Coherent phonon generation by light pulse train and Optical control of material propeties

在光散射实验中，材料中低于3 THz的频率模式被称为“低频模式”，它与材料的性质直接相关。在液体中，低频声子模和弛豫模在这一区域占主导地位。弛豫模是液体状态所特有的，在连接微观自由度和宏观量方面起着重要作用。在本工作中，我们发展了各种方法来产生多脉冲，这些方法直接由略微失谐的激光腔产生，并使用傅里叶变换脉冲整形装置。具体地说，我们将放大的脉冲分成两部分，由幅度调制独立地施加，以产生双色同步脉冲。利用这些脉冲，我们观察到了四波混频结构中运动光栅的衍射。这种方法特别适合于在不激发弛豫模的情况下选择性地激发声子模。我们进一步利用同步振荡的调Q YLF激光器作为探针，进行了频域四波混频(FD4WM)光谱。结果表明，该光谱能有效地提供泵浦脉冲激发下的频率响应信息，适合于观察选择性激发后的声子动力学。

The frequency modes below 3 THz in materials are called "low frequency modes" in light scattering experiment and are known to relate directly with the material properties. In liquid, the low frequency phonon modes and the relaxational mode dominate over this region. The relaxational mode is peculiar to liquid state and is believed to play an important role in connecting the microscopic degrees of freedom with the macroscopic quantities.In the present work, we have developed various methods to create multipulses; directly created by slightly detuned laser cavity and by employing the Fourier transform pulse shaping apparatus. Particularly we have divided an amplified pulse into two, which are applied independently by the amplitude modulations to create two-color synchronous pulses. Using these pulses, we have observed the diffraction from the moving grating in the four-wave mixing configuration. This method is particularly suitable for the selective excitation of the phonon modes without exciting the relaxational mode.We further perform the frequency-domain four-wave mixing (FD4WM) spectroscopy by employing a Q-switched YLF laser synchronously oscillating as a probe. It is found that this spectroscopy efficiently offers the information on the frequency response excited by the pump pulses and becomes suitable to observe the phonon dynamics after selective excitation.