Extension of high-harmonic generation cutoff in solids to 50 eV using MgO

High-harmonic generation (HHG) in solids driven by femtosecond lasers is a promising method for the compact production of coherent extreme ultraviolet (XUV) radiation but so far has been limited to photon energies below 40 eV. Here, we report the highest ever recorded photon energy for a harmonic in a solid sample, reaching 50 eV (31st harmonic) in 100-µm-thick MgO, using a 780 nm, 30 fs driving pulse. This is achieved through optimization of the spectrometer and detection efficiency, as well as an increase in emission efficiency enabled by a larger excitation area and the use of a multi-cycle pulse. We observe that the harmonic cutoff exhibits nontrivial behavior as a function of laser field strength, suggesting that an extension to our existing understanding of the generation process may be needed. This work demonstrates further the potential for compact XUV sources beyond 50 eV based on solid-state media.