Spatiotemporal profile of yoked superfluorescence from Rb vapor in the strong-excitation regime

We investigated both spatial and temporal profiles of the 420-nm yoked superfluorescence (YSF) from the atomic vapor of rubidium by driving the $5S-5D$ two-photon transition with an ultrashort laser pulse. By increasing the pump-pulse power beyond the saturation intensity, the spatial profile of the 420-nm YSF periodically changed between a central bright spot and a ring-shaped radial profile. The temporal profile of the 420-nm YSF was measured with a sampling oscilloscope with a time resolution of 50 ps. The experimental results were successfully reproduced by simulations using the Maxwell-Bloch equations. The simulated results confirmed that, in the strong excitation regime, the 420-nm emission from the lower transition component of the YSF is significantly delayed with respect to the 5.2-$\mu \mathrm{m}$ emission from the upper transition, while they are overlapped in the weak excitation regime.

Figure 1

(a) Energy diagram of Rb and the related transitions. (b) Schematic of the experimental setup.

Figure 2

(a) Experimental (upper row) and simulated (lower row) blue-light profiles at different pump-pulse conditions. A–D were recorded at pump-pulse power of 49 mW, 122 mW, 166 mW, and 272 mW, respectively. ${\mathrm{A}}^{\prime }-{\mathrm{D}}^{\prime }$ were simulated at the pump-pulse intensities indicated in Fig. 4. (b) Experimental blue-light profiles recorded at different distances from the cell center, indicated in the bottom panel of each column, at the pump-pulse power values of A, C, and D as in (a).

Figure 3

Temporal profiles of the blue light at different temperatures. (a) Experimental results at the pump-pulse power in the C case of Fig. 2. (b) Simulated results at the pump-pulse intensity of 74 GW/${\mathrm{cm}}^2$.

Figure 4

(a) Two-dimensional plot of the squared Rabi frequency $|{\mathrm{\Omega }}_{6P_{3/2}5S}{|}^2$ with respect to the time after the pump-pulse excitations (vertical axis) and the peak intensity of the pump pulse (horizontal axis). (b) Time-integrated squared Rabi frequency. The integration range, indicated by the dashed line in (a), is $0-135$ ps. ${\mathrm{A}}^{\prime }-{\mathrm{D}}^{\prime }$ indicate the intensities used in the simulations shown in the lower row in Fig. 2.

Figure 5

Intensity of blue light (BL) (a) and IR (b) as well as atomic populations plotted as a function of the time delay with respect to the pump-pulse excitations. The pump-pulse parameters were the same as those in Fig. 3, while the temperature was fixed at $170{}^{\circ }\mathrm{C}$. Time evolution of the intensity of blue light (c) and IR (d) as well as the atomic populations at a pump-pulse peak intensity of 0.53 GW/${\mathrm{cm}}^2$ at $170{}^{\circ }\mathrm{C}$.