Single-crystalline hexagonal ZnO microtube optical resonators

High quality ZnO microtubes with hexagonal cross sections, fabricated via an oxidation-sublimation process, are studied as novel optical resonators. Whispering gallery modes, Fabry-Pérot modes, and an additional set of modes with different polarizations are directly observed in the visible spectral range at room temperature by using the spatially resolved spectroscopic technique. The diameter and wall thickness of the microtube are crucial for light modulation. The experimental results are explained and fitted with a plane wave interference model and Cauchy dispersion formula for refractive indices.