Lenses made of plasma open up new ultrafast applications

Attosecond experiments – a Nobel Prize-winning area of research – may have gotten an additional boost thanks to a novel idea from the world of plasma accelerators. Pulses of light lasting a billionth of a billionth of a second drive attosecond experiments, which can track the movements of electrons. However, those pulses are notoriously difficult to focus. A team of scientists from DESY and the Max Born Institute (MBI) in Berlin has produced a new and flexible solution: Use a lens made of plasma, the highly energized state of matter found in lightning or on the surface of the sun. The resulting setup, described in Nature Photonics, not only focuses the attosecond pulses without significant absorption but also has a negligible effect on the duration of the pulses. The results potentially enable a wider range of attosecond experiments, which are useful in understanding the movement of electrons in novel and complex materials and elucidating the electronic roots of chemical reactions important for industrial and pharmaceutical processes.

The cutting edge of ultrafast methods and experiments is attosecond technology. Light pulses lasting an attosecond are used not only for experimental purposes, but also to initiate plasma acceleration in miniaturised accelerator modules. However, focusing the necessary light pulses, which lie in the extreme-ultraviolet (XUV) or X-ray region of the electromagnetic spectrum, is highly challenging due to the lack of suitable optics. “To make a pulse as short as 100 attoseconds, you need a very large range of wavelengths or colours of light,” says DESY scientist Jonathan Wood, one of the authors of the paper. “In a typical lens, different colours travel at significantly different speeds, causing the pulse to stretch out in time.” Additionally, and crucially, the electrons in the atoms in the lens itself absorb some of the light, meaning that even fewer quantum particles of light, or photons, make it into the experiment. “The more photons you have, the more detail you get in your experiment. So these changes have big effects on what you can accomplish in diagnosing signal to noise,” Wood says.

Researchers at MBI and DESY have solved this problem by shaping plasma into a focusing element. The team used an attosecond pulse setup based on that used to drive the plasma accelerator in the FLASHForward project at DESY. To create their plasma-based optic, the team sent strong electrical pulses through hydrogen gas inside a tiny tube within a block of sapphire. This process strips the hydrogen atoms of their electrons, creating a plasma. The electrons naturally move outward toward the edges of the tube, where the temperature is considerably lower, shaping the plasma into a concave lens in the process. Normally, such a lens would spread light out rather than focus it. However, since plasma bends light differently than ordinary materials, it instead focuses the attosecond pulses.

Additionally, the researchers show that the focus of the lens can be adjusted by changing the pressure of the gas and, consequently, the density of the plasma. Importantly, the team found that the plasma lens serves as an effective filter for the infrared driving pulses, which normally require thin metal filters. This means those filters are no longer necessary, allowing more of the XUV light to reach the experiment. This may enable the generation of stronger attosecond pulses, opening new opportunities for experiments that are currently limited by low numbers of photons.

“The technique offers an alternative approach to constructing an attosecond beamline, characterized by simple alignment, wavelength tunability, and, most importantly, high throughput”, says first author Evaldas Svirplys, a doctoral student at MBI. “The ability to generate stronger attosecond pulses opens the door to a wide range of potential applications in ultrafast spectroscopy and could contribute to the development of the next generation of attosecond light sources.”

To better understand how the focused attosecond pulses behave over time, the scientists ran computer simulations. They discovered that the pulses stretch only slightly, from 90 to 96 attoseconds after being focused by the plasma lens. Under more realistic conditions – where different colors of the initial attosecond pulse arrive at slightly different times (a phenomenon known as chirp) – the plasma lens actually shortened the pulses. In this case, the pulse duration decreased from 189 to 165 attoseconds.

“This is a first demonstration, but the results are promising enough that this could be applied to attosecond experiments soon,” Wood says. “It’s really good to have these sorts of collaborations like this between us and MBI.”

“Results like this show that technologies developed for and in relation to plasma accelerators have broad effects far beyond the actual accelerator cell – the plasma lens could be exceptionally useful for future attosecond photon science experiments,” says DESY Director for Accelerators Wim Leemans. “This further confirms the revolutionary nature of plasma accelerator technologies, and that DESY is at the absolute frontier of developments in accelerator technology and ultrafast physics.”