Generating attosecond hard X-ray pulses

Once only a part of science fiction, lasers are now everyday objects used in research, healthcare and even just for fun. Previously available only in low-energy light, lasers now come in wavelengths from microwaves through X-rays, opening up a range of different downstream applications. In a study in Nature, a team led by UW–Madison scientists generated the shortest hard X-ray pulses to date through the first demonstration of strong lasing phenomena. The resulting pulses can lead to several potential applications, from quantum X-ray optics to visualizing electron motion inside molecules. “We have observed strong lasing phenomena in inner-shell X-ray lasing and been able to simulate and calculate how it evolves,” says Uwe Bergmann, physics professor at UW–Madison, and senior author on the study. The inner-shell X-ray lasing process is similar as it is in optical lasing, just at much shorter wavelengths. Because inner-shell electrons are tightly held, powerful X-ray pulses, like those from X-ray free-electron lasers (XFEL), are required to excite enough of them simultaneously to result in lasing. In turn, the photons they emit in this process are also at X-ray wavelengths. But XFEL pulses are generally “dirty,” with each pulse really being made of several short, intense spikes in time, and a range of spikes with different wavelengths, limiting some of their applications. “They’re just not clean, beautiful pulses (like visible lasers),” says Thomas Linker, joint postdoctoral researcher at UW–Madison and the Stanford PULSE Institute at SLAC and lead author of the study. “But it’s the only thing we have.” Here, the researchers tightly focused XFEL pulses onto a sample made of copper or manganese. The input pulse is still dirty, but very short and incredibly powerful: the equivalent of focusing all the sunlight that hits the Earth into one square millimeter. The emitted X-ray photons hit instrumentation that disperses them by wavelength, much like a prism disperses visible light into a rainbow, reflects it based on its angle, then is read by a detector. Their results show that emitted light contained all of the expected wavelengths, but spatially, it showed a few hotspots instead of the expected smooth signal. Applying a 3D simulation, Linker calculated that the emitted X-rays underwent filamentation, a strong lasing phenomenon. When they further increased the intensity of the input pulse, they saw another unexpected result: instead of seeing hotspots of one wavelength, they observed spectral broadening and sometimes multiple spectral lines. They ran the simulation on this new data and realized that this result can only be explained by another lasing phenomenon called Rabi cycling, where the pulse is so strong that the sample will cyclically absorb photons and emit them by stimulated emission. They used their simulation to plot the emitted pulse intensity over time and found that their dirty input pulses resulted in extremely short stimulated emission pulses — the shortest hard X-ray pulses observed by anyone to date. “We have generated hard X-ray pulses, 60 to 100 attoseconds in duration, with these strong lasing phenomena,” Linker says. An attosecond is one quintillionth of a second, and this extremely short pulse duration is what could drive new, advanced LASER applications. “If you want to see electron dynamics, how they move inside their orbitals, that’s the attosecond timescale,” Linker says. Adds Bergmann: “There are so many nonlinear technologies and phenomena that the laser community uses now, but very few of those have dared to have been tried with hard X-rays. This work is a step towards pushing the exciting field of real laser science into this powerful hard X-ray regime.”