Cascade and yoked superfluorescence detected by sum frequency generation spectroscopy.

We investigated the superfluorescent decay process of dense rubidium atomic vapor in a cell. Using a femtosecond laser pulse, the atoms were excited from the 5S ground state to the 6P state. The 2.73m and 1.37m fields were generated on the cascaded decay, 6P → 6S → 5P, which further stimulated the 780 nm forward emission on the 5P → 5S transition. Using sum frequency generation (SFG) spectroscopy, we observed all emission fields and the time delay between them, with sufficient temporal resolution. The experimental results were successfully reproduced using semiclassical simulations.