Arbitrarily copropagating superluminal and slow light by controllable one-photon detuning in trapped cold atoms

Absorption spectra of cold cesium atoms confined in a magneto-optical trap are observed around the D2 transition line (6S(1/2), F=4-->6P(3/2), F'=5) with the probe laser frequency detuned near the trapping laser frequency. We observe the dispersion-like profile resulted from the stimulated Raman processes with the sub-natural linewidth, and study the behavior of the group velocity of light in trapped cold atoms. By only changing the blue and red detuning of the probe frequency from the trapping laser frequency, we are able to arbitrarily control the speed of a light pulse from subluminal to superluminal velocity.