Observation of arbitrary group velocities of light from superluminal to subluminal on a single atomic transition line

Abstract

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.