Interfacial epitaxy of single-crystalline Al₂(MoO₄)₃ flakes for anisotropic phonon polaritons

The unique phonon characteristics of metal molybdates give rise to intriguing anisotropies in thermal conductivity and mechanical elasticity. However, direct nanoscale imaging of these phonon modes and their interactions with light remains challenging, as these materials have so far been available only as isomorphic polycrystalline films. Here, we develop a cost-effective interfacial epitaxy strategy for synthesizing thin-layer aluminum molybdate (Al₂(MoO₄)₃, AMO) single crystals. This is achieved by confining a molybdenum foil between two sapphire substrates, which directs uniform nucleation and epitaxial growth of high-quality monoclinic AMO. The strong phonon-photon coupling fosters hyperbolic phonon polaritons (PhPs), whose propagation can be tailored by excitation frequency and crystal symmetry. Crucially, biaxial epitaxial strain induces anisotropic edge-launched PhPs. This work establishes a general approach for synthesizing high-quality ternary metal molybdates, providing a versatile platform for polaritonics and nanophotonics.