For half a century, scientists have been trying to figure out how to build a cost-effective and reasonably sized X-ray laser that could, among other things, provide super-high-resolution imaging, according to Henry Kapteyn, a CU-Boulder physics professor and fellow at JILA, a joint institute of CU-Boulder and the National Institute of Standards and Technology. Such a device also could be used by scientists to peer into a single cell or chemical reaction to gain a better understanding of the nanoworld.
CU-Boulder researchers have created a tabletop device that uses atoms in a gas to efficiently combine more than 5,000 low-energy mid-infrared laser photons to generate each high-energy X-ray photon.
CU-Boulder physics professors and JILA fellows Henry Kapteyn and Margaret Murnane stand next to one of their laser devices. (Photo by Glenn Asakawa/University of Colorado)
Science - Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers
High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo–electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.
The tabletop device -- an X-ray tube in the soft X-ray region -- produces a bright, directed beam of X-rays by ensuring that all of the atoms in a multi-atmosphere pressure gas emit X-rays, according to Kapteyn.
“As an added advantage, the X-rays emerge as very short bursts of light that can capture the fastest processes in our physical world, including imaging the motions of electrons,” Kapteyn said.
Laser beams, which are visible light, represent one of the best ways to concentrate energy and have been a huge benefit to society by enabling the Internet, DVD players, laser surgery and a host of other uses.
“However, the same revolution that happened for visible light sources that made it possible to create laser-like beams of light for widespread use instead of multidirectional light from a light bulb, is only now happening for X-rays,” Kapteyn said.
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