The ability to compress laser pulses to ultrahigh powers could open the way for compact lasers in the order of exawatt and above. Such powerful lasers would enable scientists to push the limits of physical science by deepening their study of the nature of matter. A new method for laser pulse compression, developed jointly by researchers at the University of Strathclyde, Ulsan National Institute of Science and Technology (UNIST), and Gwangju Institute of Science and Technology (GIST), exploits the dispersive property of inhomogeneous plasma to compress laser pulses. According to the researchers, the method is simpler and more efficient than other plasma-based schemes for compressing laser pulses. The plasma-based approach holds the potential to generate exawatt-to-zettawatt lasers from a compact compressing device. Courtesy of Hyojeong Lee. The researchers simulated the reflection of a long, negatively frequency-chirped laser pulse off the density ramp of an over-dense plasma slab. They created a mirror that not only reflected light pulses, but also compressed them in time by a factor of more than 200. By tailoring the plasma density, the researchers found that they could make the reflection path of high-frequency components longer than the path of low-frequency components in a frequency-chirped, long laser pulse. As the density increased longitudinally, high-frequency photons at the leading part of the laser pulse penetrated more deeply into the plasma region than lower-frequency photons, resulting in pulse compression. In essence, the gradient in the density of plasma, which was fully ionized, caused the photons to bunch together, like a line of cars driving up a steep hill. The researchers verified this concept using 1D particle-in-cell simulations showing that a picosecond laser pulse could be compressed to the duration of a few femtoseconds, with an efficiency exceeding 99%. Proof-of-principle simulations predicted compression of a 2.35-ps laser pulse to 10.3 fs — a 225-fold increase. The results indicated that a small plasma volume, only 10 cm in diameter, would be sufficient to handle extremely high powers of up to 7.5 exawatts. The new method for laser pulse compression shares the common principle of group-delay dispersion with dispersive mirrors, but with the major difference of using plasma instead of a dielectric. The researchers predict that a long, chirped pulse could be compressed several hundred times, with almost no energy loss, using a millimeter-size plasma grating, which is orders of magnitude smaller than those of conventional chirped pulse amplification (CPA) systems. Unlike the solid-state gratings commonly used in CPA, plasma is robust and resistant to damage at high intensities. “Plasma can perform a role similar to traditional diffraction gratings in CPA systems, but is a material that cannot be damaged,” professor Hyyong Suk said. “It will therefore enhance traditional CPA technology by including a very simple add-on. Even with plasma of a few centimeters in size, it can be used for lasers with peak powers exceeding an exawatt.” When light increases in intensity, it becomes capable of transforming matter. For example, when electrons are subject to laser intensities above 1018 W/cm2, they approach the speed of light and enter the realm of relativistic optics. At intensities of 1024 W/cm2 and above, protons approach the speed of light, and particles experiencing intense laser fields react to their own radiation fields — the current intensity frontier in physics. At intensities above 1029 W/cm2, known as the Schwinger limit, particles are produced directly from vacuum, that is, light can be directly transformed into matter. Exawatt-to-zettawatt lasers are required to reach intensities of this level. The computer simulations performed by the joint research group demonstrate a potential way to increase the intensity of light to the point where it is powerful enough to extract particles from vacuum for the study of matter. With further optimization, the researchers believe that even zettawatt powers could be obtained from a compact system. Understanding the nature of matter and vacuum at intensities above 1024 W/cm2 are among the outstanding challenges of modern physics. “An important and fundamental question is what happens when light intensities exceed levels that are common on Earth,” professor Dino Jaroszynski said. “High-power lasers allow scientists to answer basic questions on the nature of matter and the vacuum and explore what is known as the intensity frontier. “Applying terawatt-to-petawatt lasers to matter has enabled the development of next-generation laser-plasma accelerators, which are thousands of times smaller than conventional accelerators,” he said. “Providing new tools for scientists is transforming the way science is done.” High-power lasers also enable the study of astrophysical phenomena in the laboratory, providing glimpses into the interior of stars and the origin of the universe. “The results of this research are expected to be applicable in various fields, including advanced theoretical physics and astrophysics,” professor Min Sip Hur said. “It can also be used in laser fusion research to help address the energy issues facing humanity. Our combined Korean and U.K. teams plan to experimentally test the ideas in the lab.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01321-x).