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3thz的频率模式被称为“低频模式”，它与材料的特性直接相关。在液体中，低频声子模式和弛豫模式在该区域占主导地位。弛豫模式是液体所特有的，在连接微观自由度和宏观量方面起着重要作用。在目前的工作中，我们已经开发了各种方法来创建多脉冲；由稍微失谐的激光腔直接产生，并采用傅立叶变换脉冲整形装置。特别地，我们已经把一个放大的脉冲分成两个，这是由振幅调制独立应用，以创建双色同步脉冲。利用这些脉冲，我们观察了移动光栅在四波混频结构下的衍射。这种方法特别适合于在不激发弛豫模式的情况下，对声子模式进行选择性激发。我们进一步进行频域四波混频（FD4WM）光谱，采用调q YLF激光器同步振荡作为探针。结果表明，该光谱学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.