Epsilon-near-zero Resonance Excitation in AZO nano-coated Optical Fiber Waveguide*

We experimentally demonstrate a novel optical waveguide design of side-polished fiber coated with a nano-film of aluminum-doped zinc oxide (AZO). The flat platform of the side-polished fiber is provided by etching/polishing out the cladding of a conventional single mode fiber apart from the doped core. AZO nano-films with epsilon-near-zero (ENZ) wavelength (real part of permittivity of the material crosses zero) at telecommunication wavelengths are fabricated onto the D-shaped platform by atomic layer deposition. Due to the evanescent field coupling between the optical fiber core mode and the modes supported by the AZO nano-layer, a highly confined ENZ mode in the AZO nano-film on the D-shaped platform could be excited. Furthermore, we investigate the polarization-dependent ENZ mode coupling properties with ambient refractive indices. Our results show the first experimental demonstration on the excitation of highly confined ENZ mode on optical fibers. These hybrid ENZ-optical fibers have potential applications for zero-index photonics, for instance, for studying enhanced nonlinear ENZ effects in fiber, quantum emission in ENZ media, and subwavelength mode in-fiber optical- and bio-sensing.