Quantum beating in uv radiation generation by ultrashort laser pulses via four-wave mixing

Abstract

We show that quantum beating appears in the intensity of Stokes and uv radiation generated via hyper-Raman scattering (HRS) and four-wave mixing (FWM), respectively, when a large number of close-lying upper states of atoms or molecules is excited by two-photon interaction with consecutive ultrashort laser pulses. The quantum beating at frequencies correlated with the energy difference between atomic states results from a Ramsey-like interference of atomic wave packets created by the pump and probe laser fields. We reveal that the destructive interference between HRS and FWM is superimposed on the wave packet interference, leading to substantial enhancement of the beating fringe visibility. We examine the effect of laser phase fluctuations and show that they strongly limit the number of beating modes. An analytical solution of transient Maxwell-Bloch equations is obtained, and the oscillatory dependence of uv emission on the delay between two laser pulses is revealed, allowing atomic wave packets to be detected optically. A comparison to available pump-probe experiments is made.