Rabi oscillations in the spatial profiles of superfluorescent pulses from rubidium vapor

In this study, we investigate 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium (Rb) by driving the Rb 5S - 5D two-photon transition with an ultrashort pulsed laser. When the pump pulse is close to its transform limit (~ 100 fs) or down-chirped up to around 200 fs, the 420-nm YSF appears as a low-divergence beam with a ring-shaped radial profile. Although such a beam profile is less sensitive to the vapor pressure of Rb in a cell, its diameter rigorously varies as a function of the pump-pulse power. By numerically solving a time-dependent Schrödinger equation for a single-Rb atom, we well reproduce our experimental observation, indicating that a single-atom Rabi oscillation is responsible for the spatial beam profile of the 420-nm emission.