Slow and fast light in plasma

Extreme manipulation of light's group velocity in optical media leads to ``fast'' and ``slow'' light, where pulses propagate superluminally or slow to an almost complete stop. Both phenomena have been found in a range of nonlinear optical media, including atomic gases, photorefractive crystals, and optical fibers. Plasmas are attractive for extreme non-linear optics as they tolerate fluences many orders of magnitude beyond the damage thresholds of traditional optical media, but high required powers and the complexity of the laser-produced plasma make precision control of group velocity difficult. We report the first experimental demonstration of slow and fast light in a plasma. We control the group velocity of light between 0.14c and -0.27c in a fully-ionized He/H2 plasma via optical wave mixing of an auxiliary pump laser and the manipulated probe beam, mediated by the wavelength-detuning-dependent ion-acoustic plasma response. Besides unveiling the potential of plasmas to generate slow and fast light at extreme fluences, these results might also impact inertial confinement fusion or high-energy density physics experiments, where complex wave-mixing processes between dozens of laser beams may impact the group velocities and temporal profiles of these beams.