We report on spiking dynamics of frequency upconverted emission at 420 nm generated on the 6 P 3 / 2 − 5 S 1 / 2 transition in Rb vapor two-photon excited to the 5 D 5 / 2 level with laser light at 780 and 776 nm. The spike duration is shorter than the natural lifetime of any excited level involved in the interaction with both continuous and pulsed pump radiation. The spikes at 420 nm are attributed to temporal properties of the directional emission at 5.23 µm generated on the population-inverted 5 D 5 / 2 − 6 P 3 / 2 transition. A link between the spiking regime and cooperative effects is discussed. We suggest that the observed stochastic behavior is due to the quantum-mechanical nature of the cooperative effects rather than random fluctuation of the applied laser fields.
We report a study on the transfer of intensity correlation properties in frequency upconversion via four-wave mixing (FWM) in rubidium (Rb) vapor. The 5S1/2−5P3/2−5D5/2 two-photon transition in Rb85 leads to collimated blue light (CBL) generation at 420 nm by absorbing two input photons of wavelengths 780 nm and 776 nm. The g(2)(τ) intensity correlation measurement of the CBL field reveals that the intensity correlations of the input fields are transferred to the CBL field due to strong atomic coherence in the FWM loop. We also demonstrate that the measured values of g(2)(τ) of the CBL field are reduced by the effect of Doppler broadening of Rb vapor.
Temporal spiking of directional infrared and ultraviolet cooperative emission from sodium vapors excited to the
4D5/2 level with a continuous-wave resonant laser pump is analyzed. Mirrorless lasing at 2207 and 2338 nm on cascade population-inverted transitions and radiation at 330 nm generated due to four-wave mixing demonstrate a high degree of intensity correlation.
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
5d2D5∕2 and
7s2S1∕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 $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.