Quantum beating in Rb at 18.3 THz (608 cm{sup -1}) detected by parametric six-wave mixing and sum-frequency generation in LiIO{sub 3}

Axially phase-matched parametric six-wave mixing (PSWM) has been observed in Rb vapor by two-photon excitation of the atom (5s{yields}{yields}7s,5d) with {approx}150 fs pulses, centered at {lambda}{approx}769 nm and generated by a Ti:Al{sub 2}O{sub 3} laser system. For a peak laser pulse intensity of 30 GW cm{sup -2}, PSWM is observed for Rb number densities ([Rb])>2.5x10{sup 15} cm{sup -3} whereas, for [Rb]=2.2x10{sup 16} cm{sup -3}, the PSWM process is detectable for pump intensities above {approx}3 GW cm{sup -2}. Upconversion of the 1.367 {mu}m and 1.323 {mu}m signal waves into the visible ({lambda}{approx}494 nm) has been accomplished by sum frequency generation in LiIO{sub 3}, and quantum beating in Rb at 18.3 THz ({approx}608 cm{sup -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 et al. [Opt. Lett. 23, 70 (1998)] in that a variety of coherent nonlinear optical processes ({chi}{sup (3)},{chi}{sup (5)},...) appear to be suitable for observing the amplitude and phase of atomic (or molecular) wave packets.