Optically Induced Ultrashort Transient Dynamics in Condensed Materials

在光学注入相干光声子和极化子后，测量化合物半导体折射率的变化。通过相干拉曼激发（CRE）过程，声子/极化子态的注入发生在亚ps时间尺度上。注入的声子几乎是单色的，具有已知的波矢量和偏振，具有初始固定的相对相位，并且初始空间定位于两个泵浦激光器的重叠区域。将使用光束偏转或光致光栅技术来监测折射率的变化。该效应将作为泵浦激光器的强度和频率、相干声子态传播矢量与半导体晶体轴的相对方向以及晶格缺陷的影响的函数进行研究。其他感兴趣的测量包括相干声子/极化子脱相时间和三阶非线性电子磁化率的实部和虚部的测量。在一个相关的实验中，等离子体过剩能量对相干LO声子的消相时间的影响将使用脉冲（50 fs），可调谐激光源来测量GaP。化合物半导体光学性质的变化可以在极短的时间尺度上测量。这些变化是由两个激光脉冲引起的，这两个激光脉冲聚焦在半导体的同一区域重叠。两个激光器的频率相差的量正好等于半导体的某些振动模式的特征频率。这些振动模式将反过来改变入射到半导体上的另一束光的透射或反射。该实验对研究超高速光开关器件的基本机理具有重要意义。＊＊＊

9311175 Bron Changes in the index of refraction of compound semiconductors will be measured after optical injection of coherent optical phonons and polaritons. The injection of the phonon/polariton states occurs on a subps time scale through a coherent Raman excitation (CRE) process. The injected phonons are nearly monochromatic, have known wave vectors and polarizations, have initially fixed relative phase, and are initially spatially localized to the overlap region of the two pump lasers. A beam deflection or a photoinduced grating technique will be used to monitor the changes in the index of refraction. The effect will be studied as a function of the intensity and frequency of the pump lasers, the relative orientation of the propagation vector of the coherent phonon state and the crystal axes of the semiconductor, and the effects of lattice defects. Other measurements of interest include the coherent phonon/polariton dephasing time and the measurement of the real and imaginary components of the third order nonlinear electronic susceptibility. In a related experiment, the effect of plasma excess energy on the dephasing time of coherent LO phonons will be measured for GaP using a pulsed (50 fs), tunable laser source. %%% Changes in the optical properties of a compound semiconductor will be measured on extremely short time scales. These changes are induced by two laser pulses which are focused to overlap the same region of the semiconductor. The frequency of the two lasers differs by an amount which is exactly equal to a characteristic frequency of certain vibrational modes of the semiconductor. These vibrational modes will in turn alter the transmission or reflection of another light beam which is incident on the semiconductor. The experiment can yield important insight into the basic mechanisms which are essential to the development of ultrafast optical switching devices. ***