Optical pulse propagation with group velocities larger than the speed of light in vacuum, c, or negative, have been demonstrated theoretically and experimentally in a variety of systems . The anomalous dispersion required for generating fast light occurs near the center of absorption lines and on the wings of gain lines . We have recently shown that the nondegenerate four-wave mixing process in hot rubidium vapor may result in pulse propagation with large negative group velocities . An advantage of using this system is that no optical cavity is needed, so the process is inherently multi-spatial-mode and can support images [13, 14]. Here we present the experimental demonstration of the superluminal propagation of multi-spatial-mode images, in which all spatial sub-regions propagate with negative group velocities. We investigate the spatial mode properties and temporal reshaping of the fast light images, and show large relative pulse peak advancements of up to 64% of the input pulse width. The present results are applicable to temporal cloaking devices that require strong manipulation of the dispersion relation , where one can envision temporally cloaking various spatial regions of an image for different durations. Additionally, the modes involved in a four-wave mixing process similar to the present experiment have been shown to exhibit quantum correlations and entanglement [16, 17]. The results presented here provide insight into how to tailor experimental tests of the behavior of these quantum correlations and entanglement in the superluminal regime.
Fast light, four-wave mixing, superluminal images