  In twisted layered materials (t-LMs), an inter-layer rotation can break
inversion symmetry and create an interfacial array of staggered out-of-plane
polarization due to AB/BA stacking registries. This symmetry breaking can also
trigger the formation of edge in-plane polarizations localized along the
perimeter of AB/BA regions (i.e., saddle point domains). However, a
comprehensive experimental investigation of these features is still lacking.
Here, we use piezo force microscopy to probe the electromechanical behavior of
twisted hexagonal boron nitride (t-hBN). For a parallel stacking alignment of
t-hBN, we reveal very narrow (width ~ 20 nm) saddle point polarizations, which
we also measure in the anti-parallel configuration. These localized
polarizations can still be found on a multiply-stacked t-hBN structure,
determining the formation of a double moir\'e. We also visualize a t-hBN
moir\'e superlattice in the topography maps with atomic force microscopy,
related to the strain accumulated at the saddle point domains. Our findings
imply that polarizations in t-hBN do not only point in the out-of-plane
direction, but also show an in-plane component, giving rise to a much more
complex 3D polarization field.
