Property Variation in Wavelength-thick Epsilon-Near-Zero ITO Metafilm for Near IR Photonic Devices

Thin indium tin oxide (ITO) films have been used as a medium to investigate epsilon-near-zero (ENZ) behavior for unconventional tailoring and manipulation of the light-matter interaction. However, the ENZ wavelength regime has not been studied carefully for ITO films with thicknesses larger than the wavelength. Thick ENZ ITO film would enable the development of a new family of ENZ-based opto-electronic devices that take full advantage of the ENZ behavior. Here, we demonstrated wavelength-thick ITO films reaching the ENZ regime around a wavelength of 1550 nm, which permit the design of such devices operating in the common optical telecommunications wavelength band. We discovered that the permittivity of the film was non-uniform with respect to the growth direction. In particular, after annealing at a sufficiently high temperature, the real part of the permittivity showed a step change from negative to positive value, crossing zero permittivity near the middle of the film. Subsequently, we conducted comprehensive microanalysis with X-ray diffraction, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) to investigate the correlation of the permittivity variation with variations in the ITO crystallite morphology and relative concentrations of different atom species. The result of this study will allow us to design a new family of opto-electronic devices where ITO can be used as the cladding that guides light within an air-core waveguide to provide a new platform to explore ENZ properties such as environment insensitivity, super-coupling, and surface avoidance. We have also provided a comprehensive method to determine the permittivity in a non-uniform ENZ material by using an advanced physical model to the fit experimental data.