Tuning epsilon-near-zero wavelength of indium tin oxide film via annealing

Indium tin oxide (ITO) has a wide range of applications at its epsilon-near-zero (ENZ) wavelength due to its unique optical properties. Post-annealing is a simple way to tune the ENZ wavelength. We show that the ENZ wavelength of ITO films could be red-shifted over a wide range from 1200 nm to 1550 nm by thermal annealing in air for durations up to 130 min at 330 °. Optical transmittance and reflectance spectra were measured for these ITO samples, along with electron densities, to extract the Drude model parameters of plasma frequency, damping factor, electron mobility and effective mass. The results show that the changes in electron density and effective mass collectively cause the red-shift in the plasma frequency and ENZ wavelength. The oxygen uptake and crystallite size increase during the annealing in the air are the main reasons for the change in electronic properties. This versatile method of tuning the ITO's ENZ wavelength can expand the wavelength range for applications and adapt it to working at wavelengths of plasmonic devices in the telecommunication wavebands.