ULTRAFAST VIBRATIONAL DYNAMICS BY TIME-DELAYED NONDEGENERATE OPTICAL 4-WAVE-MIXING IN CONDENSED MATTER

Ultrafast dynamical phenomena in an electronic system coupled to the atomic vibrational degrees of freedom of a condensed matter environment are shown to be revealed in time-delayed nondegenerate optical four-wave mixing (OFWM) experiments. We perform a numerical multiple integration and obtain both the time-dependent and time-integrated signal intensity as a function of the time interval tau(s), between the two excitation light pulses. In contrast to the Markovian relaxation case where the vibrational degrees of freedom simply play the role of a thermal reservoir, our analysis, which takes into consideration the nonequilibrium dynamics of the vibrational system, shows that the time-dependent signal intensity is asymmetric with respect to the positive and negative off-resonance frequency Omega(2) of the second excitation pulse, even when the first excitation pulse is on resonance. It is also found that the time-integrated intensity measured as a function of tau(s), shows a delayed peak which becomes more pronounced as Omega(2) is decreased. These phenomena are manifestations of the nonequilibrium atomic vibrations generated by the optical excitation and the electron-vibration interaction. It is emphasized that time-delayed nondegenerate OFWM is the most direct and efficient tool for investigating ultrafast dynamics of optically generated nonequilibrium states.