  One challenge of using nonlinear optical phenomena for practical applications
is the need to perform phase-matching. Recently, epsilon-near-zero materials
have been shown to demonstrate strong optical nonlinearities, in addition to
their other unique properties. As suggested by their name, the permittivity of
the material is close to zero for a certain wavelength range. We demonstrate
that this small permittivity allows for efficient three-wave mixing
interactions to take place in indium-tin-oxide thin films without the need for
phase matching the pump and signal beams. The efficiency of the second-order
nonlinear interactions is characterized, and cascaded three-wave mixing is
demonstrated.
