Polarization of signal wave radiation generated by parametric four-wave mixing in rubidium vapor: Ultrafast

The polarization characteristics of the signal wave produced in Rb vapor by difference-frequency, parametric four-wave mixing (FWM) has been investigated for either ultrafast \par\par(88\'bc150\parfs)\par\par or nanosecond time-scale excitation of the \par\par5s86\rquote 86\rquote 5d\par\par, \par\par7s\par\par two photon transitions. The electronic configurations of the \par\par5d\par\parD\par588\bullet 2\par2\par\par\par and \par\par7s\par\parS\ par188\bullet 2\par2\par\par\par states of Rb, as well as their energy separation \par\par(88\'bc608\par\parcm88\rquote 1\par)\par\par, offers the opportunity to examine separately the resonantly enhanced \par\par5s86\rquote 86\rquote 7s\par\par, \par\par5d86\rquote 6p86\rquote 5s\par\par 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 \par\par(88\'bc10\parns)\par\par excitation of the \par\par5s86\rquote 86\rquote 5d\par\par \par\par(J=588\bullet 2)\par\par transition produces a signal wave \par\par(\parbbs\par88\'bc420\parnm)\par\par having the same ellipticity as the driving optical field. Two photon excitation of Rb \par\par(7s)\par\par on the same time scale, however, generates an elliptically polarized signal when the pump is linearly polarized \par\par(b5=1)\par\par, a result attributed to \par\par7s86\rquote 6p\par\par, \par\par5p\par\par amplified spontaneous emission at \par\par88\'bc4\parbcm\par\par and \par\par88\'bc741\parnm\par\par, respectively. Simultaneous excitation of the \par\par5s86\rquote 86\rquote 7s\par\par, \par\par5d\par\par transitions with \par\par88\'bc150\parfs\par\par pulses centered at \par\par88\'bc770\parnm\par\par 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 \par\par88\'bc5\'c3\emdash \par1014\par\parcm88\rquote 3\par\par\par.\par