Ultrafast Permittivity Engineering Enables Broadband Enhancement and Spatial Emission Control of Harmonic Generation in ZnO.

Moderate efficiencies of nonlinear optical processes can be one of the challenges limiting even more widespread applications. Here we demonstrate a broadband and giant enhancement of nonlinear processes in ZnO through ultrafast permittivity engineering. A remarkable enhancement of the second and third harmonic generation of up to 2 orders of magnitude can be observed over a broadband range of driving wavelengths. Moreover, this nonlinearity enhancement is reversible with a recovery time of ∼120 fs. Additional experiments and simulations confirm that the observed enhancement originates from a permittivity change induced by the photocarrier population. Our results provide the opportunity to actively customize materials with a larger nonlinearity for nanophotonics on ultrafast time scales over broadband wavelength ranges. Utilizing this finding, we also demonstrate a relevant application, where a transient wave-guiding effect is induced by a donut-shaped photocarrier-excitation pulse, which both reduces the width of the spatial profile of harmonic emission below the diffraction limit and simultaneously increases its central emission strength.