Isotope-Engineered Symmetry Breaking in Polar van der Walls Crystal

Polaritons—hybrid light–matter quasiparticles arising from resonant photon–phonon coupling—enable extreme optical anisotropy and nanoscale light manipulation in polar crystals. Traditionally, shear polaritons have been associated with intrinsically low-symmetry materials such as β-Ga₂O₃ or CdWO₄, where non-orthogonal vibrational modes give rise to asymmetric dispersion and directional energy flow. However, achieving tunable control over such symmetry-broken polaritonic responses remains challenging due to material constraints.

In this work, we demonstrate that shear-like polariton behavior can be induced even in a nominally symmetric van der Waals (vdW) crystal, α-MoO₃, through a combination of isotopic loss engineering and interlayer twist control. By systematically varying the isotopic composition of molybdenum and oxygen, we tailor the phonon damping while maintaining the underlying lattice symmetry. When two isotopically distinct α-MoO₃ layers are twisted with respect to one another, the resulting heterostructure exhibits pronounced asymmetry in polariton propagation—shear behavior emerging purely from controlled loss imbalance and twist geometry. Our analytical and numerical models establish a generalized framework to quantify shear polaritons independent of crystal thickness, revealing that the phenomenon depends solely on permittivity anisotropy and twist-induced phase mismatch, confirmed by the experimental results as well. This approach opens a new paradigm for realizing symmetry-breaking polaritonic phenomena in otherwise symmetric materials, offering a scalable and tunable pathway to engineer anisotropic light transport in vdW systems.