Fast and slow light in thin gas cells

We theoretically propose a strategy to use a travelling monochromatic wave in a simple two level atomic system to tune the group velocity over a wide range. Both subliminal and superluminal group velocity are demonstrated from the interaction of a light wave with atoms in the case where the cell length is much less than the light beam diameter. The tunable control of the light group velocity for the negative (superluminal light) to positive (subluminal light) are achieved in different physical conditions. In fact, we control the group velocity by tuning the relaxation parameters such as, the collision between atoms, the incident intensity and the beam diameter. This allows us to switch from subluminality to superluminality and back in a sub-Doppler system.