  We realize and investigate a nonlinear metasurface taking advantage of
intersubband transitions in ultranarrow GaN/AlN multi-quantum well
heterostructures. Owing to huge band offsets, the structures offer resonant
transitions in the telecom window around 1.55 $\mu$m. These heterostructures
are functionalized with an array of plasmonic antennas featuring
cross-polarized resonances at these near-infrared wavelengths and their second
harmonic. This kind of nonlinear metasurface allows for substantial
second-harmonic generation at normal incidence which is completely absent for
an antenna array without the multi-quantum well structure underneath. While the
second harmonic is originally radiated only into the plane of the quantum
wells, a proper geometrical arrangement of the plasmonic elements permits to
redirect the second-harmonic light to free-space radiation, which is emitted
perpendicular to the surface.
