Optical anisotropy of Bi₂SeO₅ and near-field characterization of its waveguide modes

Anisotropic van der Waals (vdW) materials have attracted increasing attention in nanophotonics due to their unique optical properties. As a stable vdW material with a high dielectric constant (κ) and a wide bandgap, Bi₂SeO₅ shows significant potential for semiconductor applications, but its optical properties have not been extensively studied. In this work, we first measured the refractive index of Bi₂SeO₅ using an imaging spectroscopic ellipsometer and revealed its in-plane and out-of-plane anisotropy. Then, we studied the planar waveguide modes supported by Bi₂SeO₅ flakes using photoemission electron microscopy (PEEM), showing good agreement with theoretical results of anisotropic waveguides. Near-field modes excited by ring slits were also studied and were significantly different from the simulated modes of isotropic materials. These results confirm the anisotropy of Bi₂SeO₅, suggesting its potential applications in nanophotonic and photoelectronic devices.