Coherent initiation of superfluorescence by a weak continuous laser field

Superfluorescence (SF) is a collective spontaneous emission where the radiators spontaneously synchronize to yield an intense single-pulse emission; because SF is triggered by fluctuations in the vacuum field, its phase is random from shot to shot. We demonstrate that this phase can be controlled by seeding SF with a continuous wave (CW) laser which is resonant with superfluorescence.

Using a femtosecond laser as the pump beam source, rubidium (Rb) atoms in a heated glass cell were excited from the 5S ground state to the 6P state. The superfluorescence fields at 2.73 μm and 1.37 μm were emitted in a cascade decay from the 6P to 6S to 5P state, with a coherent light at 780 nm emitted by the nonlinear polarization generated between the 5P and 5S states. The emission process was a fast phenomenon that took less than 1 ns to complete, and we measured the time profiles of the three emissions with a sufficient temporal resolution by applying sum frequency generation spectroscopy. In order to control the superfluorescence at 1.37 μm, we irradiated a CW laser light resonant with the superfluorescence as a seed light. The results demonstrated that the 1.37-μm superfluorescence was coherently driven by the seed light. At the same time, the instantaneous intensity of the seed light was coherently amplified by 7 orders of magnitude by the superfluorescence, indicating that the superfluorescence can serve as a quantum optical amplifier.