Catching a light ride on a plasma wave.

Catching a light ride on a plasma wave

Replace the waves of Waikiki with relativistic waves in an ionized gas, and substitute, a bunch of photons for a surfer. The result is a novel method for increasing the frequency of short pulses of laser light. Such a technique, if practicable, could provide an efficient, flexible way of generating coherent X-rays.

The idea of using plasma waves to "accelerate" photons comes from physicist John M. Dawson and graduate student Scott C. Wilks of the University of California, Los Angeles. Their theoretical analysis of how such a photon accelerator would work appears in the May 29 PHYSICAL REVIEW LETTERS.

"Of course, nothing goes faster than the speed of light [in a vacuum]," Dawson says. "But in a plasma, light travels slower than it does in a vacuum, so one can in fact increase its velocity."

When a bunch of electrons, accelerated to nearly the speed of light, whips through an ionized gas, or plasma, it launches a rapidly moving wave in the plasma, much like the wake generated by a speedboat. Injecting into the plasma wave a pulse of laser light, consisting of a group of photons, transfers energy from the wave to the photons. As they ride the wave, the photons increase their frequency substantially and gain a little speed.

Researchers have already observed plasma wakes, and computer simulations indicate that transferring energy from such wakes to photons is possible in principle. "Now we need to demonstrate that the phenomenon exists," Dawson says.

Using plasma waves to accelerate photons may be most useful for generating high-frequency light. "From our studies, we believe [plasma acceleration] should have high efficiency," Dawson says. "It looks like it might be a cheap way to get coherent X-rays. We could convert low-energy [microwave] photons, which are cheap and easy to make, into expensive photons, shifting them into the ultraviolet or X-ray frequencies."

In contrast, synchrotron light sources, which rely on electron acceleration to generate coherent X-rays, convert into light only a small proportion of the total energy required to run the system.

The notion that relativistic plasma waves can modify photon frequencies also has intriguing implications for astrophysics. Such waves could shift the frequency of light coming from pulsars and other astronomical objects, complicating the interpretation of such observations.

Dawson sees the possibility of not only accelerating but also manipulating photons, perhaps using them to make X-ray movies of molecular vibrations. "It's a tremendously rich field," he says. "There are so many possibilities that experimental programs are just barely scratching the surface. Practice is lagging way behind theory."