There are many exciting experiments — such as tests of fundamental physical theories — that require high-precision measurements in the extreme ultraviolet (XUV) region of the radiation spectrum. However, the laser sources available for generating such wavelengths are not only complex and expensive, but also are characterized by poor spectral resolution. So the prospect of making precise spectroscopic measurements in the XUV seemed unlikely. But, in this week’s issue of Nature, Christoph Gohle and colleagues1 show how a ‘frequency comb’ could pave the way to a new generation of compact high-resolution XUV sources.

Laser pulses in the XUV spectral region can be generated by a technique known as high harmonic generation (HHG) — a process that converts, for example, a near-infrared laser pulse into an XUV pulse by focusing it into a nonlinear medium. This technique, however, delivers XUV pulses only at rather low repetition rates, typically a few kiloherz. This is because the input pulses have to be amplified before being directed through the target, which is a relatively slow procedure.

Gohle et al. have found a way to step up the pace by coupling the driving laser pulses to a high-quality optical cavity containing the nonlinear medium. Power that is not converted after a single pass through the target is reflected back and can contribute in a subsequent pass. This recycling procedure increases the overall conversion efficiency to a degree that the full repetition rate of laser can be used — typically a few hundred megahertz. At these high-repetition rates, the output is expected to show a comb structure with well-separated narrow lines, thus opening the door to precision frequency metrology in the XUV.

The authors claim that their new scheme will not only bring high-resolution XUV spectroscopy within reach, but also — owing to the excellent spatial coherence of the resulting light — enable further applications such as XUV interferometry, holography and lithography.