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

Abstract

Twist‑polaritonics provides precise control of light-matter states through the stacking of atomically smooth, anisotropic layers but has been restricted to van der Waals (vdW) crystals. Non‑vdW crystals, despite their symmetry‑broken dielectric responses ideal for exotic polaritons, are difficult to thin into suitable flakes due to their rigid 3D bonding networks, thus limiting the implementation of deep‑subwavelength twist‑polaritonics. Here, we establish 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 exhibit deep‑subwavelength polariton confinement beyond λ/20. Moreover, their atomic‑scale flatness enables the assembly of twisted bilayers, in which we observe 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.