In the process of high-harmonic generation (HHG), the frequency of an intense laser pulse is converted into new frequencies many multiples that of the fundamental.  In gas phase targets, these new frequency components can extend into the XUV or even soft X-ray regime. However, a significant challenge exists in using HHG to produce high brightness beams as the overall conversion efficiency is typically very low.
   
In this study, Rutledge and coauthors investigate the high-harmonic generation process in waveguides constructed from chirped periodically poled lithium niobate crystals, driven by a 3,000 nm, 100 MHz repetition rate laser, with pulse energies up to 12 nJ. Harmonics up to the 9^th order are observed, along with a remarkable total conversion efficiency of 23% into all harmonics. Further, the authors show that this originates not from direct high-harmonic generation, but rather from a series of low-order, perturbative harmonic generation steps. This cascaded process is made possible through the favorable guiding and phase matching conditions of the system. Overall, this work shows an innovative and powerful technique for the production of stable, high-harmonic pulse trains with high repetition rates.

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