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

飞秒脉冲激光（fs脉冲）由于其极高的能量密度而引起非常有趣的现象。对物质的影响不是热的，而是多光子吸收。当这种fs脉冲的多光子吸收作用于水时，会产生不同于普通激光（如纳米脉冲激光）引起的空化现象。在本研究中，通过几种类型的透镜，将持续时间为50 ~ 150fs、重复频率为1kHz、脉冲能量为0.2 ~ 1.4μ j的fs脉冲聚焦在玻璃池中的超纯水上。通过分束器从原始光束中分离出来的fs脉冲被用作探测光。通过调整探测光（fs脉冲）的光程长度，实现了飞秒级时间分辨光学测量。通过有效地利用这种测量技术，我们阐明了水的折射率变化，在飞秒间隔内气泡的成核和生长过程，以及在毫秒间隔内气泡的性质。基于这些结果，我们阐明了这些与fs脉冲峰值强度之间的关系。此外，我们还研究了飞秒到皮秒量级的成核和生长过程。最后，我们发现受多光子吸收激发的水分子分解为H_2和O_2分子。我们发现了一种新的制氢方法。此外，基于上述结果，研究了固体透明材料（石英）与fs脉冲之间的相互作用。利用光纤脉冲制备了光纤，并研制了一种新型光纤探头。该探针在微液滴/气泡测量中表现出优异的性能（论文获得2010年物理所优秀论文奖）。

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).