Physics
An X-ray laser is a device that amplifies electromagnetic radiation in the X-ray portion of the spectrum, producing coherent, highly directed beams with wavelengths on the order of nanometers or less. Unlike conventional lasers that operate in the visible or infrared, X-ray lasers require extreme pumping energies and have no mirrors for typical cavity feedback, so most operate on a single-pass, amplified spontaneous emission principle. They enable imaging and probing of matter at atomic length scales and femtosecond time scales, with applications ranging from biology to materials science.
X-ray lasers produce coherent X-rays by creating a population inversion in a medium, typically a hot, dense plasma, and amplifying spontaneous emission in a single pass because no mirrors exist for X-rays at normal incidence. The gain medium is usually created by striking a solid target with a powerful optical laser, generating a plasma column with neon-like or nickel-like ions that have the necessary electronic level structure. The first laboratory demonstration in 1985 at Lawrence Livermore National Laboratory used a selenium plasma pumped by the Nova laser, achieving lasing at 20.6 nm.1 Since then, tabletop X-ray lasers using shorter-pulse drivers have been developed, but they remain limited to wavelengths above about 10 nm.
X-ray free-electron lasers (XFELs) are a distinct class of X-ray lasers that use relativistic electron beams passing through periodic magnetic structures (undulators) to generate coherent X-rays via self-amplified spontaneous emission (SASE). The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, operational since 2009, was the first hard X-ray FEL, producing pulses with wavelengths down to about 0.15 nm and durations of tens of femtoseconds.2 European XFEL in Germany and SACLA in Japan are other major facilities. XFELs have enabled single-particle imaging, time-resolved studies of chemical reactions, and the determination of protein structures from nanocrystals, as demonstrated in the first serial femtosecond crystallography experiments.
X-ray lasers are powerful tools for probing matter at atomic scales and ultrafast timescales. In biology, XFELs have been used to determine the structures of membrane proteins and to capture the dynamics of photosystem II during water splitting.3 In materials science, they allow studies of phase transitions, shock compression, and the behavior of matter under extreme conditions. X-ray lasers also enable X-ray interferometry and holography, which can image nanoscale structures with high resolution. In plasma physics, they are used to diagnose dense plasmas and to study warm dense matter, relevant to inertial confinement fusion research.
Beyond the headline facilities, X-ray lasers have a rich history of niche developments. The first X-ray laser concept was proposed in 1967 by John Madey, who later built the first FEL in the infrared.4 Plasma-based X-ray lasers have been demonstrated in many elements, including nickel-like ions of silver and cadmium, achieving wavelengths as short as 3.9 nm. A notable edge case is the 'water window' (2.3–4.4 nm), where X-ray lasers can image biological samples in their natural aqueous environment with high contrast. Additionally, X-ray lasers have been used to create 'X-ray movies' of laser-driven shock waves, and tabletop systems have been developed for laboratory-scale applications, though they remain less powerful than XFELs.
X-ray lasers are at the forefront of ultrafast science, enabling observations of atomic and molecular dynamics that were previously impossible.
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