Abstract
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.
Highlights
Materials where the permittivity, ε, approaches zero (ENZ) have attracted substantial interest in the academic community due to numerous interesting phenomena predicted to occur within this regime
We studied the permittivity and microstructure of several wavelength-thick indium tin oxide (ITO) films annealed at temperatures ranging from 250 C to 400 C
In order to create samples with different plasma frequencies, we annealed samples diced from this film stack for 10 minutes at temperatures ranging from 250 C to 400 C in N2 atmosphere
Summary
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. As we show in the Supplementary Material, if the imaginary part of permittivity is less than 0.6, the waveguide simulation indicated that the light will be index guided in the air-core and have relatively low propagation loss This loss could be reduced in a disk resonator design since it has one less ITO sidewall. In the samples annealed at 350 C or greater, the permittivity depth profile showed a sharp transition mid-way through the film; at a wavelength of 1550 nm, the bottom portion of the film had negative permittivity, whereas the top portion had positive permittivity, and a thin layer in between had permittivity near zero These samples may be suitable for specialized devices that take advantage of this steep transition. The result provided a method to determine the epsilon profile in an anisotropic ITO film
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