Steep dispersion and group velocity belowc3000in coherent population trapping

Abstract

We have measured the index of refraction in the region of a coherent population trapping resonance in a cesium vapor cell with an interferometric technique. We find very high normal dispersion (up to $\frac{\mathrm{dn}}{\mathrm{df}}=9.7\mathrm{}\times {10}^{-12\mathrm{}}$ ${\mathrm{Hz}}^{-1\mathrm{}}$) at low absorption. From our spectra we obtain very small group velocities below $\frac{c}{3000}$. This corresponds to a delay time of more than 200 ns for a cesium cell 20 mm in length, equivalent to more than 60 m of propagation in vacuum.