All-dielectric χ(2) metasurfaces: recent progress

Optical metasurfaces, i.e. arrays of nanoantennas with sub-wavelength size and separation, enable the manipulation of light-matter interactions in miniaturized optical components with no classical counterparts. Six decades after the first observation of the second harmonic generation (SHG) in bulk crystals, these devices are expected to break new ground in the field of nonlinear optics, shifting the focus from the phase matching approach achieved within long propagation distances to that of near-field resonances interplay in leaky nanocavities. Here we review the recent progress in SHG with all-dielectric metasurfaces. We discuss the most used technological platforms which underpinned such advances and analyze different SHG control approaches. We finally compare their performances with other well-established technologies, with the hope to delineate the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities.