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Physical Review A

Observation of superfluorescent emissions from laser-cooled atoms

Éric Paradis, B. Barrett, Anantharaman Kumarakrishnan, R. Zhang, Georg A. Raithel

Published Apr 25, 2008• 43 citations
Publisher

Abstract

We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the $5{D}_{5/2}$ level through the $6{P}_{3/2}$ level to the $5{S}_{1/2}$ ground level. The atomic system is initially excited to the $5{D}_{5/2}$ level from the ground state via two-photon excitation through the intermediate $5{P}_{3/2}$ 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.

DOI
10.1103/physreva.77.043419

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We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the 5D5/25{D}_{5/2}5D5/2​ level through the 6P3/26{P}_{3/2}6P3/2​ level to the 5S1/25{S}_{1/2}5S1/2​ ground level. The atomic system is initially excited to the 5D5/25{D}_{5/2}5D5/2​ level from the ground state via two-photon excitation through the intermediate 5P3/25{P}_{3/2}5P3/2​ level. The fluorescence on the 6P\ensuremath-5S6P\text{\ensuremath{-}}5S6P\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∼N\ensuremath−1\ensuremath{\sim}{N}^{\ensuremath{-}1}\ensuremath∼N\ensuremath−1, where NNN is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since NNN is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the 5D\ensuremath-6P5D\text{\ensuremath{-}}6P5D\ensuremath-6P transition at 5.2 \ensuremathμm5.2\text{ }\ensuremath{\mu}\text{m}5.2 \ensuremathμm, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the 5D5D5D to the 6P6P6P level via SF at 5.2 \ensuremathμm5.2\text{ }\ensuremath{\mu}\text{m}5.2 \ensuremathμm. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the 5.2 \ensuremathμm5.2\text{ }\ensuremath{\mu}\text{m}5.2 \ensuremathμm 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 NNN, suggesting that the 5D5D5D 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 5D5D5D state from the 5S5S5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P6P6P state and then to the 5S5S5S state. The SF emission at 420 nm on the 6P6P6P--5S5S5S 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 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{6P}_{1/2}6P1/2​ hyperfine splitting.
We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).
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.