  Nonlinear optical phenomena are at the heart of various technological domains
such as high-speed data transfer, optical logic applications, and emerging
fields such as non-reciprocal optics and photonic time crystal design. However,
conventional nonlinear materials exhibit inherent limitations in the
post-fabrication tailoring of their nonlinear optical properties. Achieving
real-time control over optical nonlinearities remains a challenge. In this
work, we demonstrate a method to switch third harmonic generation (THG), a
commonly occurring nonlinear optical response. Third harmonic generation
enhancements up to 50 times are demonstrated in zinc oxide films via the
photoexcited state generation and tunable electric field enhancement. More
interestingly, the enhanced third harmonic generation follows a quadratic
scaling with incident power, as opposed to the conventional cubic scaling,
which demonstrates a previously unreported mechanism of third harmonic
generation. The THG can also be suppressed by modulating the optical losses in
the film. This work shows that the photoexcitation of states can not only
enhance nonlinearities, but can create new processes for third harmonic
generation. Importantly, the proposed method enables real-time manipulation of
the nonlinear response of a medium. The process is switchable and reversible,
with the modulations occurring at picosecond timescale. Our study paves the way
to boost or suppress the nonlinearities of solid-state media, enabling robust,
switchable sources for nonlinear optical applications.
