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Physical Review A

Nonadherence to the conventional group velocity for nanosecond light pulses in Rb vapor

T. Kohmoto, Hirokazu Tanaka, Shingo Furue, K. Nakayama, Masakazu Kunitomo, Yukio Fukuda

Published Aug 12, 2005• 5 citations
Publisher

Abstract

Propagation of nanosecond light pulses in a resonantly absorbing medium is studied. Deviation from the conventional group velocity and superluminal-to-subluminal transition were observed for the natural atomic transitions of Rb. The observed frequency and atomic density dependences of the propagation delay in the anomalous dispersion region, where the conventional concept of the group velocity breaks down, are reproduced well by the theory of the net group delay.

DOI
10.1103/physreva.72.025802

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Quantum Electronics and Laser Science Conference•Nov 13, 2002•1 citation
Propagation of nanosecond light pulses in a resonantly absorbing medium is studied. Deviation from the conventional group velocity and superluminal-to-subluminal transition were observed for the natural atomic transitions of Rb. The observed frequency and atomic density dependences of the propagation delay in the anomalous dispersion region, where the conventional concept of the group velocity breaks down, are reproduced well by the theory of the net group delay.
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Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D1{D}_{1}D1​ transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results.
We have examined the propagation of femtosecond laser pulses in an absorbing dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to subluminal transition in the propagation velocity keeping its initial shape almost intact. It is verified that the peak velocity is well described by a modified group velocity v(S) defined within the framework of the saddle-point method as well as by a recent prediction of the net group delay of surviving frequency.
Summary form only given. The propagation of light in a dispersive medium is characterized by the group velocity /spl nu//sub g/=c(n+/spl nu//sup dn//d/spl nu/)/sup .1/ where n is the refractive index and /spl nu/ the optical frequency. Depending on the sign and magnitude of /sup dn//d/spl nu/ in the medium, the group velocity can be positive, infinite or negative. The group velocity corresponds to the speed of the propagation of the maximum of an optical pulse travelling into the medium. This remains true whether the dispersion in the medium is normal (/sup dn//d/spl nu/>0) or anomalous (/sup dn//d/spl nu/<0). It has been demonstrated that coherently prepared atomic systems can exhibit steep dispersion leading to small absolute values of the group velocity (slow light). In recent work, we have demonstrated that narrow resonances arising from the coherent interaction of two optical fields with a closed atomic transition between two degenerate levels may correspond to a steep anomalous dispersion and thus to slow and negative group velocity. We report on the experimental study of the propagation of an amplitude modulated wave in a medium presenting negative group velocity.
Summary form only given. We use gain-assisted linear anomalous dispersion to demonstrate superluminal light pulse propagation with a negative group velocity through a transparent atomic medium. We place two Raman gain peaks closely to obtain an essentially lossless anomalous dispersion region that results in a superluminal group velocity. The group velocity of a pulse in this region exceeds c and can even become negative,while the shape of the pulse is preserved. We measured a negative group velocity index of n/sub g/=-315(/spl plusmn/5). Experimentally, a light pulse propagating through the spin-polarized atomic vapor cell exits from it earlier than propagating through the same distance in vacuum by a time difference that is 315 times of the vacuum light propagation time L/c=0.2 ns.