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Journal of the Optical Society of America B

Coherent double-switching between superluminal and subluminal propagation of probe pulse via AC Stark shift

Saswata Ghosh

Published Aug 27, 2013• 5 citations
Publisher

Abstract

The group index and its lineshape parameters of a relatively weak double-resonance probe pulse (signal) propagating through an open doubly driven M-type five-level atomic system are analytically formulated. It is shown for the first time, to our knowledge, that the group velocity status of the signal can be altered (between superluminal and subluminal propagation) by the combined effect of atomic coherence and linear AC Stark shift through coherent coupling. The distinct feature in this scheme is that, although it does not rely on electromagnetically induced transparency, a double-switching effect (i.e., from superluminal to subluminal and vice versa) is observed with negligible absorption/gain at two different frequency regimes.

DOI
10.1364/josab.30.002540

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Journal of the Optical Society of America B•May 14, 2015•5 citations
The group index and its lineshape parameters of a relatively weak double-resonance probe pulse (signal) propagating through an open doubly driven M-type five-level atomic system are analytically formulated. It is shown for the first time, to our knowledge, that the group velocity status of the signal can be altered (between superluminal and subluminal propagation) by the combined effect of atomic coherence and linear AC Stark shift through coherent coupling. The distinct feature in this scheme is that, although it does not rely on electromagnetically induced transparency, a double-switching effect (i.e., from superluminal to subluminal and vice versa) is observed with negligible absorption/gain at two different frequency regimes.
Contrary to popular belief, we demonstrate the feasibility of generating superluminal (and subluminal) probe (and signal) light via electromagnetically induced transparency in a medium comprising coupled double-ladder systems. This scheme can be realized in both homogeneously (radiative) as well as in inhomogeneously (Doppler) broadened atomic systems. Unlike more intricate earlier schemes, our scheme is based simply on steady-state propagation dynamics resulting from compensation of the inevitable absorption losses by large nonlinear gain generated through appropriate choice of the pump and coupling fields. We show how easily in this scheme the speed of weak probe (and signal) fields can be switched from subluminal to superluminal by simply varying the strengths of the coherent pump and coupling fields. Furthermore, it is shown that under these conditions both the signal and probe fields are intensity matched and both propagate with the same subluminal (or superluminal) group velocity without suffering loss or gain for long distance in the medium.
The interaction of a weak probe laser with an inverted Y-type four-level atomic system driven by two additional coherent fields is investigated theoretically. Under the influence of the coherent coupling fields, the steady-state linear susceptibility of the probe laser shows that the system can have single or double electromagnetically induced transparency windows depending on the amplitude and the detuning of the coupling lasers. The corresponding index of refraction associated with the group velocity of the probe laser can be controlled at both transparency windows by the coupling fields. The propagation of the probe field can be switched from superluminal near the resonance to subluminal on resonance within the single transparency window when two coupling lasers are on resonance. We propose an atomic 87Rb system for experimental observation.
A scheme is proposed for control of pulse propagation from subluminal to superluminal in a four-level Λ-type atomic system. With the relatively intense probe laser and the appropriate detuning of control field, the medium exhibits much lower absorption in the spectral ranges where the dispersion changes from normal to abnormal in the double electromagnetically induced transparency system than in the single electromagnetically induced transparency system. The transmission intensity can be enhanced by several times for the subluminal pulse propagation and by several orders of magnitude for the superluminal pulse propagation without the large distortion in the former compared with those in the latter. We attribute the dramatic absorption reduction to the enhanced nonlinear effects.
Abstract The propagation of a weak probe field through a four-level inverted-Y type atomic system driven by two additional coherent fields, namely the control field and the repump field, is studied. The simulated probe absorption shows that electromagnetically induced transparency and the line shape of the transparency window under a Doppler-broadened condition are dependent on the residual Doppler averaging due to the wavelength mismatch between the control and probe fields. For 87 Rb the probe propagation changes from subluminal on-resonance under a Doppler-free condition to superluminal on-resonance under a Doppler-broadened condition. For both 87 Rb and 133 Cs, the narrow subluminal window occurring in the group index profile in a Doppler-free environment offers an excellent delay bandwidth for probe field propagation. Under this condition, the order of the delay in probe field propagation can be as long as a few hundred µ s and seems quite promising in the field of optical delay generation and light storage. The analysis shows that the residual Doppler averaging has a greater impact on the switching window under the variation of the Rabi frequency of the control field as compared to that of the repump field.
The group velocity of the probe light pulse (GVPLP) propagating through an open Λ-type atomic system with a spontaneously generated coherence is investigated when the weak probe and strong driving light fields have different frequencies. It is found that adjusting the detuning or Rabi frequency of the probe light field can realize switching of the GVPLP from subluminal to superluminal. Changing the relative phase between the probe and driving light fields or atomic exit and injection rates can lead to GVPLP varying in a wider range, but cannot induce transformation of the property of the GVPLP. The absolute value of the GVPLP always increases with Rabi frequency of the driving light field increasing. For subluminal and superluminal propagation, the system always exhibits the probe absorption, and GVPLP is mainly determined by the slope of the steep dispersion.