We investigated the polarization properties of superfluorescence (SF) emitted from dense cesium atomic vapor in a cell. The atoms were excited from the 6 S ground to the 8 P state using a femtosecond laser pulse. The SF fields generated on the cascaded decay, 8 P → 8 S → 7 P , mediated the nonlinear optical process. We observed 4.2-µm and 456-nm forward directional emissions generated on the 8 S → 7 P and 7 P → 6 S transitions, respectively. The polarizations of the two fields were correlated in each laser shot, and their directions fluctuated from shot to shot, reflecting the noise that initiated the 4.2-µm emission.
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 5 S ground state to the 6 P state. The 2.73 μ m and 1.37 μ m fields were generated on the cascaded decay, 6 P → 6 S → 5 P , which further stimulated the 780 nm forward emission on the 5 P → 5 S 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.
We investigated nonlinear optical processes driven by superfluorescent (SF) fields and their controllability with external laser fields. Cesium atoms were excited from the
6S ground state to the
8P state by a femtosecond laser pulse. The cascaded SF decay, $8P\ensuremath{\rightarrow}8S\ensuremath{\rightarrow}7P$, created coherence between the
7P and ground states, and led to the emission of a 455-nm pulse. A nanosecond laser pulse was applied to switch the $8S\ensuremath{\rightarrow}7P$ transition to the $8S\ensuremath{\rightarrow}6P$ one and newly generate an 852-nm pulse. At the same time, the nanosecond pulse suppressed the 852-nm emission through the inhibition of the development of the SF field between the
8P and
8S states, which led to the saturation of the 852-nm power with the increase of the nanosecond pulse power. The presented results offer an approach to investigate cooperative radiation processes in many-body systems and to develop an alternate light source.
We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the
5D state from the
5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the
6P state and then to the
5S state. The SF emission at 420 nm on the
6P--
5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.
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 $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, 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 $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the
5D to the
6P level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ 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.
We experimentally investigate the second-order quantum coherence function of a superradiant burst in a cascaded quantum system. We chirally (i.e., direction dependently) couple about 900 cesium atoms to the forward-propagating mode of an optical nanofiber. We then prepare the ensemble close to the maximally excited state, where the subsequent collective emission of a burst is known as superfluorescence. Here, we observe that second-order coherence emerges in the course of the decay. This is a clear signature of the underlying collective dynamics that is also at the origin of the superradiant burst itself. We furthermore study the dynamics of the second-order coherence function of the emission in relation to the initial average dipole moment of the ensemble. In addition, by investigating the correlation of early and late photon emission events, we obtain evidence for fundamental shot-to-shot fluctuations in the delay of the emission of the burst. Our findings thus reveal surprising similarities between superradiance in cascaded and symmetrically coupled systems, despite the fact that they are described by fundamentally different coupling Hamiltonians.
A laser pulse two-photon resonant with the ${6S}_{1/2}\ensuremath{-}{6D}_{3/2}$ 442 nm transition in Cs vapor generates delayed superfluorescence (SF) on the upper ${6D}_{3/2}\ensuremath{-}{6P}_{1/2}$ 876 nm and lower ${6P}_{1/2}\ensuremath{-}{6S}_{1/2}$ 894 nm transitions. Yoked SF is observed in the forward (along the laser) direction as evidenced by the simultaneous appearance of SF on both transitions. Cascade SF is observed in the backward direction, where SF on the lower transition only evolves after it has terminated on the upper. The four SF signals exhibit quantum beats associated with the
6P1/2 hyperfine splitting.