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Chinese Physics

Experimental investigation of slow light propagation in degenerate two-level system

Wang Li-Rong, Yanting Zhao, Jie Ma, Jianming Zhao, Liantuan Xiao, Jia Suo-Tang

Published Jan 16, 2006• 5 citations
Publisher

Abstract

Slowing a light pulse in a degenerate two-level system is observed with a double-frequency sweeping technique. The effects of coupling beam intensity, cell temperature and frequency detunings of the coupling and probe beams in resonance, on the slowing of light propagation in such a system are investigated. It is found that group velocities depend strongly on polarization combinations. A group velocity v g =6760m/s of light pulses in caesium vapour is obtained under the optimal parameters.

DOI
10.1088/1009-1963/15/2/022

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Slowing a light pulse in a degenerate two-level system is observed with a double-frequency sweeping technique. The effects of coupling beam intensity, cell temperature and frequency detunings of the coupling and probe beams in resonance, on the slowing of light propagation in such a system are investigated. It is found that group velocities depend strongly on polarization combinations. A group velocity v g =6760m/s of light pulses in caesium vapour is obtained under the optimal parameters.
We investigate the weak probe pulse propagation through the atomic medium, in the presence of the strong control laser field, under the regime of the electromagnetically induced transparency (EIT). These fields couple to the medium by forming the 3-level ladder configuration. We focus on two distinct cases: closed system, which is a non-degenerate, and open system, where the middle level is 3-fold degenerate, and the additional levels are coupled to the other levels only via spontaneous emission. The spatio-temporal profile of the probe pulse envelope is obtained by using the formalism of Maxwell-Bloch equations with the help of the Fourier transform method. We study the influence of the control field Rabi frequency, the spectral half-width of the initial pulse and decay rates on the propagation of the probe pulse. Several important pulse parameters are also calculated and discussed. It is shown that the group velocity of light can be controlled in such a manner that one can switch from slow to fast light and vice versa when changing the control Rabi frequency and spectral pulse half-width. Moreover, the possibility of replacing the open with an effective closed system is also discussed.
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.
Under weak probe field, rotating wave and electric dipole approximations, we have derived analytic expressions for absorption and dispersion coefficients, group index and group velocity in a four-level cascade atomic medium interacting with the probe and pump laser fields. The model is applied to 85Rb atomic system to study the control of the fast and slow light propagation of the probe beam according to the intensity and frequency of the pump laser beam. With this excitation model, we find two transparent domains and thus the light group velocity is also controlled at these two frequency domains. In particular, the amplitude and the sign of the group index are easily changed by adjusting the intensity or frequency of the pump laser. That is, the probe beam propagation can be changed between fast and slow light modes by adjusting the intensity or frequency of the pump laser. Our analytical results can be useful for related experimental studies.
This thesis presents theoretical models and results of numerical simulations describing the propagation of optical pulses through dense, thermal atomic vapours. In particular we investigate the nonlinear effects of optical solitons due to self-induced transparency (SIT) in two-level systems, optical simultons in V-type three-level systems and electromagnetically induced transparency (EIT) in Λ-type systems, including the storage and retrieval of dark-state polaritons. An investigation is made into two-photon excitation of the 5D states of rubidium in a high-intensity beam including the hyperfine structure of the relevant atomic levels. Decay from these states to the 6P manifolds is ruled out as a cause of experimentally observed fluorescence due to the amount of power broadening associated with intensities necessary to provide any significant level of population in these highly excited states. We combine the nonlinear effects of optical solitons and EIT to explain experimentally observed steepened pulses in a V-type system in a micron-length cell. We explain the behaviour as the early formation of a simulton pulse drawn from a CW probe field by a strong coupling pulse, due to coherent population trapping. We predict that in a longer cell it may be possible to facilitate propagation of matched pulses, even when the transitions in the system have different propagation coefficients, as long as decoherence from collision broadening can be controlled. The fact that weak pulses can propagate with this scheme suggests an approach to achieving transparent propagation of single or few photon pulses distinct from, but related to, both SIT and EIT.
The effect of slowing down pulses of probe radiation in a medium consisting of two-level quantum systems whose longitudinal and transverse relaxation times strongly differ is studied. The conditions for the occurrence of “slow light” in a transparency window are obtained. Regimes are discovered in which the medium becomes transparent or amplifies the probe radiation, with the high dispersion of the refractive index being conserved. The parameters at which the delay time of the probe radiation reaches a maximum or the group velocity of this radiation decreases to a maximal possible value under the condition of exact resonance with the strong field are found.
We show theoretically that the propagation of light can be slowed down considerably using the method of coherent hole-burning in a Doppler broadened three-level lambda-type atomic medium without the Doppler-free configurations. The reduction of group velocity of light pulse is achieved by the application of a saturating beam and a strong coupling beam which produce a narrow spectral hole-burning at resonance. We can obtain a larger group index than that using the method of saturation absorption spectroscopy in Doppler-broadened two-level atomic systems.