  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 time scales. Combined with a third 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 [1] 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.
