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 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.
A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.
A dense sample of atomic Rb vapor excited by a 4 ps laser pulse two-photon resonant with the $5S\ensuremath{-}5D$ transition in a large Fresnel number geometry emits a delayed coherent omnidirectional ir pulse on the $5D\ensuremath{-}6P$ transition. This superfluorescence emission involves a coherent population transfer to the
6P level and results in simultaneous directional uv emission and coherent population transfer back to the ground
5S state. This uv emission is a manifestation of parametric time-delayed four-wave mixing. For a composite noncollinear, $|{\stackrel{\ensuremath{\rightarrow}}{k}}_{1}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}|{\stackrel{\ensuremath{\rightarrow}}{k}}_{2}|$, excitation, the uv radiation pattern is conical and the cone angle depends on the angle between the excitation pulses.
Two closely spaced 778 nm, 4-ps pulses, two-photon resonant with the 5S-5D transition in Rb vapor generate a ground state grating, which later is excited by a similar third pulse, producing conical yoked superfluorescence echo on the 5S-6P, 420-nm transition. The intensity of this emission as a function of the relative delay between the leading excitation pulses is governed by the dynamics of Doppler dephasing and rephasing. This is the first observation of an echo effect induced by a spontaneous relaxation process.