Researchers have produced bright spatially-coherent X-rays from a tabletop source.
X-ray of the head of a damselfly, produced from a tabletop laser plasma interaction with a 30-fs exposure time.
A tabletop X-ray source is 1,000 times brighter than other laser-driven plasma wakefield accelerator sources, opening the door to finer temporal and spatial resolution X-ray imaging. Stefan Kneip, Zulfikar Najmudin, and other researchers at Imperial College London, the University of Michigan, and Instituto Superior Técnico Lisbon report in Nature Physics that they have produced bright spatially-coherent X-rays from a tabletop source (doi: 10.1038/NPHYS1789).
Potential applications include phase-contrast and lensless imaging (previously limited to synchrotron facilities), imaging of ultrafast atomic or molecular interactions, and dramatically improved medical X-ray resolution.
Today, synchrotrons are the standard source of bright coherent X-rays, but these particle accelerators are big and expensive. Laser plasma wakefields produce high accelerating fields and offer a route to creating much more compact (and potentially cheaper) systems. This method uses intense laser pulses to generate a wake in a plasma, similar to the wake generated behind a speeding boat.
The wakefield can generate electric fields at least 1,000 times stronger than those generated by conventional particle accelerators. This allows a wakefield to accelerate particles into the GeV range over a distance of 1 cm. The plasma wave also makes the particles oscillate in the transverse direction and give off radiation. At high energies, this radiation is in the X-ray regime. Although soft X-rays have been observed before, the scientists have now managed to produce hard X-rays, entering a regime that increased the brightness of the X-rays by three orders of magnitude.
The output of the new system has two promising qualities, in addition to brightness: First, it emits roughly 30-fs X-ray pulses. Second, the emission comes from a point only about 1 µm across. This results in a pencil-thin beam of hard X-rays that allows researchers to see fine details in their samples. Thus, the system improves the possible resolution of images as well as providing the ability to capture ultrafast interactions.
The X-ray system fits on a tabletop—or, more accurately, within a cubic meter-sized vacuum chamber—but this current system required the use of one of the most advanced ultrafast lasers in the world: The University of Michigan's HERCULES laser. The Ti-sapphire–based laser produced 32-fs linearly polarized pulses with a central wavelength of 800 nm, with energy of 2.3 J. The beam was focused to an 11-mm-diameter spot, with a peak intensity of 4.7 × 1019 W/cm2.
Improvements to high-energy ultrafast laser technology could thus lead to better, and more-affordable, bright X-ray sources. Researcher Stefan Kneip at Imperial College says, "For some applications, it [our system] will enable important measurements which have not been possible until now."
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.