the surface of solids. The signal detected at the light-irradiated front surface has a large value and a good signal-to-noise ration compared with that at the back surface. Laser-induced surface structural changes are clearly observed using this system.4. SURFACE TRANSFORMATIONS AFTER HIGH-POWER LASER IRRADIATIONCrack formation in glass surfaces and explosive crystallization in a-Si were observed after high-Zower laser irradiation. The damage in the irradiated region was caused by laser ablation and plasma formation, but cracks and liquid drops also appeared outside the irradiated region over a distance of many millimeters. Delayed crack formation is caused by chemical reactions of component of glass with atmospheric water vapor at microcracks in the surface induced by laserーdriven shock waves. We also observed explosive crystallization of aーSi initiated by shock waves.