Optics Letters

Superluminal light propagation assisted by Zeeman coherence

Huijuan He, Zhengfeng Hu, Yuzhu Wang, Li‐Gang Wang, Shi-Yao Zhu

Published Jul 25, 2006• 6 citations

Abstract

We have observed a dispersionlike absorption (or gain) spectrum at the D1 transition in a Rb vapor cell filled with a buffer gas, due to Zeeman coherence of the ground states in a double Lambda configuration. Meanwhile, we have also observed superluminal pulse propagation. It is experimentally demonstrated that the front speed of a light pulse still equals the light speed c in vacuum, although the group velocity of the light pulse is(-2.2+/-0.6) x 10(4) m/s.

DOI: 10.1364/ol.31.002486

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Optics Letters

Superluminal light propagation assisted by Zeeman coherence

Huijuan He, Zhengfeng Hu, Yuzhu Wang, Li‐Gang Wang, Shi-Yao Zhu

Published Jul 25, 2006• 6 citations

Publisher

Abstract

DOI: 10.1364/ol.31.002486

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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

6{}^{2}{S}_{1/2}F=4

and $6{}^{2}{P}_{3/2}{F}^{\ensuremath{'}}=5$ in the

{D}_{2}

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

{v}_{g}=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.

c/41000)

in a Cs vapor cell is reported. Energy exchange between copropagating light components through long-lived Zeeman coherence with enhanced absorption or transmission has been observed.

We experimentally study the group time delay for a light pulse propagating through hot

^{87}\mathrm{Rb}

vapor in the presence of a strong coupling field in a $\ensuremath{\Lambda}$ configuration. We demonstrate that the ultraslow pulse propagation is transformed into superluminal propagation as the one-photon detuning of the light increases due to the change in the transmission resonance line shape. Negative group velocity as low as $\ensuremath{-}c∕{10}^{6}=\ensuremath{-}80\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$ is recorded. We also find that the advance time in the regime of the superluminal propagation grows linearly with increasing laser field power.