Fig. 7

Meta-optics for HHG. a Left: SEM image of the metal-sapphire nanostructures. Right: the measured EUV spectra with different pump-laser intensity [150]. b Left: SEM image of the plasmon-assisted metasurface composed of Au nanoantenna array on a silicon substrate. Right: the high-harmonic spectrum of the antenna array with the polarization parallel (red) and perpendicular (green) to the long axis of the antennas, and high-harmonic spectrum of the unpatterned Si substrate (black) [151]. c Left: SEM image of the silicon metasurface on a sapphire substrate that supports high-Q Fano resonance. Right: the high-harmonic spectra when the polarization of excitation pulses is parallel (red) and perpendicular (green) to the bar. The dotted black line is the emission from unpatterned Si [152]. d Left: schematic of HHG from a CdO-based metafilm with an ENZ response. Right: the spectrum extends from 3rd to 9th under p-polarized (red and blue) and s-polarized (orange) excitation [153]. e Left: illustration of nanostructured and ion-implanted semiconductors that generate tailored high-harmonic wavefronts. Middle: the far-field diffraction of the third (red) and fifth (blue) harmonics. Right: the third-harmonic emission pattern [154]