Polarization of signal wave radiation generated by parametric four-wave mixing in rubidium vapor: Ultrafast (∼150−fs) and nanosecond time scale excitation

Abstract

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 $(\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ or nanosecond time-scale excitation of the $5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}5d$, $7s$ two photon transitions. The electronic configurations of the $5d\phantom{\rule{0.2em}{0ex}}^{2}D_{5∕2}$ and $7s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}$ states of Rb, as well as their energy separation $(\ensuremath{\sim}608\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$, offers the opportunity to examine separately the resonantly enhanced $5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}7s$, $5d\ensuremath{\rightarrow}6p\ensuremath{\rightarrow}5s$ 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 $(\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ excitation of the $5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}5d$ $(J=5∕2)$ transition produces a signal wave $({\ensuremath{\lambda}}_{s}\ensuremath{\sim}420\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ having the same ellipticity as the driving optical field. Two photon excitation of Rb $(7s)$ on the same time scale, however, generates an elliptically polarized signal when the pump is linearly polarized $(\ensuremath{\epsilon}=1)$, a result attributed to $7s\ensuremath{\rightarrow}6p$, $5p$ amplified spontaneous emission at $\ensuremath{\sim}4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and $\ensuremath{\sim}741\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, respectively. Simultaneous excitation of the $5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}7s$, $5d$ transitions with $\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses centered at $\ensuremath{\sim}770\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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 $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.