3-D Wave Mixing Process Explored

March 29, 2006, UMEA, Sweden -- The concept that particles can be created in a vacuum is the focus of physicists from Umea University in Sweden and the Rutherford Appleton Lab in Oxfordshire, England, who hope to explore the vacuum by aiming three powerful laser streams at each other.

Mattias Marklund and Gert Brodin, associate professors at Umea University, initiated the research. Marklund said the team proposes making use of four-wave mixing, in which three laser pulses interact to produce a fourth pulse of photons by "stimulating" the vacuum.

"Under normal circumstances, beams of light are completely uninterested in each other," Marklund said. "The fact that light-light interactions in vacuum is possible at all stems from the remarkable quantum properties of empty space. So-called virtual particles that come in and out of existence over very short periods become prominent components of light propagation when the light intensity comes close to the Schwinger limit, which is roughly 10^16 V/cm. However, even before this happens, the vacuum becomes weakly nonlinear, and light-light interaction is enabled by these virtual particles."

As the high-intensity light beams cross, the photons may exchange energy and momentum via the virtual particles, thus giving rise to photons with a new, but well-defined, frequency and propagation direction, Marklund said.

"Previous suggestions for detecting light-light interactions in vacuum (using lasers) have involved a two-dimensional geometry where three laser beams interact in a plane," he said. "We have shown that this is not optimal in any way for a realistic setup, and we have instead stressed that a fully three-dimensional setup, where the laser pulses come from three different directions, is better."

Moreover, the use of the Astra Gemini Laser (which will be in operation at Rutherford in 2007) will enable the team to make the setup even more competitive, he said. The Astra Gemini will consist of two 0.5 petawatt lasers, something unique in the laser world, where such high-intensity lasers normally do not have a "partner," Marklund said. "By splitting one of the laser beams, we obtain three independently configurable high-intensity lasers, which can be used to probe the quantum vacuum and light-light interactions. The four-wave process would yield 0.07 photons per shot at Astra Gemini, and with a repetition rate of three pulses per minute, the necessary statistics could be achieved over a reasonable short period of time."

Light-light interactions might also be used to explore various hypotheses related to dark energy. The Swedish-British experiment, which will be carried out over the next year at the Rutherford Appleton Lab, is described in the March 3 Physical Review Letters.

For more information, visit: www.phys.umu.se/personal/mattias.htm