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Comptes Rendus Physique

Electromagnetically-induced transparency, slow light, and negative group velocities in a room temperature vapor of 4He∗

Fabienne Goldfarb, T. Lauprêtre, Jérôme Ruggiero, Fabien Bretenaker, Joyee Ghosh, Rupamanjari Ghosh

Published Dec 1, 2009• 20 citations
Publisher

Abstract

4 He ∗ at room temperature is a particularly interesting system as velocity changing collisions (VCCs) are necessary to observe ultra-narrow (less than 10 kHz) EIT windows for purely electronic spins in the presence of Doppler broadening. Such narrow resonances are known to be linked to a dramatic reduction of the group velocity of a probe pulse, although the medium is transparent. The evolution of the delay is recorded with respect to the coupling beam intensity and to small Raman detunings. We also demonstrate that it is possible to use optically detuned resonances ( Fano-like profiles ) to see a transition from slow light to negative group velocity. All these measurements are found to be in good agreement with a simple model based on an effective homogeneous linewidth.

DOI
10.1016/j.crhy.2009.10.004

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4 He ∗ at room temperature is a particularly interesting system as velocity changing collisions (VCCs) are necessary to observe ultra-narrow (less than 10 kHz) EIT windows for purely electronic spins in the presence of Doppler broadening. Such narrow resonances are known to be linked to a dramatic reduction of the group velocity of a probe pulse, although the medium is transparent. The evolution of the delay is recorded with respect to the coupling beam intensity and to small Raman detunings. We also demonstrate that it is possible to use optically detuned resonances ( Fano-like profiles ) to see a transition from slow light to negative group velocity. All these measurements are found to be in good agreement with a simple model based on an effective homogeneous linewidth.
Electromagnetically-induced transparency (EIT) in three-level Λ-systems is based on quantum interference effects involving coherence between the two lower levels, which allow propagation of a resonant probe light beam in the presence of a strong coupling field. We have observed transparency in ultra-narrow (<10 kHz) windows at the resonant 1.083 μm transition for purely electronic spins in gaseous 4 He* at room temperature. Slow light is an interesting outcome of the EIT phenomenon due to extreme dispersion within the narrow transparency window. We have obtained group delays of about 4 μs in a 2.5 cm long He* cell. A complete theoretical analysis explains our observations emphasizing the positive role of collisions. Slow light with 1-GHz Doppler broadening opens the door to applications of controllable large-bandwidth delays in radars.
We present a realistic theoretical treatment of a three-level Λ system in a hot atomic vapor interacting with a coupling and a probe field of arbitrary strengths, leading to electromagnetically-induced transparency and slow light under the two-photon resonance condition. We take into account all the relevant decoherence processes including collisions. Velocity-changing collisions (VCCs) are modeled in the strong collision limit effectively, which helps in achieving optical pumping by the coupling beam across the entire Doppler profile. We take into account a dynamic rate of influx of atoms in the two lower levels of the Λ, and an outflux from all the three levels. The steady-state expressions for the atomic density-matrix elements are numerically evaluated to yield the experimentally measured response characteristics. Our predictions are in excellent agreement with the reported experimental results for 4He*. The role played by the VCC parameter is seen to be distinct from that by the transit time or Raman coherence decay rate.
Dicke narrowing is a phenomenon that dramatically reduces the Doppler width of spectral lines, due to frequent velocity-changing collisions. A similar phenomenon occurs for electromagnetically induced transparency (EIT) resonances, and facilitates ultranarrow spectral features in room-temperature vapor. We directly measure the Dicke-like narrowing by studying EIT line shapes as a function of the angle between the pump and probe beams. The measurements are in good agreement with an analytic theory with no fit parameters. The results show that Dicke narrowing can increase substantially the tolerance of hot-vapor EIT to angular deviations. We demonstrate the importance of this effect for applications such as imaging and spatial solitons using a single-shot imaging experiment, and discuss the implications for the feasibility of storing images in atomic vapor.
We experimentally studied Zeeman electromagnetically induced transparency (EIT) resonances and slow light propagation in hot Rb vapor. Propagation of weak σ− polarized light pulses in the presence of stronger σ+ polarized background through Rb vapor was realized using a single laser beam and the Pockels cell. The dependences of slow light group velocity and fractional pulse delay on the overall laser beam intensity and temporal pulse length showed that lower optical power and longer light pulses lead to improved EIT and slow light features. The connection between EIT and slow light was also investigated showing that narrower and more contrasted EIT resonances are necessary for further decreasing a group velocity.