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. ***
