Study on phonon generation and phonon coherence in dielectric.

通过详细的低温光谱研究，我们发现在585 nm处NdGaO_3晶体中观测到的调制四波混频（FWM）信号是由几条吸收线之间的量子拍引起的，在本项目开始时，我们将其归因于声子产生的结果。FWM信号有一个有趣的特征，即即使在低温下，吸收谱线也主要均匀地展宽。我们对该信号进行了详细的分析，并将结果发表在《物理学》杂志上。启13。然而，这些结果与本项目的结果是不同的主题。利用喇曼激发声子产生的方法，对SrTiO_3和BaTiO_3两种介质中的声子产生和相干性进行了研究。BaTiO_3晶体质量较差，无法获得较好的信噪比。声子产生的激励源是脉冲能量为800 （muJ…More）、脉冲持续时间约为130 （fs）的钛蓝宝石再生放大器的输出，对应的峰值功率为6.2x10^9 (W)。再生放大器的输出分为两个强激发光束（波矢量k_1， k_2）和一个弱探测光束（波矢量k_3）。我们用BOXCARS配置激发样例。测量了k_3 + k_2 - k_1方向上的信号与激发束和探测束之间的延迟时间tau的关系。(1)为了检验我们的激励和探测系统的时间分辨率和灵敏度，我们在CS_2中测量了信号。信号呈现双指数衰减曲线，时间常数分别为0.4 （ps）和0.7 （ps）。由CS_2的K err效应和测量的时间常数合并的信号与CS_2分子中取向极化弛豫的时间常数相对应。在此基础上，系统的时间分辨率为0.2 （ps）。(2)在室温下测量SrTiO_3晶体中的信号时，在tau = 0处，我们得到了k_3 + k_2 - k_1方向上非常强的信号，比CS_2中的信号大一个数量级。该信号主要来源于SrTiO_3晶体的非线性电子极化，表明SrTiO_3晶体是一种具有高磁化率的高效非线性材料。然而，我们无法观察到信号及其对tau >0的调制，这是在声子产生时预计会出现的。由于声子的产生，即使使两束激励光束的偏振方向垂直激发e_2声子模式，也无法获得信号。然而，我们最近观察到，在10k时，当激发光束的偏振方向相互垂直时，在τ >处的小信号对声子的产生似乎是微弱的。我们要研究这个信号与激发光束的偏振和温度的关系。少

We have found that the modulated four-wave-mixing (FWM) signal observed in NdGaO_3 crystal at 585 nm results from the quantum beats between the several absorption lines based on the detailed spectroscopic study at low temperature, which, in the beginning of this project, we attributed to the result of phonon generation. The FWM signal consists of interest features that the absorption lines are predominantly homogeneously broadened even at low temperature. We have performed the detailed analysis of the signal and published its results in Phys. Rev. 13. however, these results are different subject from that of this project.We have performed the study on phonon generation and phonon coherence in two dielectric, SrTiO_3, BaTiO_3 by mean of the phonon generation with Raman excitation. BaTiO_3 crystal had too worse quality to get the good signal to noise ratio.The excitation source for phonon generation is the output from a Ti-sapphire regenerative amplifier with the pulse energy of 800 (muJ … More ) and the pulse duration of about 130 (fs), corresponding to the peak power of 6.2x10^9 (W). The output from the regenerative amplifier is split into two strong excitation beams (wave vector k_1, k_2) and a weak probe beam (wave vector k_3). We excite the sample with the BOXCARS configuration. The signal in the direction of k_3 + k_2 - k_1 is measured as a function of the delay time tau between the excitation and probe beam.(1) In order to examine the time resolution and sensitivity of our excitation and detection system, we measured the signal in CS_2. The signal exhibited the double-exponential decay curve with the time constants, 0.4 (ps) and 0.7 (ps). The signal merged from the K err effect of CS_2 and the measured time constants correspond to that of relaxation of orientational polarization in CS_2 molecule. Based on this examination, the time resolution of our system was concluded to be 0.2 (ps).(2) When we measured the signal in SrTiO_3 crystal at room temperature, we obtained the very strong signal in the direction of k_3 + k_2 - k_1 at tau = 0, which is greater by about one order of magnitude than that in CS_2. This signal emerges mainly from the nonlinear electronic polarization in SrTiO_3 and indicates that the SrTiO_3 crystal can be one of the efficient nonlinear materials with a large susceptibility. However, we could not observe the signal and its modulation for tau > 0, which is expected to appear if the phonon generation takes place. We could not obtain the signal due to the phonon generation even when the polarization of the two excitation beams is made perpendicular to excite the E_2-phonon mode. However, we recently observed that the small signal at tau > 0 appears doe to the phonon generation at 10 K, with the polarization of the excitation beams perpendicular to each other. We are going to study the dependence of this signal on the polarization of the excitation beams and temperature. Less