The efficiency with which light of one frequency is converted into a higher frequency in a nonlinear material increases when both frequencies travel through the material with the same phase velocity. This condition is known as phase matching. This technique is traditionally used in solids, but few solids are transparent at extreme ultraviolet and soft X-ray wavelengths. Although gases are transparent at these wavelengths, it has been difficult to achieve phase-matching in them. However, by adjusting the pressure of the gas in the fibre and/or the diameter of the fibre, the phase velocities can be made the same.

So far the Michigan group has generated X-ray pulses with energies in excess of 0.2 nanojoules and intensities greater than 1014 W cm-2. The team are confident that they can increase the X-ray output power and extend the technique to shorter wavelengths.