  If we stack up two layers of graphene while changing their respective
orientation by some twisting angle, we end up with a system that has striking
differences when compared to single-layer graphene. For a very specific value
of this twist angle, known as magic angle, twisted bilayer graphene displays a
unique phase diagram that cannot be found in other systems. Recently, high
harmonic generation spectroscopy has been successfully applied to elucidate the
electronic properties of quantum materials. The purpose of the present work is
to exploit the nonlinear optical response of magic-angle twisted bilayer
graphene to unveil its electronic properties. We show that the band structure
of magic-angle twisted bilayer graphene is imprinted onto its high-harmonic
spectrum. Specifically, we observe a drastic decrease of harmonic signal as we
approach the magic angle. Our results show that high harmonic generation can be
used as a spectroscopy tool for measuring the twist angle and also the
electronic properties of twisted bilayer graphene, paving the way for an
all-optical characterization of moir\'e materials.
