Observation of superfluorescent emissions from laser-cooled atoms

We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the 5D5/2 level through the 6P3/2 level to the 5S1/2 ground level. The atomic system is initially excited to the 5D5/2 level from the ground state via two-photon excitation through the intermediate 5P3/2 level. The fluorescence on the 6P-5S transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as ~N^-1, where N is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since N is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the 5D-6P transition at 5.2 μm, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the $5D$ to the $6P$ level via SF at 5.2 μm. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the 5.2 μm transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as N, suggesting that the $5D$ level is regeneratively pumped during the SF decay.