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Physical Review A

Ultraslow and superluminal light propagation in a four-level atomic system

Cui-Li Cui, Ji-Kui Jia, Jin-Wei Gao, Yan Xue, Gang Wang, Jin‐Hui Wu

Published Sep 14, 2007• 44 citations
Publisher

Abstract

We theoretically study the coherent interaction of a tripod-type atomic system with three laser fields, i.e., one probe and two coupling, or two probe and one coupling field. Our analytical and numerical results show that, when all three fields are on two-photon resonance, in addition to the Rabi frequencies of the coupling fields, the probe group velocities also depend on the spontaneous decay rates and the initial population distribution. So we can manipulate the probe pulses to achieve controllable ultraslow group velocities by preparing different initial atomic states. With an incoherent pump field applied, a narrow transparent window accompanied by abnormal dispersion may occur in the probe absorption spectrum, which then allows us to switch a probe pulse from ultraslow to superluminal propagation.

DOI
10.1103/physreva.76.033815

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We theoretically study the coherent interaction of a tripod-type atomic system with three laser fields, i.e., one probe and two coupling, or two probe and one coupling field. Our analytical and numerical results show that, when all three fields are on two-photon resonance, in addition to the Rabi frequencies of the coupling fields, the probe group velocities also depend on the spontaneous decay rates and the initial population distribution. So we can manipulate the probe pulses to achieve controllable ultraslow group velocities by preparing different initial atomic states. With an incoherent pump field applied, a narrow transparent window accompanied by abnormal dispersion may occur in the probe absorption spectrum, which then allows us to switch a probe pulse from ultraslow to superluminal propagation.
We investigate dispersion and absorption properties of a weak probe field in a four level atomic system through interacting dark resonances. We show that two narrow peaks appears in the optical spectra due to the presence of interacting dark resonances. This two narrow peaks leads to the superluminal light propagation with strong absorption. However, if a weak incoherent pump field is exerted to the probe transition, then the peaks structures can be changed such that two absorption peaks transformed to two gain deeps. We Show that both subluminal and superluminal light propagation occur in this system, where controlled by the incoherent pumping strength.
The coherent superposition of two-atomic levels induced by coherent population trapping is employed in a standard ΛΛΛ type scheme to form a tripod-like system. A weak probe pulse scanning across the system is shown to experience a crossover from absorption to transparent and then to amplification. Consequently the group velocity of the probe pulse can be controlled to propagate either as a subluminal, a standard, a superluminal or even a negative speed. It is shown that the propagation behavior of the light field is entirely determined and controlled by the initial states of the coherent superposition.
The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence (SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self- and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature, the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.
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.
When the interactions of the two coupling transitions from an excited level to two nearly degenerate ground levels with the same vacuum occur in a four-level tripod-scheme atomic system, vacuum-induced coherence (VIC) results from the quantum interference between the two spontaneous transitions. We investigate the effects of VIC and the relative phase between the two coupling fields on the probe absorption and dispersion properties as well as the group velocity of the probe field. We find that by adjusting the incoherent pumping field and VIC as well as the relative phase, the central absorption peak in the double transparency spectrum reduces, even develops into a gain dip, and the probe field propagates at a velocity from superluminal to subluminal. It is particularly pointed out that the subluminal light propagation with gain without inversion can be realized in such a system. The physical interpretation of the phenomena has been given.
The propagation of a weak probe field in a laser-driven four-level atomic system is investigated. We choose mercury as our model system, where the probe transition is in the ultraviolet region. A high-resolution peak appears in the optical spectra due to the presence of interacting dark resonances. We show that this narrow peak leads to the superluminal light propagation with strong absorption, and thus by itself is only of limited interest. But if in addition a weak incoherent pump field is applied to the probe transition, then the peak structure can be changed such that both sub- and superluminal light propagation or a negative group velocity can be achieved without absorption, controlled by the incoherent pumping strength. A suitable choice of laser propagation directions allows us to preserve these results under Doppler averaging.