Abstract
A 'comb' of photons at evenly spaced frequencies in the extreme ultraviolet has been generated. It will allow a more precise search for variation in the fine-structure constant, which sets the strength of the electromagnetic force. See Letter p.68 Laser-based optical frequency combs, so called because they emit evenly spaced spectral lines, are used in precision spectroscopy and other applications requiring accurate measurements, such as atomic clocks. Efforts to extend this capability to shorter wavelengths in the extreme ultraviolet — which would open up exciting new applications, including searches for variation in fundamental constants — have lacked sufficient power for the purpose until now. Jun Ye and co-workers demonstrate a new approach, using a high-power, high-repetition pulsed infrared laser coupled into an optical cavity, to produce an improved extreme UV comb. In a first precision spectroscopy demonstration, they use direct frequency comb spectroscopy to determine argon and neon atomic transitions with ultra-high precision.
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