Tunable optical anisotropy in epitaxial phase-change VO thin films.

We theoretically and experimentally demonstrate a strong and tunable optical anisotropy in epitaxially-grown VO thin films. Using a combination of temperature-dependent X-ray diffraction, spectroscopic ellipsometry measurements and first-principle calculations, we reveal that these VO thin films present an ultra-large birefringence (Δ > 0.9). Furthermore, leveraging the insulator-to-metal transition of VO, we demonstrate a dynamic reconfiguration of optical properties from birefringent to hyperbolic, which are two distinctive regimes of anisotropy. Such a naturally birefringent and dynamically switchable platform paves the way for multi-functional devices exploiting tunable anisotropy and hyperbolic dispersion.