We report on time-resolved pulsed four-wave mixing (FWM) signals in a thermal Rubidium vapor involving a Rydberg state. We observe FWM signals with dephasing times up to 7 ns, strongly dependent on the excitation bandwidth to the Rydberg state. The excitation to the Rydberg state is driven by a pulsed two-photon transition on ns timescales. Combined with a cw de-excitation laser, a strongly directional and collective emission is generated according to a combination of the phase matching effect and averaging over Doppler classes. In contrast to a previous report (Huber et al. in Phys Rev A 90: 053806, 2014) using off-resonant FWM, at a resonant FWM scheme we observe additional revivals of the signal shortly after the incident pulse has ended. We infer that this is a revival of motion-induced constructive interference between the coherent emissions of the thermal atoms. The resonant FWM scheme reveals a richer temporal structure of the signals, compared to similar, but off-resonant excitation schemes. A simple explanation lies in the selectivity of Doppler classes. Our numerical simulations based on a four-level model including a whole Doppler ensemble can qualitatively describe the data.
We present resonant four-wave mixing (FWM) signals involving a Rydberg state in a thermal Rb vapor. The dynamic behavior of the FWM signals exhibits revival peaks shortly after the incident pulse, which are due to coherent collective emission among all Doppler classes. The FWM signals have dephasing times up to 7 ns, and strongly depend on the excitation bandwidth to the Rydberg state. Our numerical simulations based on a four-level model including the whole Doppler broadened ensemble can describe the data quite well.
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 $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
We present new results on the coherent control of a four-wave mixing signal in rubidium vapor involving Rydberg levels. By varying the relative polarizations of the laser beams responsible for the nonlinear process, we control atomic excitation routes and thus quantum interference in the medium. We observed interference fringes as a function of the relative polarizations and further developed a theoretical model based on the density-matrix formalism that is in very good agreement with our experimental results.
Probing the interactions between a mode-locked laser pulse and saturated rubidium vapor with a pump–probe technique has resulted in observations of coherences within the atom that persist longer than 150 ps. Quantum beats due to the interference between the 7S and 5D_{5/2} energy states were observed by a four-wave mixing process. The difference frequency between the 5D–5P_{3/2} and the 5P_{3/2}–5S transitions was also observed for interpulse delays up to 180 ps. The temporal evolution of the beat amplitude indicates the existence of a superfluorescent process and a scattering process detected simultaneously with the four-wave mixing process. A model was developed to explain the processes and was used to predict the range of pulse parameters for which the scattering process and four-wave mixing could be simultaneously detected.
We investigate the transient response of the generated light from four-wave mixing (FWM) in the diamond configuration using a step-down field excitation. The transients show fast decay times and oscillations that depend on the detunings and intensities of the fields. A simplified model taking into account the thermal motion of the atoms, propagation, absorption, and dispersion effects shows qualitative agreement with the experimental observations with the energy levels in rubidium (
5S1/2,
5P1/2,
5P3/2, and
6S1/2). The atomic polarization comes from all the contributions of different velocity classes of atoms in the ensemble modifying dramatically the total transient behavior of the light from FWM.