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arXiv:quant-ph/0106107 · Physical Review A

Knob for changing light propagation from subluminal to superluminal

G. S. Agarwal, Tarak Nath Dey, Sunish Menon

Published Oct 10, 2001• 251 citations
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Abstract

We show how the application of a coupling field connecting the two lower metastable states of a $\ensuremath{\Lambda}$ system can produce a variety of effects on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from regions of anomalous dispersion and gain in susceptibility.

DOI
10.1103/physreva.64.053809

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We show how the application of a coupling field connecting the two lower metastable states of a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ system can produce a variety of effects on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from regions of anomalous dispersion and gain in susceptibility.
We show that the group velocity of a weak pulse can be manipulated by controlling the phases of two weak optical fields applied to a V-shaped three-level system. Such control can even cause the probe propagation to change from subluminal to superluminal. We consider two schemes: in the first, the excited states are coupled by decay-induced coherence, which is an inherent property of the medium, and in the second, quantum coherence is created by coupling the excited states to each other by a strong microwave field. We also discuss the group velocity reduction experienced by a single weak propagating probe due to decay-induced coherence.
We were able to arbitrarily control the speed of a light pulse from subluminal to superluminal velocity by changing only the power of the laser for coupling coherently the single transition between 62S1/2F=46{}^{2}{S}_{1/2}F=462S1/2​F=4 and $6{}^{2}{P}_{3/2}{F}^{\ensuremath{'}}=5$ in the D2{D}_{2}D2​ line of a Cs atomic vapor system. With weak coupling power, a Gaussian light pulse was propagated superluminally with a negative group velocity ${v}_{g}=\ensuremath{-}c/14400,$ which is caused by a highly anomalous dispersion related to an electromagnetically induced absorption. By increasing the coupling power at the same laser frequency, the pulses were propagated with a vacuum speed at the middle power and a subluminal group velocity vg=c/3000{v}_{g}=c/3000vg​=c/3000 at high power, which is caused by a normal dispersion related to an electromagnetically induced transparency. It was also found that group velocities depend largely on polarization combinations.
The propagation of pulses through dispersive media was investigated by solving Maxwell's equations without any approximation. It was demonstrated that the superluminal or subluminal pulse propagation originates essentially from the interference of the different frequency components of the pulse. The coherence of the different frequency components plays an important role in superluminal propagation. When the pulse becomes partially coherent, the propagation changes from superluminal to subluminal. Since superluminality always exists as long as the spectrum of the coherent pulse is within the anomalously dispersive region whatever the shape of the pulse is, such a coherent pulse must be transmitted superluminally.
Abstract Light-matter interaction between a light field and an atomic medium could be used to manipulate the medium’s dispersive properties, leading to subluminal and superluminal regimes. Utilizing the same idea, this work investigates the propagation of structured superluminal light and anomalous phase shifts in a Λ- type three-level atomic medium, where a structured control field governs the atomic system. The absorption and dispersion properties are modulated by the azimuthal quantum number (ℓ) of the control field. The maxima and minima of the absorption spectrum, along with the corresponding normal and anomalous dispersion peaks, are enhanced with a (2ℓ) dependence on the azimuthal quantum number in the medium. The negative group index and anomalous phase shift vary within the ranges −1000 ≤ ng ≤ −900 and −6.5 × 10 5 ≤ ϕ ≤ −5.6 × 10 5 rad, respectively. The corresponding group velocity lies in the range −3.33 × 10 5 ≤ vg ≤ −3.0 × 10 5 m/s within the spatial region −2λ ≤ x, y ≤ 2λ. Furthermore, the anomalous phase shift is significantly enhanced in regions exhibiting superluminal behavior, following the (2ℓ) azimuthal quantum number dependence. The findings of this work may have potential applications in optical storage devices, imaging, coding, and the design of advanced photonic technologies.