Study of Coherent Optical Phonons Through Nonlinear Optics.

The dynamical process involved in the generation and decay of coherent longitudinal optical (LO) phonons (near zone center) in GaP and ZnSe has been investigated experimentally. The coherent LO phonons are generated by using nanosecond and picosecond lasers. It is found that a force associated with nonlinear polarization can excite coherent LO phonons in addition to the well-known Raman excitation. Vibronic sideband spectroscopy has been used to determine the frequency of longitudinal acoustic (LA) phonons produced by the decay of coherent LO phonons in GaP. The dephasing time of coherent LO phonons in GaP and ZnSe are also measured directly as a function of temperature through the coherent anti-stokes spectroscopy (CARS) in the picosecond time domain. It is observed that coherent LO phonons in GaP decay into two half-energy LA phonons with equal but opposite wavevectors, and the results of the temperature dependence of dephasing time show that only anharmonic phonon-phonon interaction contributes to the decay of coherent LO phonons under the present experimental condition. The temporal dependence, in the picosecond domain, of the CARS signal associated with the LO Raman resonance in GaP and ZnSe is used to obtain the complex components of the third-order nonlinear susceptibility (chi)('(3)). Under certain experimental conditions the measurements in the temporal domain are markedly advantageous compared with the traditional measurements in the spectral domain.