Spectral control of high order harmonics through non-linear propagation effects

High harmonic generation (HHG) in crystals has revealed a wealth of perspectives such as all-optical mapping of the electronic band structure, ultrafast quantum information, and the creation of all-solid-state attosecond sources. Significant efforts have been made to understand the microscopic aspects of HHG in crystals, whereas the macroscopic effects, such as non-linear propagation of the driving pulse and its impact on the HHG process, are often overlooked. In this work, we study macroscopic effects by comparing two materials with distinct optical properties, silicon (Si) and zinc oxide (ZnO). By scanning the focal position of 85 fs duration and 2.123 μm wavelength pulses inside the crystals, (Z-scan) we reveal spectral shifts in the generated harmonics. We interpret the overall blueshift of the emitted harmonic spectrum as an imprint of the spectral modulation of the driving field on the high harmonics. This process is supported with numerical simulations. This study demonstrates that through manipulation of the fundamental driving field through non-linear propagation effects, precise control of the emitted HHG spectrum in solids can be realized. This method could offer a robust way to tailor HHG spectra for a range of applications.