Abstract

Recent progress has been made in the theoretical and experimental study of the amplification of optical signals without population inversion (AWI) of the participating electronic levels. Induced coherence among appropriate levels in the active medium is the necessary condition for obtaining AWI. Similarly, the generation of wave packet states in atoms and molecules also relies on the coherent superposition of several states. It is feasible, therefore, that AWI is possible in a system which has been excited into a wave packet state. We have thus demonstrated experimentally such amplification in an extended sample of sodium dimers and we term this process wave packet amplification (WPA). An important characteristic of WPA is that the gain is doubly phase-sensitive—it is modulated at both the wave packet oscillation period and the optical period. The former time-dependence is a result of the periodic revivals of the pump-induced electronic polarization and the latter optical phase-sensitivity is a manifestation of the coherent nature of the molecular wave packet. The experiments to be described use a time-resolved upconversion method to detect the change in energy of a 50 fs probe pulse after it passes through a sodium heat pipe that has been pumped with another 50 fs pulse. The maximum saturated energy gain in this system is four and corresponds to all of the available energy being transferred from the wave packet to the probe pulse.

© 1993 Optical Society of America