Tightly-focused femtosecond laser interaction with water

摘要

Tightly-focused femtosecond laser pulse (>35fs, 800nm) irradiation up to sub-peta-W/cm~2 at the focus onto thin waterflow (10-15μm thick) in air results in plasma formation associated with dynamic and macroscopic laser ablationphenomena. Under such experimental conditions, hard X-ray, THz wave, and sound/ultrasound emission were measuredwith a Geiger counter, time-domain spectroscopy with a ZnTe(110) crystal (1mm-thick) based on electro-opticalsampling method, and microphone/transducer, simultaneously. Under the single pulse irradiation, intensities of X-ray,THz wave, and sound show their maxima at the same position of the water flow along the laser irradiation axis (Z-axis),while the width of the intensity profile along the Z-axis is narrower at 44μm in X-ray than that of THz wave at 225μmand sound (10Hz-20kHz) at 400μm when the laser intensity is at 0.4mJ/pulse and focused by an off-axis parabolic mirror(f = 5cm). Under the double pulse irradiation conditions with the time delay between the main pulse (p-pol.) and the prepulse(s-pol.) up to 15ns, those emissions are enhanced, which can be ascribed to time-dependent various phenomena oflaser ablation such as pre-plasma formation and transient surface modifications induced by the pre-pulse irradiation.