Relativistic plasma could boost intensity of petawatt laser

A vacuum isn’t empty space. According to quantum electrodynamics, virtual particles and antiparticles flit in and out of existence across exceedingly small spaces and in short periods of time.

One way to study quantum vacuum fluctuations is to use an extremely strong electromagnetic field. For intensities greater than 10²⁹ W/cm², known as the Schwinger limit, the field is able to spontaneously create virtual electron–positron pairs from the quantum vacuum. No source, however, has yet generated an electromagnetic field of that intensity. Robin Timmis and Peter Norreys (University of Oxford), Brendan Dromey (Queen’s University Belfast), and their colleagues have now used the Gemini laser system at the Rutherford Appleton Laboratory in the UK to generate bright, coherent extreme-UV light with an intensity of 10²¹ W/cm². ¹ Although still eight orders of magnitude away, the team argues that the same approach, if implemented at other, high-power laser facilities, could scale up and reach the sought-after Schwinger limit.

A key component for generating a high-intensity field is a plasma that is relativistic—free electrons oscillate near the speed of light—and well shaped. The researchers created such a plasma by shining 5 J, 50 fs pulses from Gemini’s petawatt laser onto a glass target. The plasma interacts with the same laser pulse that generated it, and that interaction leads to the emission of coherent high-frequency harmonics of the laser. Under the right conditions, the harmonics become tightly focused. The result is a train of spatially compressed attosecond pulses whose intensity is many orders of magnitude greater than that of the incident laser.

The challenge encountered in past attempts to generate coherent harmonic focusing was that the harmonic spectrum decayed too fast. Timmis, Dromey, and their colleagues overcame that problem by implementing a specialized plasma mirror. It functions essentially as an optical switch that increases or decreases the reflectivity of the plasma created at the glass target’s surface. For years, plasma mirrors have been used for boosting laser intensity. Using material coatings to carefully tune the plasma mirror’s reflectivity, the researchers enabled the laser pulse to generate a coherent harmonic beam more efficiently.

Simulations of the Gemini laser’s interaction with the relativistic plasma by Timmis, Dromey, and colleagues suggest that the intensity of the incident laser pulse can be boosted even further, to 10²³ W/cm². The coherent harmonic focus should scale with laser intensity, so reaching the Schwinger limit could, in principle, be done with a higher-powered laser. The lasers at the ELI-NP (Extreme Light Infrastructure Nuclear Physics) facility in Romania are 10 PW, an order of magnitude more powerful than Gemini’s. Laser systems that are even more powerful, including the 25 PW NSF OPAL (Optical Parametric Amplifier Line) lasers at the University of Rochester and the 100 PW system at the Station of Extreme Light in Shanghai, China, are currently in development.