Sub-luminal and super-luminal light propagation in inverted-Y system with wavelength mismatching effects

We investigate the electromagnetically induced transparency phenomenon in a four-level inverted-Y system, i.e. composite of three-level Λ and Ξ systems, using the density matrix formalism theoretically in stationary as well as moving atoms. Most of the EIT studies have taken the probe and coupling fields with wavelength matched condition to minimize the effect of Doppler-broadening. In this work we show a variation of absorption and dispersion profiles with wavelength mismatching factor because in real atoms for multilevel system the matched wavelength condition is difficult to obtain. This mismatching results in Doppler-broadening. We show that for moving atoms at the room temperature, a narrow window is also present within the wide transparency window for the probe absorption and dispersion for the mismatched cases. The group index modifies the probe propagation from super-luminal to sub-luminal within a very small detuning range near resonance. The variation of the group index with the coupling fields shows optical switching behavior when one of the coupling field is slightly detuned.