Investigation of the interactions between femto-second pulse laser and ultrapure water

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Femtosecond-pulse lasers (fs pulses) cause very interesting phenomena due to their extremely high energy density. The effects on substances are not thermal, but multi-photon absorption. When this multi-photon absorption of fs pulses operates on water, extraordinary phenomena different from laser-induced cavitation by a usual laser such as a nano-pulse laser are induced. In the present investigation, fs pulses of 50-150fs in duration, 1kHz in repetition rate and 0.2-1.4μJ in pulse energy were focused at ultrapure water in a glass cell through several types of lens. The fs pulses split from original beams through a beam splitter were used as probe light. The femtosecond-order time-resolved optical measurement was realized by adjusting a light path length of the probe light (fs pulses). By effectively using this measurement technique, we elucidated the changes of refraction index of the water, the bubble nucleation and growth processes at an interval of femtoseconds, and the bubble properties at an interval of milliseconds. Based on these results, we have elucidated a relationship between those and fs-pulse peak intensity. In addition, we investigated the nucleation and growth processes in femtosecond- to picosecond-order. Finally, we have discovered the water molecules excited by the multi-photon absorption were decomposed into H_2 and O_2 molecules. We found out a new method of hydrogen production.Furthermore, based on the above results, the interactions between a solid transparent material (quartz) and the fs pulses were investigated. An optical fiber was microfabricated by the fs pulses, and a new type optical fiber probe was created. The excellent performances of the probe were demonstrated in micro droplet/bubble measurement (the paper received the Outstanding Paper Award from Institute of Physics, 2010).