  We report second-harmonic generation (SHG) from thick hexagonal boron nitride
(hBN) flakes with approximately 109-111 layers. The resulting effective
second-order susceptibility is similar to previously reported few-layer
experiments. This confirms that thick hBN flakes can serve as a platform for
nonlinear optics, which is useful because thick flakes are easy to exfoliate
while retaining a large flake size. We also show spatial second-harmonic maps
revealing that SHG remains a useful tool for the characterization of the layer
structure even in the case of a large number of layers.
