  High harmonic generation (HHG) opens a window on the fundamental science of
strong-field light-mater interaction and serves as a key building block for
attosecond optics and metrology. Resonantly enhanced HHG from hot spots in
nanostructures is an attractive route to overcoming the well-known limitations
of gases and bulk solids. We demonstrate a nanoscale platform for highly
efficient HHG driven by strong mid-infrared laser pulses: an ultra-thin
resonant gallium phosphide (GaP) metasurface. The wide bandgap and the lack of
inversion symmetry of the GaP crystal enable the generation of even and odd
harmonics covering a wide range of photon energies between 1.3 and 3 eV with
minimal reabsorption. The resonantly enhanced conversion efficiency facilitates
single-shot measurements that avoid material damage and pave the way to
controllable transition between perturbative and non-perturbative regimes of
light-matter interactions at the nanoscale.
