&Bullet; physics 14, s60

Interactions between two laser beams in a plasma allow precise control of the speed of light.

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Light has a strict speed limit, but under certain conditions this limit can be exceeded for individual light pulses. Heretofore, light has been made to travel beyond its speed limit in media, including atomic gases and optical fibers. Clément Goyon of Lawrence Livermore National Laboratory, California, and colleagues are now showing how the speed of light in a plasma can be adjusted to produce a change in speed on the order of magnitude of light. The demonstration could help improve control over inertial confinement fusion experiments and could lead to plasma-based optics for high-power lasers.

The speed at which light pulses travel through a material can differ greatly from the speed c This light moves in a vacuum. This speed, known as the group speed, can be either higher or lower than cand it takes into account how the shape of a pulse of light propagates and warps as it moves through a material.

For their demonstration, Goyon and colleagues first generated a hydrogen-helium plasma by ionizing a gas beam with a polarized laser beam. Then they aimed a second laser beam at the plasma. Where the paths of the two beams crossed, the horizontal component of the second laser pulse slowed in response to a change in the index of refraction of the plasma. This slowdown resulted from interactions between the two lasers and the plasma.

When measuring the time lag between the horizontal and vertical components of the second laser pulse, the team found that they had different speeds. By adjusting the frequency difference between the two beams, they found that they could set that speed of 0.995c up to between 0.12c and – 0.34cwhich indicates that the top of the pulse is faster than c.

–Rachel Berkowitz

Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.


  1. C. Goyon et al., “Slow and fast light in the plasma with optical wave mixing” Phys. Rev. Lett.126205001 (2021).

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