Beam shaping and frequency conversion in nonlinear all-dielectric metasurfaces

We demonstrate ultrafast tunable, near-infrared to ultraviolet frequency conversion in a chalcogenide glass metasurface based on Mie resonances and quasi-bound states in the continuum resonances, enabled by a phase-locking mechanism between the pump and the inhomogeneous portion of the third harmonic signal. Through phase locking, the pump pulse and the inhomogeneous harmonic component can co-propagate, resulting in the acquisition of the same refractive index and absorption coefficient as the pump. If this process occurs within a cavity, efficient frequency conversion can take place, even in the presence of strong material absorption at the wavelengths of the harmonics. As for all nonlinear processes, a resonant condition at the pump field boosts the nonlinear interactions. Finally, we experimentally show the simultaneous generation of phase-locked structured light beams, including optical vortices and Hopf-links at fundamental and tripled frequencies in all-dielectric nonlinear optical metasurfaces even though the tripled frequency corresponds to the region of high absorption of the dielectric material.