Slow and fast light, or big changes in the group velocity of light have been seen in a variety of optical media. However, the refractive index required to induce an observable effect has not yet been accomplished in a plasma. (“The group velocity g is the velocity with which the envelope of a pulse propagates in a medium, assuming a long pulse with narrow bandwidth (so that higher-order chromatic dispersion is not relevant) and the absence of nonlinear effects (i.e., low enough optical intensities. Under certain circumstances, the group velocity can be higher than the vacuum velocity of light. However, this does not allow for superluminal transmission of information, which would amount to a violation of causality. There are also cases with a strongly reduced group velocity dispersion (usually in the vicinity of some narrow resonance) (slow light, a hot topic of current research.” Wikipedia).
Scientists from Lawrence Livermore National Laboratory (LLNL) and the Laboratory for Laser Energetics (LLE) recently published a report titled “Slow and Fast Light in Plasma Using Optical Wave Mixing," in Physical Review Letters that described how a laser-plasma system can be configured to produce large and measurable changes in the group velocity of light.
Clément Goyon is the lead author of the report. He said that their team achieved both slow and fast light inside plasmas. This demonstrates the ability to custom tailor the refractive index of a laser-plasma system. He also said, “Slow and fast light is the tip of the iceberg. The community has increased its understanding of optical nonlinear plasma properties over the last decades. Being able to predict and use plasma properties to our advantage is critical for high-energy laser experiments in high-energy density physics and inertial confinement fusion.”
Goyon went on to explain that the cross-beam energy transfer utilized by the National Ignition Facility relies on correctly predicting the nonlinear optical properties of plasma using linear theory. Additionally, plasma-based replacements for a variety of standard optical components would enable the manipulation of light at extreme fluences. (Fluence refers the number of particles crossing a unit area expressed as particles per second.)
The reported experiment was conducted at the Jupiter Laser Facility where an energetic pump beam and a low-energy probe beam were crossing inside a helium/hydrogen plasma. By turning the wavelength difference between the two beams, the Goyon’s team was able to change the pulsed light group velocity from 0.995c to 0.12c and -0.34c. (The letter c refers to the speed of light in a vacuum or approximately three hundred thousand kilometers per second.) Pierre Michel’s group helped provide funding for the project. He said that laser-plasma interactions are extremely difficult to predict and control. He said, “However, by demonstrating slow and fast light in plasmas, I feel we have achieved a new stepping stone for the applications of plasmas as an optical medium for high-power lasers, by reproducing one of the most confounding and delicate achievements of modern nonlinear optics using plasma. This helps advance the case for using plasma as a medium in the design of future generations of high-power lasers.”