Temporal and spatial evolution of picosecond phonon-polariton pulses in crystals

The creation and the detection of short polariton pulses by use of the nonlocal time-resolved coherent anti-Stokes Raman scattering (CARS) technique is analyzed using a classical model. The spatiotemporal evolution of the CARS signal is calculated and the relations between the measured parameters and the material characteristics governing the polariton dynamics are explicitly derived. The results are applied to the investigation of the ordinary polariton in ${\mathrm{LiIO}}_3$ on both sides of a forbidden band. The dispersions of the polariton group velocity and dephasing time are directly measured in the time domain and the results are compared to static investigations. The frequency and temperature dependences of the measured polariton dephasing rates are interpreted in terms of anharmonic coupling with phonons on the basis of a theoretical model including both electrical and mechanical anharmonicity.