A comb in the extreme ultraviolet

Abstract

Because the laser intensity required for this extreme nonlinear process to occur is high (more than 1013 watts per square centimetre), amplified femtosecond lasers at low repetition rates (10 Hz to 100 kHz) with 10-3 joules per pulse are typically used. [...] simply detecting HHG radiation is not sufficient to verify that comb structure exists. [...] both groups11,12 mixed third-harmonic radiation at a wavelength of 266 nm with an independently derived reference signal to observe beat signals that have a spectral linewidth of a few hertz, much less than the roughly 1012 Hz expected from the uncertainty principle for 50-fs pulses.