Observation of giant group index in a multi-level ⁸⁵Rb atomic medium at room temperature

We report an experimental measurement of giant group index in the ⁸⁵Rb atomic medium at room temperature via a four-level V-type scheme on the D₂ line. Our experiment uses co-propagation configuration of probe and coupling lasers through an atomic sample. In this configuration, three sharp EIT windows with significant transparency depths are observed on the probe absorption spectrum. By establishing a Mach-Zehnder interferometer, we measure the dispersive curve and hence obtain the group index curve with three enhanced positive peaks at the locations of the EIT windows, interspersed with negative peaks. The amplitude of the group index curve is increased as the temperature decreases and is decreased as the temperature increases. We estimate from our experimental results the good values of the group index to be n_g = 5.8 × 10⁴ (slow light regime) and n_g = -4.0 × 10⁴ (fast light regime). We also show that the experimental measurements are in good agreement with the theoretical results.