Researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Synchrotron SOLEIL near Paris have made a major breakthrough in the development of laser-plasma free-electron lasers (FELs). For the first time, they have successfully operated these devices in the high-gain regime, a state where the generated radiation is amplified significantly. This achievement, published in the journal Physical Review Letters, represents a key step toward creating more compact and affordable FELs for scientific research. Free-electron lasers generate extremely short and intense flashes of light that are crucial for studying atoms, molecules, and new materials. However, access to these powerful tools is currently limited, as only a few large-scale facilities exist worldwide, and the waiting times for experiments are long. While conventional FELs have achieved operational stability, laser-plasma FELs—considered a more compact alternative—had not until now. A team led by Dr. Arie Irman from HZDR's Institute of Radiation Physics has now overcome this challenge after years of research. In their experiment, the researchers produced ultraviolet light flashes with a wavelength of 272 nanometers and high pulse energy. The FEL power showed exponential growth, a hallmark of the high-gain regime, marking a significant improvement over their earlier results from 2023. Dr. Marie Labat from Synchrotron SOLEIL notes that in laser-plasma FELs, electrons can gain the energy needed to produce intense light flashes in just a few millimeters, compared to the much longer distances required in traditional accelerators—up to about 2 kilometers. The success of the experiment hinged on precisely controlling the interaction between the laser and the plasma. Using HZDR's high-performance laser, DRACO, the team generated the necessary plasma in a thin gas beam. Dr. Susanne Schöbel of HZDR explains that this was a complex task due to the nonlinear processes involved in the laser-plasma interaction. However, by fine-tuning the laser to match the plasma, the team was able to produce stable and reproducible ultraviolet light flashes using their undulator—a device that guides the electron beam to emit light. A key feature of the experiment was the first observation of exponential growth in radiation output power. The researchers now aim to further optimize the laser-plasma interaction and the undulator setup to enhance the quality of the light flashes. Dr. Irman mentions that ultraviolet light is just the starting point, with the next goal being extreme ultraviolet (EUV) light, which could be used for more efficient quality control of nanostructured computer chips. While it may take several years before compact laser-plasma FELs become widely available, the potential impact of this technology has already sparked considerable interest in the scientific community.