  We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces
loaded by highly nonlinear multi-quantum well substrates, establishing a
platform to control the nonlinear wavefront at will based on giant localized
nonlinear effects. We apply this approach to design flat nonlinear metasurfaces
for efficient second-harmonic radiation, including beam steering, focusing, and
polarization manipulation. Our findings open a new direction for nonlinear
optics, in which phase matching issues are relaxed, and an unprecedented level
of local wavefront control is achieved over thin devices with giant nonlinear
responses.
