  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 novel 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 effects of the driving pulse inside the dense solid
media and its impact on the HHG process is 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, 2.123 $\mu$m 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 driving field spectral
modulation occurring during the propagation inside the crystal. This 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 realised.
This method could offer a robust way to tailor HHG spectra for a range of
spectroscopic applications.
