  Time-dependent nonlinear media, such as rapidly generated plasmas produced
via laser ionization of gases, can increase the energy of individual laser
photons and generate tunable high-order harmonic pulses. This phenomenon, known
as photon acceleration, has traditionally required extreme-intensity laser
pulses and macroscopic propagation lengths. Here, we report on a novel
nonlinear material$-$an ultrathin semiconductor metasurface$-$that exhibits
efficient photon acceleration at low intensities. We observe a signature
nonlinear manifestation of photon acceleration: third-harmonic generation of
near-infrared photons with tunable frequencies reaching up to
$\approx3.1\omega$. A simple time-dependent coupled-mode theory, found to be in
good agreement with experimental results, is utilized to predict a new path
towards nonlinear radiation sources that combine resonant upconversion with
broadband operation.
