Dipole-dipole interactions between excited Rb atoms at long range (approximately 300a0-2150a0) have been observed with molecular wave packets and a coherent nonlinear optical process. Fourier analysis of the parametric four-wave mixing (PFWM) signal wave intensity produced in femtosecond pump-probe experiments demonstrates the appearance of sidebands associated with the Rb 7s-5d(5/2) quantum beating frequency of approximately 18.3 THz. Calculations show that the observed sideband splittings and Fourier domain profiles result from multiple atom, dipole-dipole interactions, and ensembles comprising five or fewer Rb (7s, 6p) atoms account for virtually all of the data.

No abstract

The polarization characteristics of the signal wave produced in Rb vapor by difference-frequency, parametric four-wave mixing ͑FWM͒ has been investigated for either ultrafast ͑ϳ150 fs͒ or nanosecond time-scale excitation of the 5s → →5d, 7s two photon transitions. The electronic configurations of the 5d 2 D 5/2 and 7s 2 S 1/2 states of Rb, as well as their energy separation ͑ϳ608 cm −1 ͒, offers the opportunity to examine separately the resonantly enhanced 5s → →7s, 5d → 6p → 5s FWM pathways on the nanosecond time scale and then to drive both channels simultaneously with an ultrafast pulse of sufficient spectral width. As expected, dye laser ͑ϳ10 ns͒ excitation of the 5s → →5d ͑J =5/2͒ transition produces a signal wave ͑ s ϳ 420 nm͒ having the same ellipticity as the driving optical field. Two photon excitation of Rb ͑7s͒ on the same time scale, however, generates an elliptically polarized signal when the pump is linearly polarized ͑ =1͒, a result attributed to 7s → 6p, 5p amplified spontaneous emission at ϳ4 m and ϳ741 nm, respectively. Simultaneous excitation of the 5s → →7s, 5d transitions with ϳ150 fs pulses centered at ϳ770 nm yields polarization characteristics that can be approximated as a superposition of those for the individual transitions, thus displaying weak coupling between the two FWM channels. Also, the influence of molecular contributions to the FWM signal is observed for Rb number densities above ϳ5 ϫ 10 14 cm −3 .

No abstract

No abstract