Light-Drag Enhancement by a Highly Dispersive Rubidium Vapor

The change in the speed of light as it propagates through a moving material has been a subject of study for almost two centuries. This phenomenon, known as the Fresnel light-drag effect, is quite small and usually requires a large interaction path length and/or a large velocity of the moving medium to be observed. Here, we show experimentally that the observed drag effect can be enhanced by over 2 orders of magnitude when the light beam propagates through a moving slow-light medium. Our results are in good agreement with the theoretical prediction, which indicates that, in the limit of large group indices, the strength of the light-drag effect is proportional to the group index of the moving medium.

Figure 1

Transmission spectra of natural Rb near the $D_2$ transition measured at two different temperatures. Zero detuning is set to the center of the left transmission peak observed at $160°\mathrm{C}$. The label beneath each spectral feature gives the isotope and ground-state hyperfine level responsible for that feature [30]. The shaded area shows the region where the experiment is performed (see Fig. 3).

Figure 2

Ring interferometer used to measure the phase shift induced by light drag. LP is a linear polarizer and PBS is a polarizing beam splitter. The laser is operated at the Rb $D_2$ transition line, around zero frequency detuning shown in Fig. 1. The inset shows a sample fringe pattern with the maximum fringe displacement observed at $160°\mathrm{C}$. The upper and lower fringes are the fringe patterns as the cell moves to the left and right, respectively.

Figure 3

Change in phase velocity due to light drag, $\mathrm{\Delta }u$, as a function of (a) temperature and (b) frequency detuning at a Rb temperature of $160°\mathrm{C}$. The contribution of dispersion $n_{g}v$ is shown with blue hollow circles. In (a) the laser is operated at $-0.49\mathrm{GHz}$ frequency detuning (see Fig. 1).

Figure 4

Experimental setup to measure the group index of the Rb vapor. An electro-optic modulator (EOM) is used to generate laser pulses of 10-ns width, and the time delay is measured by an oscilloscope.