  Transparent conducting oxides are highly doped semiconductors that exhibit
favourable characteristics when compared to metals, including reduced material
losses, tuneable electronic and optical properties, and enhanced damage
thresholds. Recently, the photonic community has renewed its attention towards
these materials, recognizing their remarkable nonlinear optical properties in
the near-infrared spectrum, a feature previously overlooked despite their
long-standing application in photovoltaics and touchscreens. The exceptionally
large and ultra-fast change of the refractive index, which can be optically
induced in these compounds, extends beyond the boundaries of conventional
perturbative analysis and makes this class of materials the closest
approximation to a time-varying system, and a unique playground for studying a
variety of novel phenomena within the domain of photon acceleration. Here we
report the spatio-spectral fission of an ultra-fast pulse trespassing a thin
film of aluminium zinc oxide with a non-stationary refractive index. By
applying phase conservation to this time-varying layer, our model can account
for both space and time refraction and explain in quantitative terms, the
spatial separation of both the spectrum and energy. Our findings represent an
example of extreme nonlinear phenomena on subwavelength propagation distances
and shed light on the nature of several nonlinear effects recently reported not
accounting for the full optical field distribution. Our work also provides new
important insights into transparent conducting oxides transient optical
properties which are critical for the ongoing research in photonic time
crystals, on-chip generation of nonclassical states of light, integrated
optical neural networks as well as ultra-fast beam steering and frequency
division multiplexing
