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Physical Review A

Picosecond superradiance in a three-photon resonant medium

Gombojav O. Ariunbold, Wenlong Yang, Alexei V. Sokolov, Vladimir A. Sautenkov, Marlan O. Scully

Published Feb 27, 2012• 17 citations
Publisher

Abstract

We investigate picosecond cooperative emission in an ensemble of rubidium atoms coherently excited by 100-fs laser pulses that are three-photon resonant to the $6P$-$5S$ transition. The emitted 420-nm light is recorded by a streak camera with 2-ps resolution. The resultant pulse shape shows ringing typical for superradiance in extended sample. The pulse shape is measured as a function of input power and is compared to the solutions of Maxwell-Bloch equations. The observed superradiance, which is in the low-excitation regime, is found to be closely related to optical precursor behavior.

DOI
10.1103/physreva.85.023424

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Communications Physics•Oct 6, 2026•0 citations
We investigate picosecond cooperative emission in an ensemble of rubidium atoms coherently excited by 100-fs laser pulses that are three-photon resonant to the 6P6P6P-5S5S5S transition. The emitted 420-nm light is recorded by a streak camera with 2-ps resolution. The resultant pulse shape shows ringing typical for superradiance in extended sample. The pulse shape is measured as a function of input power and is compared to the solutions of Maxwell-Bloch equations. The observed superradiance, which is in the low-excitation regime, is found to be closely related to optical precursor behavior.
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.
Optical precursor is the sharp optical pulse front that does not show delay in absorptive media. In this thesis, optical precursor behavior in rubidium (Rb) vapor was investigated in the picoseconds regime. An amplified femtosecond laser was shaped to a 7-ps square pulse with sharp rising and trailing edges. This pulse was then sent into a hot rubidium vapor, and the center frequency of the laser pulse was absorbed. The output pulses were measured by a fast streak camera with 2-picosecond resolution. By varying the temperature of the Rb vapor, the measured pulse shapes showed the progression of formation of optical precursors. The measured pulses shapes showed good agreement with theory.\n \nOn the other hand, a connection between optical precursors and femtosecond laser pumped 3-photon superradiance was investigated in this thesis. Maxwell-Bloch equations were numerically solved in two steps with commercial software Mathematica 8. A good agreement was found between simulation and experiment. It was confirmed that, at low excitation regime, superradiance generated from hot rubidium vapor, which were pumped by a femtosecond laser, can be understood as the formation of optical precursors.
We report the observation of coherent UV light pulses by the coherent scattering of IR pulses from atomic rubidium vapor. Rubidium atoms were first excited by a 100 fs pulse from the 5S ground state to the 5D state via a two-photon transition. The atoms were then pumped by an IR pulse resonant to the 5D–12P transition. The presence of the IR pulse triggered the instantaneous emission of a UV light pulse on the 12P–5S transition. The pulse had a time duration of tens of picoseconds, which was measured by a picosecond-resolution streak camera. The temporal shape of the generated light is explained by a simplified atom–field interaction theory.
We study backward cooperative emissions from a dense sodium atomic vapor. Ultrashort pulses produced from a conventional amplified femtosecond laser system with an optical parametric amplifier are used to excite sodium atoms resonantly on the two-photon 3S –4S transition. Backward superfluorescent emissions (BSFEs), both on the 4S –3P and 4S –3P transitions, are observed. The picosecond temporal characteristics of the BSFE are observed using an ultrafast streak camera. The power laws for the dependencies of the average time delay and the intensity of the BSFEs on input power are analyzed in the sense of cooperative emission from nonidentical atomic species. As a result, an absolute (rather than relative) time delay and its fluctuations (free of any possible external noise) are determined experimentally. The possibility of a backward swept-gain superfluorescence as an artificial laser guide star in the sodium layer in the mesosphere is also discussed.
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).
Observation of pulsed superradiance from two-level systems under direct excitation is challenging because the much stronger excitation pulse overlaps with the emitted radiation in time, space, and spectrum. Previous demonstrations have therefore mostly relied on excitation via auxiliary levels or multiphoton excitation at different wavelengths, limiting applicability. Here we present an interferometric pump-probe technique that enables observation of picosecond superradiance from a moderately-dense-but-optically-thin ensemble of two-level atoms resonantly excited under substantial Doppler and collisional broadening. Sequential pump and probe pulses tuned to the 555.6-nm E1 transition of 174 Yb allowed us to reconstruct the temporal waveform of the superradiant field from time-averaged transverse fluorescence. The fluorescence displayed interference fringes as a function of the pump-probe delay, whose envelope corresponds to the superradiant polarization P ( t ); taking its time derivative yields the radiated electric field. Pulse widths extracted from the reconstructed field follow a power-law scaling of 1/N00.441/{N}_{0}^{0.44}1/N00.44​ , where N 0 is the number of atoms in the interaction volume. The measured temporal dynamics and scaling behavior are in good agreement with simulations based on the coupled Maxwell-Schrödinger equations. This technique provides direct access to ultrafast superradiant dynamics without requiring auxiliary transitions, multiphoton excitation, or ultrafast detectors.