Design and characterization of an athermal ENZ-ITO clad, air-core micro-ring resonator

We report our work on the fabrication and testing of an indium-tin-oxide (ITO) clad air-core micro-ring resonator (MRR). The ITO that makes up the cladding has been processed after deposition so that its real and imaginary permittivity are both in the epsilon-near-zero (ENZ) regime around 1550nm wavelength. This allows the MRR to index guide light in air which has a low thermo-optic coefficient and reduces the temperature dependent shifts of the resonances. The MRR cross-section was shown to support guided modes using numerical simulations and the structure was fabricated using two-photon polymerization 3D printing. We demonstrate the athermal behavior of this air-core MRR by coupling light into it with a tapered fiber and we measure a temperature dependent wavelength shift of 1 pm/°C.