A non-van der Waals platform for deep-subwavelength twist-polaritonics based on β-Ga₂O₃ nanoflakes

Twist-polaritonics provides precise control of light-matter states through the stacking of atomically smooth, anisotropic layers, but has been restricted to the van der Waals (vdW) crystals. Non-vdW crystals, despite their symmetry-broken dielectric responses ideal for exotic polaritons, are challenging to prepare as suitable flakes due to their rigid 3D bonding networks, thus limiting the implementation of deep-subwavelength twist-polaritonics. We established a non-vdW polaritonic platform using ultrathin, single-crystalline β-Ga₂O₃ nanoflakes synthesized by exploiting its anisotropic bonding hierarchy on the quasi-layered (100)B plane. These flakes exhibited deep-subwavelength polariton confinement beyond λ/20. Moreover, their atomic-scale flatness enabled the assembly of twisted bilayers, in which we observed a topological transition of the polariton dispersion from hyperbolic to elliptical, directly controlled by the twist angle. This work positions β-Ga₂O₃ as a high-performance nanophotonic platform beyond the vdW family, while proposing that anisotropic bonding hierarchy provides a general strategy to unlock non-vdW twist-polaritonic functionality in a wide range of bulk crystals.