  Moir\'e superlattices formed by vertically stacking van der Waals layers host
a rich variety of correlated electronic phases and function as novel photonic
materials. The moir\'e potential of the superlattice, however, is fixed by the
interlayer coupling of the stacked functional layers (e.g. graphene) and
dependent on carrier types (e.g. electrons or holes) and valleys (e.g. {\Gamma}
vs. K). In contrast, twisted hexagonal boron nitride (hBN) layers are predicted
to impose a periodic electrostatic potential that may be used to engineer the
properties of an adjacent functional thin layer. Here, we show that this
potential is described by a simple theory of electric polarization originating
from the interfacial charge redistribution, validated by its dependence on
supercell sizes and distance from the twisted interfaces. We demonstrate that
the potential depth and profile can be further controlled by assembling a
double moir\'e structure. When the twist angles are similar at the two
interfaces, the potential is deepened by adding the potential from the two
twisted interfaces, reaching ~ 400 meV. When the twist angles are dissimilar at
the two interfaces, multi-level polarization states are observed. As an example
of controlling a functional layer, we demonstrate how the electrostatic
potential from a twisted hBN substrate impedes exciton diffusion in a
semiconductor monolayer. These findings suggest exciting opportunities for
engineering properties of an adjacent functional layer using the surface
potential of a twisted hBN substrate.
