Practical x-ray lasers would provide a revolutionary advance in medical imaging and advanced lithography, but several fundamental roadblocks have prevented their development.

Now scientists at the University of Colorado at Boulder have demonstrated that phase-matched high-order harmonic generation can produce coherent extreme-UV radiation, and they predict that the technique will be extended to produce a laserlike beam of coherent x-ray radiation from a tabletop device.

Generation of high-order harmonics in an ionized gas is not new, but what is new is the ability to phasematch the process. By using counter-propagating pulses to modulate the main pulse of fundamental radiation in the ionized gas, the scientists successfully demonstrated a variation of the quasi-phase-matching technique that has become common in crystals.

As described in the Feb. 25 online edition of Nature Physics, they have seen phase-matching enhancement factors of ∼300, generating fluxes of 10¹⁰ photons per second at 70 eV (15 nm). In future experiments, they hope to extend their demonstration to photon energies well into the soft x-ray region.

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