Thickness-dependent properties of transparent conductive oxides for epsilon-near-zero applications

In recent years, epsilon-near-zero (ENZ) materials have attracted much attention due to their unique properties that can be tuned under electrical and optical signals. Furthermore, they allow for a strong enhancement of a nonlinearity close to the ENZ regime, which can have a direct impact on many fields ranging from telecommunications, sensing, quantum optics, to neuromorphic computing. Among the many materials belonging to the ENZ class, transparent conductive oxides (TCOs) are of particular interest because they are very well known to the community and possess very well understood electrical and optical properties. This work extends the direct control of the optical properties of transparent conductive oxides by tailoring the film thickness, which opens new possibilities for ENZ-enhanced photonic applications. It is shown that the thickness-dependent ENZ resonance in the TCO films is equivalent to the power-dependent ENZ resonance as for the same amount of power provided to the TCO film the electric field confinement in the thinner films is highly enhanced for TM polarized light, resulting in the higher electrons effective mass. Thus, the optical properties of TCO can be optically tuned in the ENZ regime, which opens possibilities for a new type of devices that can operate under all-optical switching mechanism.