Abstract

Light tuned to the 6p ²P_½ → 8s ²S_1/2 excited state transition wavelength 761.09 nm in a region of (ground state) spectral transparency of Cs causes abrupt appearance of population in the 6p state after delays of up to five seconds following the introduction of radiation into the vapor cell. These nonlinear dynamics occur at laser intensities well below that necessary for laser-induced plasma formation or saturation, and only at vapor densities exceeding 10¹⁶ cm^-3. The temporal evolution of p state fluorescence is reproduced by calculations of a laser-induced critical reaction (avalanche) involving resonant energy exchange between excited 6d ²D_3/2 and ground state atoms during collisions. Experimental threshold density and the variation of avalanche delay with incident intensity are shown to be in good agreement with theoretical expectations based on measured excited state collisional cross-sections. Implications of this effect, in which the rate of excited state collisions can be varied optically at constant vapor density, for unique studies of local fields in nonlinear optical interactions of dense systems and collisional contributions to excited state dephasing are suggested.

© 1993 Optical Society of America