High-Harmonic Generation Yields 4.4-nm X-Rays

A team of researchers at the University of Colorado at Boulder, the University of Sofia in Bulgaria, Lawrence Berkeley National Laboratory in Berkeley, Calif., and the University of California, Berkeley, has converted laser radiation to soft x-rays in the water window using a quasi-phase-matching process and high-harmonic generation in a gas-filled modulated waveguide. The group reported the development, which may have applications in biological imaging, in the Oct. 3 issue of Science.

The scientists demonstrated that using waveguides with an approximately sinusoidal inner diameter enables them to produce higher-energy x-rays than do straight waveguides. The technique, which they had reported previously, is believed to induce modulations in the phase and amplitude of the generated radiation that enable the process to avoid destructive interference.

In this case, they injected 22-fs pulses from a 1-kHz Ti:sapphire laser into a 2.5-cm-long modulated waveguide filled with neon to generate 4.4-nm x-rays, to which water is transparent but which carbon atoms absorb.