Extension of water-window harmonic cutoff by laser defocusing-assisted phase matching.

We extend a recently demonstrated scheme [Optica4, 976 (2017)OPTIC82334-253610.1364/OPTICA.4.000976] to overcome the limit of conventional harmonic cutoff for different pulse durations, laser wavelengths, and gas targets. By tuning the truncation of long wavelength lasers, we show that the defocusing-assisted phase matching (DAPM) can be achieved in a tightly focused beam and highly ionized short gas cell, and can be used to effectively extend the harmonic cutoff energy and optimize its yield. An analysis of phase matching reveals that at longer wavelengths, greater cutoff extension to the water window region is achieved because of the larger harmonic intrinsic phase (proportional to the cube of laser wavelength), and because DAPM works at relatively higher laser intensities using a Ne target. This scheme provides a promising method for efficiently generating intense attosecond light sources in the extreme ultraviolet to x-rays.