Construction of High-Frequency Phonon Generating System by means of Stimulated Light Scattering

在本研究中，我们研制了超短脉冲激光器，并应用于凝聚态物质中的声子激发。为此，我们已经构建了两个钛宝石激光系统，一个稳定的脉冲激光器与100 fs的持续时间和超快脉冲激光器与10 fs的持续时间。在后者中，我们采用了钛：蓝宝石棒，厚度为2毫米，而不是2厘米，这是通常采用在商业上可用的激光器。利用这种细棒，我们可以用三阶群速度色散小的石英棱镜代替腔中的高折射率棱镜对，利用这些激光系统进行了受激光散射实验，产生了高频相干声子。通过插入棱镜对来减小腔外的脉冲宽度，我们能够<-1>通过各种无序材料的受激光散射来研究高达300 cm^的声子产生。典型的结果总结如下：（1）苯衍生物显示出较强的弛豫模和较宽的低频声子模，峰值在60 cm ~ 2处<-1>，这与醇的结果相反。醇类在30 cm-2处有一个弱的弛豫模和低频声子模峰<-1>。(2)各种液体原则上分为上述两种类型。(3)聚合物的特征在于在所研究的频率范围内缺乏弛豫模式，并且在80 cm 2处存在低频模式峰<-1>。由于聚合物的低频模位于相对较高的频率区域，且宽度适中，因此聚合物的时间响应表现出近过阻尼振荡。结果表明，时域测量方法对表征弛豫模与低频声子模之间的关系特别有用。

In this study, we have developed ultrashort pulse lasers and have applied to phonon excitaion in condensed matter. For this purpose, we have constructed two Ti : sapphire laser systems ; a stable pulse laser with 100 fs duration and an ultrafast pulse laser with 10 fs duration. In the latter, we have employed a Ti : sapphire rod with 2 mm thickness instead of 2 cm, which is usually employed in commercially available lasers. By virtue of this thin rod, we can replace a prism pair having a high refractive index in the cavity with quartz prisms which show small third-order group velocity dispersion.Using these laser systems, we have performed stimulated light scattering experiment to generate high-frequency coherent phonons. With inserting a prism pair to reduce the pulse width outside the cavity, we are able to investigate the phonon generations up to 300 cm^<-1> by means of stimulated light scattering for various disordered materials. Typical results are summarized as follows : (1) Benzene derivatives show a strong relaxational mode and broad low frequency phonon modes peaked at 60 cm^<-1>, which is in contrast to the result for alcohols. The alcohols show a weak relaxational mode and low frequency phonon modes peak at 30 cm^<-1>. (2) Various liquids are in principle categorized into the above two types. (3) Polymer is characterized by the lack of relaxational modes in the frequency range investigated and by the presence of low frequency modes peak at 80 cm^<-1>. Since the low frequency modes lie relatively high frequency region with moderate width, the time responses of polymers shows a nearly overdamped oscillation. It is concluded that the time-domain measurement is particularly useful to characterize the relationship between the relaxational mode and low frequency phonon modes.