The observation of an atomic wave packet by use of a coherent, nonlinear-optical process is reported. Wave packets formed in K or Rb vapor by two-photon excitation of ns and (<i>n</i>-2)<i>d</i>states (<i>n</i>=8 for K; <i>n</i>=11 , 12 for Rb) with red (~620-nm) , 80–100-fs pulses were detected by four-wave mixing in pump-probe experiments. The temporal behavior of the wave packet is observed by monitoring the coherent UV radiation generated near the alkali <i>mp</i><sup>2</sup><i>P</i>→ <sup>2</sup><i>S</i><sub>1/2</sub> (7≤<i>m</i>≤12 for Rb; 5≤<i>m</i>≤7 for K) resonance transitions when a probe pulse is scattered by the wave packet established by the earlier (identical) pump pulse. The spatial and spectral characteristics of the UV emission are well described by axially phase-matched four-wave mixing, and all the prominent frequency components of the wave packets are associated with energy differences between pairs of excited states for which Δι=0 or Δι=2 . These results demonstrate that the wave packet modulates χ<sup>(3)</sup>of the medium, thus rendering the wave packet detectable.
© 1998 Optical Society of America
H. C. Tran, P. C. John, J. Gao, and J. G. Eden, "Interaction of atomic wave packets with four-wave mixing: detection of rubidium and potassium wave packets by coherent ultraviolet emission," Opt. Lett. 23, 70-72 (1998)