OBSERVATION OF NARROW RESONANCES INSIDE HOMOGENEOUSLY SELF-BROADENED LINES IN PUMP-PROBE REFLECTION EXPERIMENTS

We report the experimental investigation of spectral hole burning and four-wave mixing at the interface of glass and a dense cesium vapor us ing two tunable diode lasers in a pump-probe experiment. Narrow spectr al features were observed in the center of the strongly self-broadened D-2 line at Cs number density from 9x10(15) to 2.8x10(16) cm(-3). The origin of the resonances is associated with the coherent scattering o f laser beams at a self-induced population grating, and their spectral widths reflect the relaxation rate of the population difference. A si mple asymptotic expression is used to fit the spectral Line shapes and to infer a population relaxation time of 3.7(2) ns, which is almost a n order of magnitude shorter than natural lifetime of the excited stat e.