Study on phonon generation and phonon coherence in dielectric.

通过对NdGaO_3晶体在585 nm处的光谱研究发现，在低温下观察到的调制四波混频（FWM）信号是由几条吸收线之间的量子拍频引起的，在本项目开始时，我们将其归因于声子产生的结果。四波混频信号包括的利益功能，吸收线主要均匀加宽，即使在低温下。我们已经对信号进行了详细的分析，并在Phys.Rev.13上发表了结果。我们利用拉曼激发下的声子产生研究了SrTiO_3、BaTiO_3两种介质中的声子产生和声子相干性。BaTiO_3晶体的质量较差，不能获得较好的信噪比。声子产生的激发源是钛宝石再生放大器的输出，脉冲能量为800 μ J ...更多信息 脉冲宽度约为130（fs），对应的峰值功率为6.2 × 10^9（W）。来自再生放大器的输出被分成两个强激发光束（波矢量k_1、k_2）和一个弱探测光束（波矢量k_3）。我们用BOXCARS组态激发样品。在k_3 + k_2 - k_1方向上的信号被测量为激发光束和探测光束之间的延迟时间τ的函数。(1)为了检验我们的激发和检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，时间常数为0.4（ps）和0.7（ps）。由CS_2的K err效应产生的信号与测量的时间常数相对应，这是CS_2分子取向极化弛豫的结果。基于这一检查，我们的系统的时间分辨率的结论是0.2（ps）。(2)在室温下测量SrTiO_3晶体的信号时，在τ = 0时，在k_3 + k_2 - k_1方向上得到了很强的信号，比CS_2晶体的信号强一个数量级。该信号主要来自SrTiO_3晶体中的非线性电子极化，表明SrTiO_3晶体是一种具有高极化率的有效非线性材料。然而，我们无法观察到tau > 0的信号及其调制，如果发生声子生成，则预计会出现这种情况。即使使两束激发光的偏振方向垂直，激发E_2声子模，也不能得到由于声子产生而产生的信号。然而，我们最近观察到，在τ> 0的小信号似乎与10 K时的声子产生有关，激发光束的偏振相互垂直。我们将研究该信号对激发光束的偏振和温度的依赖性。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