  All-optical modulation yields the promise of high-speed information
processing. In this frame, metasurfaces are rapidly gaining traction as
ultrathin multifunctional platforms for light management. Among the featured
functionalities, they enable light wavefront manipulation and, more recently,
demonstrated the ability to perform light-by-light manipulation through
nonlinear optical processes. Here, by employing a nonlinear periodic
metasurface, we demonstrate all-optical routing of telecom photons upconverted
to the visible range. This is achieved via the interference between two
frequency-degenerate upconversion processes, namely third-harmonic and
sum-frequency generation, stemming from the interaction of a pump pulse with
its frequency-doubled replica. By tuning the relative phase and polarization
between these two pump beams, and concurrently engineering the nonlinear
emission of the individual elements of the metasurfaces (meta-atoms) along with
its pitch, we route the upconverted signal among the diffraction orders of the
metasurface with a modulation efficiency up to 90%. Thanks to the phase control
and the ultrafast dynamics of the underlying nonlinear processes, free-space
all-optical routing could be potentially performed at rates close to the
employed optical frequencies divided by the quality factor of the optical
resonances at play. Our approach adds a further twist to optical
interferometry, which is a key-enabling technique in a wide range of
applications, such as homodyne detection, radar interferometry, LiDAR
technology, gravitational waves detection, and molecular photometry. In
particular, the nonlinear character of light upconversion combined with phase
sensitivity is extremely appealing for enhanced imaging and biosensing.
