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Journal of Physics B Atomic Molecular and Optical Physics

Extremely narrowed and amplified gain spectrum induced by the Doppler effect

Lida Zhang, Yun Jiang, Ren-Gang Wan, Si-Cong Tian, Bing Zhang, Xiaojun Zhang, Jin-Yue Gao, Yueping Niu, Shangqing Gong

Published Jun 21, 2011• 17 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/44/13/135505

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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 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.
A scheme for high gain and narrow linewidth of an optical cavity with a four-level atomic system is proposed by the Doppler effect via active Raman gain (ARG) process. Atomic motion leads to Doppler frequency shift which induces constructive interference for the linear susceptibility. The enhanced normal dispersion greatly narrows the cavity linewidth, and the amplified gain gives rise to a high cavity transmission. Simulation results show that the cavity linewidth based on ARG is about one order of magnitude narrower than that based on electromagnetically-induced transparency under the same conditions, and the cavity transmission intensity could be enhanced by nearly 30 times.
We theoretically investigate the influence of a coherent pump field on the propagation of a weak light pulse of a probe field in a four-level atomic system. Due to the modulation of the pump field, the light pulse can be manipulated from subluminal to superluminal with negligible distortion. This scheme can be realized in both the ultracold and Doppler-broadened atomic systems. We also demonstrate that the spectral linewidth with an anomalous dispersion is reduced by thermal averaging; therefore, one can obtain a larger negative group refractive index in room-temperature vapor than the largest value achieved in ultracold atomic gas.
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.
An active-Raman-gain (ARG) spectrum of Rydberg atoms is investigated and proposed to measure a microwave (MW) electric (E) field. A strong Raman field couples the ground-state transition with large frequency detuning, and a probe field drives the metastable transition, whose transmission shows a large gain. When a MW field couples the Rydberg transition, the gain peak splits. It is interesting to find that the frequency splitting of gain peaks shows a linear relationship with the MW-field strength, which can be used to measure the MW E field. Moreover, the ARG spectrum has a large peak value that is about 10 times larger than that of common electromagnetically induced transparency schemes. For weak MW fields, the Doppler effect will greatly narrow the gain spectrum if the probe and Raman fields copropagate through the atomic cell, which increases the probe sensitivity dramatically. After the Doppler averaging, the probe sensitivity is enhanced by about 10 times, and the linewidth is narrowed to 1/10 compared to the case without the Doppler effect.
In a Λ-type electromagnetically induced transparency system, it shows that on the Doppler-broadened linear absorption background, as the probe intensity increases, the single narrow line-width window gradually evolves into 3 windows and 2 absorption peaks alternately. In this paper, the mechanism of probe intensity is studied in detail by using the dressed-state model. We propose that when the probe field is not so weak, the atomic Raman coherence can be manipulated by its intensity. For a Doppler-broadened system, there will appear the discontinuous energy variation of the dressed-states, and the large Raman loss due to the double resonance for dressed-states, which are the key factors for the evolution of the transparency window.