A team of researchers from the University of Oxford and Queen’s University Belfast in the UK has demonstrated that they can produce extremely intense light beams using a nonlinear optical technique called relativistic harmonic generation. This paves the way to generate electromagnetic field intensities previously unheard of even in laboratory setups. 

Just as inquiry into the quantum behavior of materials has led to the development of quantum sensing and computing, it can also help physicists better understand the world around us and the fundamental nature of the universe. 

The theory of quantum electrodynamics (QED) suggests that when light at extremely high intensities interacts with the vacuum, it can be converted into matter. The theory can be tested straightforwardly, with the only hurdle that the light of extreme intensity referred to in this theory is a million times more intense than the most powerful lasers we have developed so far. 

Einstein’s flying mirror

A research team under the leadership of Peter Norreys, a professor of inertial physics at the University of Oxford, used the Gemini Laser at the Central Laser Facility to generate coherent extreme ultraviolet (XUV) and X-ray photons using relativistic harmonic generation. 

In this approach, the researchers fired high-frequency, ultrashort, laser pulses onto a solid glass target at subpicosecond (10^-12) rates. This created a plasma that behaves like an oscillating mirror, and subsequent lasers hitting it are like hitting a mirror moving toward you at nearly the speed of light. This is often referred to as Einstein’s flying mirror. 

Doing so compresses the light reflected from the plasma, thereby increasing its intensity. Working with researchers from Queen’s University, the team concentrated this light into a small region, just a few nanometers wide, using a process called coherent harmonic focus. 

How intense was the beam? 

Unfortunately, the researchers were unable to measure the intensity directly, but theoretical estimates suggest the team may have boosted the laser intensity to 10^23 W per cm2. 

“The energy in our XUV beam was over three orders of magnitude brighter than previous measurements,” said Robbin Timmis, postdoctoral research assistant at the University of Oxford, in a press release.  “By resolving a long-standing gap between theoretical expectations and experimental results, we confirmed the required energies to support a coherent harmonic focus and therefore offer a substantial boost in intensity above that of the original laser pulse.”

The experiments demonstrate a realistic pathway to generating an extreme electromagnetic field for laboratory studies. The researchers are confident that their approach can breach the Schwinger limit, which is greater than 10^16 V per cm or 10^29 W per sq.cm, paving the way for optical studies of the quantum vacuum. 

Additionally, this could help applications of ultrafast imaging of physical and biological systems, as well as photolithography and nuclear fusion science, the researchers added. The team is now analyzing data from some follow-up experiments to decide the next steps. 

“We will be shortly publishing results about a new harmonic beam that we have discovered on that run,” added Timmis in the press release, “and future studies will focus on actively controlling the coherent harmonic focus and directly measuring its intensity.”