Observation and density dependence of the Raman echo in atomic thallium vapor

Abstract

Working on the 6P12−6P32 transition of atomic thallium vapor, we have observed Raman echoes generated by excitation and probe fields detuned from the 7S12 intermediate state by up to 50 cm-1. This constitutes the first time that echoes, generated by pure two-photon excitation, have been observed in an atomic system. The Raman echo intensity is found to increase roughly as the square of the thallium density at low densities, but at higher densities it peaks and subsequently declines. We suggest that two-photon absorption and loss of phase-matching resulting from excitation-pulse-induced population transfer are important in the high density degradation of the Raman echo signal.