Interaction of atomic wave packets with four-wave mixing: Detection of rubidium and potassium wave packets by coherent ultraviolet emission

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 (n - 2)d states (n = 8 for K; n = 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 mp²P → ²S_1/2 (7 ≤ m ≤ 12 for Rb; 5 ≤ m ≤ 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 Δl = 0 or Δl = 2. These results demonstrate that the wave packet modulates χ⁽³⁾ of the medium, thus rendering the wave packet detectable.