At JILA in Boulder, Colo., researchers have obtained x-rays as short as 5 nm by high-order harmonic generation from ions in an argon-filled fiber. The group reported its findings in the Jan. 23 issue of Physical Review Letters.

 In the work, the researchers focused femtosecond pulses of 800-nm radiation from a 1-kHz Ti:sapphire laser into a 2.5-cm-long hollow-core fiber waveguide filled with argon at a pressure of 7 t. When they used high-intensity 1.3 × 10¹⁵-W/cm² laser pulses and a fiber with a periodically modulated diameter, the setup yielded x-rays with energies as high as 250 eV. At these laser intensities, the argon gas is completely ionized, but the ions still can generate very high harmonic soft x-ray beams.

Previously, harmonic generation at these high energies emerged from neutral atoms. The scientists believe that by extending their approach to higher laser intensities, it will be possible to generate coherent x-rays at higher -- perhaps kilovolt -- photon energies.