Observation of quantum beating at 18.2 THz in Rb by parametric six-wave mixing

Abstract

Parametric Six-Wave Mixing is observed in Rb and the modulation of 1365 nm signal at 18.2 THz is detected by observing the coherent radiation at 492 nm produced by summing the pump and the signal.We have shown recently that the behavior of an atomic wavepacket can be observed by pump-probe experiments in which the wavepacket is interrogated by parametric four-wave mixing (PFWM) [1]. With axially phase-matched PFWM, quantum beating in Rb at 18.2 THz, corresponding to the 7s-5d (J�S/2) energy defect, was observed by monitoring the coherent signal at 420 nm. The temporal evolution of quantum beating at 18.2 THz was employed as a unique tool to observe the dissociation dynamics of diatomic molecules [2].Of particular interest to optoelectronics in the visible and near infrared (lR), however, is the fact that the signal wave is modulated at 18.2 THz. In this paper, we report the observation of parametric Six-Wave Mixing (SWM) in Rb as well as the observation of quantum beating at 18.2 THz by sum frequency mixing. This process has intriguing applications for modulating visible and near IR coherent radiation at precisely-specified frequencies. Figure 1 shows the Rb energy levels involved in the parametric SWM process. Of particular interest is the coherent signal wave at 1365 nm that is associated with the 62S1I2 � 52p3/2 transition of Rb. The experiments involved exciting Rb vapor with a pair of � 150 fs pulses produced by a Ti:sapphire oscillator/regenerative amplifier system operating at 1 kHz.The time delay between the pulses was computer-controlled with a Michelson interferometer. Both the pump and probe pulses possess vertically linear polarization. The pump-probe configuration, combined with parametric SWM, has the advantages of fs temporal resolution and a detection mechanism (coherent radiation at the signal frequency) that offers an extraordinary SIN ratio. A coherent superposition of the Rb 7s and 5d states is prepared by the pump pulse (centered at 770 nm) through two-photon absorption in Rb vapor. A coherent signal wave at 1365 nm is subsequently produced by two parametric SWM processes, 5 2 S1/ 2 �� 7 2 S1/ 2 � 6 2 p3/ 2 � 6 2 S1/ 2 � 5 2 p3/ 2 � 5 2 S1/ 2 and 5 2 S1/ 2 �� 5 2 Ds/ 2 � 6 2 p3/ 2 � 6 2 S1/ 2 � 5 2 p3/ 2 � 5 2 S1/ 2 . A second superposition of the 7s and 5d states is produced by the probe pulse, delayed in time with respect to the pump by L'lt. The emission at 1365 nm and the laser radiation at 770 nm are focused into a Lil03 crystal, which generates a sum frequency signal at 492 nm, as shown in Figure 2.The intensity of the sum frequency signal at 492 nm is recorded as a function of L'lt. Interference between the 7s and 5d states results in a quantum beating at 18.2 THz, which is embedded in the 1365 nm emission, and hence in the sum frequency signal at 492 nm. The quantum beating can be recovered through the Fast Fourier Transform (FFT) of the variation of the sum frequency signal with L'lt. Figure 3 shows data representative of these experiments in which the relative 492 nm signal intensit...