Harmonics Push Lasers Toward Record Intensities
An international team of scientists has demonstrated an experimental scheme that promises to multiply the intensity of the world’s most powerful lasers by thousands or even millions of times. Using the Gemini laser, a petawatt (PW or 1015 W) system in the UK, Robin Timmis of the University of Oxford and colleagues have created laser light with an intensity estimated to be 80 times greater than the incident beam [1]. Such a step change in performance suggests a pathway for reaching the intensity threshold defined by the Schwinger limit, at which point the laser’s energy density should be sufficient to create matter from empty space. Probing this limit could offer insights into the quantum states inhabiting the vacuum.
Current strategies for producing extreme-intensity light rely on compressing high peak powers into ever smaller pockets of space and time, with the most powerful systems now delivering ultrashort laser pulses with intensities approaching 1023 W/cm2. But the route to even brighter beams has so far been blocked by the diffraction limit, which dictates the minimum spot size that can be achieved for a given wavelength.
To break through that barrier, physicists have been developing methods to convert the original wavelength of the beam into a series of harmonics with much shorter wavelengths. Diffraction allows such wavelengths to be focused more tightly. But theory also predicts that constructive interference between the harmonic light fields should further compress the energy in both time and space. This so-called coherent harmonic focus (CHF) is expected to deliver attosecond (10−18 s) bursts of light with an intensity many orders of magnitude higher than the original laser pulse.
In practice, such high harmonics can be generated by firing a single, intense laser pulse at a solid target. The leading edge of this pulse creates a plasma at the target’s surface that acts like a mirror, reflecting the main part of the pulse. At its full intensity, the pulse causes the plasma mirror to oscillate at relativistic speeds, which converts the reflected light into a series of coherent harmonics at extreme ultraviolet or x-ray wavelengths. However, while attosecond trains of high harmonics have been produced, the interaction between the laser and the target has been too weak and unstable to deliver the predicted intensity boost.
In the new experiments, Timmis and her colleagues show that the efficiency of harmonic conversion can be significantly enhanced by precisely controlling the shape of the pulse’s leading edge. Specifically, the researchers used an additional pair of plasma mirrors to shorten the rise time of their pulse. Such a rapid increase in intensity is expected to create a sharper and more reflective plasma surface for the main interaction. By optimizing the configuration and material properties of the two plasma mirrors, the team was able to create a pulse with a rise time of just 350 femtoseconds and a maximum intensity of 1.2 × 1021 W/cm2.
To measure the outcome from the interaction, the researchers used a diffraction grating to separate the reflected harmonic beam into its spectral components. These individual harmonics were then detected by a camera around 1.2 m away from the target. Using a theoretical analysis of the detector response, the team estimated the energy contained in each harmonic.
This analysis revealed that the incident laser pulse was efficiently converted into coherent harmonics extending from the 12th to 47th orders, all at extreme ultraviolet wavelengths. In total, these harmonics carried an energy of around 9.5 millijoules, 3 orders of magnitude higher than previously reported and consistent with theoretical predictions for a CHF. “This advance unlocks the final condition required for coherent harmonic focusing: not just generating harmonics but doing so efficiently across many orders at once,” comments Alexander Pukhov of the University of Dusseldorf in Germany, one of a group of scientists who laid the theoretical foundations for CHF in 2005.
The beam profile of the high-frequency harmonics also changed, showing strong spreading and unusual spatial modulations that have not been observed before. The researchers explained these emergent features using a model of the light–plasma interaction, confirming that the behavior was caused by CHF. “It feels like we are just getting started in terms of understanding the rich and complex physics of this mechanism,” Timmis says.
While the resulting CHF-boosted intensity is too bright to be measured directly, simulations that match the experimental data suggest that the maximum focused intensity reached more than 1023 W/cm2, which would make it the most intense source of light yet produced. With theory predicting that the intensity gain achieved by CHF scales rapidly with the incident intensity, the results suggest that the Schwinger limit (1029 W/cm2) could be reached by one of a clutch of lasers now being built to deliver 10–50 PW in peak power. “The reward would be access to a new regime of physics, where light becomes strong enough to tear down the vacuum and create particles from nothing,” Pukhov adds.
–Susan Curtis
Susan Curtis is a freelance science writer based in Bristol, UK.
References
- R. J. L. Timmis et al., “Efficiency-optimized relativistic plasma harmonics for extreme fields,” Nature 652, 1153 (2026).