Tailoring EUV high-harmonic spectra via nonlinear propagation in MgO crystals

High-harmonic generation (HHG) in solids offers a promising pathway toward compact extreme-ultraviolet (EUV) sources; however, the influence of nonlinear propagation effects in the crystal remains underexplored. Here, we study how driving-laser propagation in bulk MgO affects HHG spectra. By analyzing the 19th, 21st, and 23rd harmonics as a function of input intensity, focal position, crystal orientation, and thickness, we observe spectral broadening and reshaping that intensifies for thicker crystals and higher intensities. These changes correlate with broadening in the transmitted infrared (IR) spectrum, suggesting that propagation effects in the driving field strongly impact the observed EUV response. Generalized nonlinear Schrödinger equation simulations, accounting for Kerr nonlinearities, reproduce the overall IR broadening trends but not the detailed spectral features, implying additional effects, such as spatiotemporal coupling and plasma dynamics. The observed dependence on thickness, orientation, and input power, together with correlations between IR and harmonic spectra, suggests that the EUV spectral modifications mainly arise from nonlinear propagation rather than the HHG process itself. These findings emphasize the critical role of propagation effects in solid-state HHG and show that careful control of crystal parameters can enable tailored EUV emission for ultrafast science applications.