  Transparent conductive oxides such as indium tin oxide (ITO) bear the
potential to deliver efficient all-optical functionality due to their
record-breaking optical nonlinearity at epsilon near zero (ENZ) wavelengths.
All-optical applications generally involve more than one beam, but the coherent
interaction between beams has not previously been discussed in materials with a
hot electron nonlinearity. Here we study the optical nonlinearity at ENZ in ITO
and show that spatial and temporal interference has important consequences in a
two beam geometry. Our pump-probe results reveal a polarization-dependent
transient that is explained by momentary diffraction of pump light into the
probe direction by a temperature grating produced by pump-probe interference.
We further show that this effect allows tailoring the nonlinearity by tuning
frequency or chirp. Having fine control over the strong and ultrafast ENZ
nonlinearity may enable applications in all-optical neural networks,
nanophotonics, and spectroscopy.
