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arXiv:quant-ph/0701164 · Physical Review A

Superluminal propagation of an optical pulse in a Doppler-broadened three-state single-channel active Raman gain medium

Kaijun Jiang, L. Deng, M. G. Payne

Published Sep 18, 2007• 26 citations
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Abstract

Using a single-channel active Raman gain medium, we show a $220\ifmmode\pm\else\textpm\fi{}20\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ advance time for an optical pulse of ${\ensuremath{\tau}}_{\mathit{FWHM}}=15.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ propagating through a $10\phantom{\rule{0.3em}{0ex}}\mathrm{cm}$ medium, a lead time that is comparable to what was reported previously using a two-mode pump field. In addition, we have verified experimentally all the features associated with this single-channel Raman gain system.

DOI
10.1103/physreva.76.033819

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Using a single-channel active Raman gain medium, we show a 220\ifmmode±\else\textpm\fi20ns220\ifmmode\pm\else\textpm\fi{}20\phantom{\rule{0.3em}{0ex}}\mathrm{ns}220\ifmmode±\else\textpm\fi20ns advance time for an optical pulse of \ensuremathτFWHM=15.4\ensuremathμs{\ensuremath{\tau}}_{\mathit{FWHM}}=15.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}\ensuremathτFWHM​=15.4\ensuremathμs propagating through a 10cm10\phantom{\rule{0.3em}{0ex}}\mathrm{cm}10cm medium, a lead time that is comparable to what was reported previously using a two-mode pump field. In addition, we have verified experimentally all the features associated with this single-channel Raman gain system.
We investigate the behavior of fast light pulse propagation in an N-type Doppler-broadened 4-level atomic system using double Raman gain processes. This system displays novel and interesting results of two controllable pairs of the double gain lines profile with a control field. The detailed physics of the processes are explored having multiple controllable anomalous regions in the medium. In this set up, the system exhibits significant enhancement in the probing Gaussian pulse through the medium as compared with Ref. [L. J. Wang, A. Kuzmich, and A. Dogariu, Nature \textbf{406}, 227(2000)]. The advance time of the retrieved Gaussian pulse is always greater than the advance time studied in the above said experiment. We analyzed that the pulse propagating through the medium with larger negative group index, 7.32×1087.32\times10^87.32×108, leaves the medium almost undistorted and sooner by time 76.12 ms76.12 \ ms76.12 ms than the pulse which leaves the medium of Wang \emph{et al.}. The Gaussian pulse always remains almost undistorted at output due to lossless characteristic of the medium. We also underlined the ways to suppress incoherences generated by the Doppler-broadening effect in the system. The limitations of the recently developed applications require to explore mechanisms for ultimate speed of a superluminal probe light pulse. In this connection, the proposed scheme may be helpful and can be easily adjusted with the current technology.
We investigate an active Raman gain scheme for significant group velocity reduction. We show that this scheme, which is fundamentally different from the electromagnetically induced transparency scheme, is capable of achieving ultraslow and distortion-free propagation of a pulsed probe field. We demonstrate the group velocity behavior that is drastically different from the conventional electromagnetically induced transparency scheme, and we show that the new scheme can be used to accurately determine the decoherence rate of a long-lived state. In addition, the Raman gain scheme has the advantage of being broadly tunable, an important feature that may have potential applications.
A scheme for enhancing precursor pulse by Doppler effect is proposed in a room-temperature active-Raman-gain medium. Due to abnormal dispersion between two gain peaks, main fields are advanced and constructively interfere with optical precursors, which leads to enhancement of the transient pulse at the rise edge of the input. Moreover, after Doppler averaging, the abnormal dispersion intensifies and the constructive interference between precursors and main fields is much strengthened, which boosts the transient spike. Simulation results demonstrate that the peak intensity of precursors could be enhanced nearly 20 times larger than that of the input.
The effect of Doppler broadening on the gain spectrum is theoretically investigated in a hot N -type active Raman gain atomic system. It is found that the gain peak in the spectrum can be extremely narrowed and amplified by two orders at room temperature as compared with that in the cold atomic system. This remarkable result originates from the shift of the dressed levels and the modification to the transition probability between dressed states when the Doppler effect is considered in the hot atomic system. The enhanced subluminal and superluminal pulse propagation for the probe field in the hot atomic system has also been obtained by numerical simulation; the probe pulse undergoes no absorption and little pulse distortion.
We make a detailed study on the dynamics of gain-assisted superluminal optical solitons in a three-state active-Raman-gain medium at room temperature. Using a method of multiple-scales we derive a high-order nonlinear Schr\"odinger equation with correction terms contributed from differential gain, nonlinear dispersion, delay in nonlinear refractive index, and third-order dispersion of the system. We show that for a long pulse with realistic physical parameters the high-order correction terms are small and can be taken as perturbations. However, for a shorter pulse these higher-order correction terms are significant and hence must be treated on equal footing as the terms in the nonlinear Schr\"odinger equation. We provide exact soliton solutions of the higher-order nonlinear Schr\"odinger equation and demonstrate that such solitons have still superluminal propagating velocity and can be generated at very low light intensity.