Quantum beating in Rb at 18.3 THz (608 cm-1) detected by parametric six-wave mixing and sum-frequency generation in LiIO3

Axially phase-matched parametric six-wave mixing (PSWM) has been observed in Rb vapor by two-photon excitation of the atom (5s→→7s,5d) with ∼150 fs pulses, centered at λ∼769 nm and generated by a Ti: Al2 O3 laser system. For a peak laser pulse intensity of 30 GW cm-2, PSWM is observed for Rb number densities ([Rb])>2.5× 1015 cm-3 whereas, for [Rb]=2.2× 1016 cm-3, the PSWM process is detectable for pump intensities above ∼3 GW cm-2. Upconversion of the 1.367 μm and 1.323 μm signal waves into the visible (λ∼494 nm) has been accomplished by sum frequency generation in LiIO3, and quantum beating in Rb at 18.3 THz (∼608 cm-1) has been observed by monitoring the upconverted signal wave intensity produced in pump-probe experiments. These results broaden the scope of the wave packet detection scheme reported by Tran [Opt. Lett. 23, 70 (1998)] in that a variety of coherent nonlinear optical processes (χ(3), χ(5),...) appear to be suitable for observing the amplitude and phase of atomic (or molecular) wave packets.