Stimulated Raman Scattering Processes in H/sub 2/ by Astigmatic Focusing Geometries

Abstract

An exhaustive study of polariton characteristics is particularly relevant to describe lattice dynamics in polar crystals , especially to address the balance between relaxational and displacive contributions from nuclei in the vicinity of a phase transition. PbTiOj perovskite has been described, for a long time, as a text book example for a displacive-type ferroelectric crystal. Nevertheless, linear spectroscopy studies of E-symmetry modes' revealed an anomalous behavior for the lowest polariton damping at a frequency value lower than 50 cm-l. Moreover, recent Rainan and 1.R. spectroscopy analysis suspected the existence of a central mode, which contribution increased up to the phase transition to cubic symmetry2.We have performed nonlinear spectroscopy experiments in the femtosecond temporal domain to have a better insight into these processes. One of the originalities of our work concerns the coherent excitation of non-symmetric phonons3 by the use of a specific polarization configuration. Moreover, the use or ne and two pump beam excitation schemes allowed us to tune the q wavevector range of observation dowii to the lowest value permitted by the anisotropy for the crystal under study. Detection of the induced transient anisotropy is measured in the time domain, using a transient grating or a Kerr gate experimental set-up ,". . . . . ... . --Fig.1: Kerr heterodyne: the signal is proportional to the real pari of the induced polarization. Fig: Transient grating: the signal is proportional to the induced polarization squared.Fitting of the time resolved signals, allowed to extract the frequencies and dampings of the two lowest polariton branches, from 160 cm*l to 3000 cm-1 q values. The numbers obtained are compared to calculated ones using different models, which can include pure relaxational motions and/or relmxntional modes coupled to specific phonons. We have shown that damping characteristics are very sensitive to the various models, compared to phonon frequencies which features could not allow to conclude to a unique theoretical model. Furthermore, whatever be the model used, and in concordance with experimental findings down to polariton frequencies of 17 cm-l,we have seen that the lowest branch polariton damping tends toward zero value with q. The apparent divergence previously observed for small q values has been modeled by invoking phonon anharmonicity.The production of multi-wavelengths by stimulated Raman scattering (SRS) in high pressure gases is receiving a great interest i n recent years. being the SRS widely used in multiparametric LIDAR systems [ I ] In ]pal-ticular, the developing of new techniques to control the energy conversion efficiency from the pump radiation to a desired Stokes order represents one of the main research objective pursued in the last years It is well known that by utilising high pressure Raman gas and a low-angle pumping geometry to reduce the four wave mixing processes, a quite high conversion to the first Stokcs (SI) can be obtained. However, as the intensity of SI become...