  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.
