In condensed-matter physics, electronic Mott insulators have triggered considerable research due to their intricate relation with high-temperature superconductors. However, unlike atomic systems for which Mott phases were recently shown for both bosonic and fermionic species, in the solid-state the fingerprint of a Mott insulator implemented with bosons is yet to be found. Here we unveil such signature by exploring the Bose-Hubbard hamiltonian using semiconductor excitons confined in two-dimensional lattices. We emphasise the regime where on-site interactions are comparable to the energy separation between lattice confined states. We then observe that Mott phases are accessible, with at most two excitons uniformly filling lattice sites. The technology introduced here allows us to program on-demand the geometry of the lattice confining excitons. This versatility, combined with the long-range nature of dipolar interactions between excitons, provide a new route to explore many-body phases spontaneously breaking the lattice symmetry.
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