  Second-order nonlinear optical response allows to detect different properties
of the system associated with the inversion symmetry breaking. Here, we use a
second harmonic generation effect to investigate the alignment of a
graphene/hexagonal Boron Nitride heterostructure. To achieve that, we activate
a commensurate-incommensurate phase transition by a thermal annealing of the
sample. We find that this structural change in the system can be directly
observed through a strong modification of a nonlinear optical signal. This
result reveals the potential of a second harmonic generation technique for
probing structural properties of layered systems.
