title: Observation of large group index enhancement in Doppler-broadened rubidium vapor.
abstract: We report experimental observation of large group index across the Lamb dips of ground hyperfine states in Doppler-broadened 87Rb vapor. By sweeping the laser frequency through each hyperfine transition we measure the saturated absorption and optical phase shift using a phase-locked Mach-Zehnder interferometer. Our measurements provide a direct demonstration of the theoretical prediction by Agarwal et al. [G. S. Agarwal and T. N. Dey, Phys. Rev. A 68, 063816, (2003)] for the first time. An enhancement factor as large as 1005 in group index was observed for Rb vapor at temperature of 85 °C. The experimental data are in good agreement with the theory.
authors.forename: Bo-Xun
authors.initials: BX
authors.lastname: Wang
authors.forename: Chih-Yuan
authors.initials: CY
authors.lastname: Liu
authors.forename: D J
authors.initials: DJ
authors.lastname: Han