We design and fabricate a metasurface composed of gold cut-disk resonators that exhibits a strong coherent nonlinear response. We experimentally demonstrate all-optical modulation of both second-and third-harmonic signals on a subpicosecond time scale. Pump−probe experiments and numerical models show that the observed effects are due to the ultrafast response of the electronic excitations in the metal under external illumination. These effects pave the way for the development of novel active nonlinear metasurfaces with controllable and switchable coherent nonlinear response.

The spin angular momentum of light plays an important role in nonlinear interactions in optical systems with rotational symmetries. Here, the existence of the nonlinear geometric Berry phase is demonstrated in the four‐wave mixing process and applied to spin‐controlled nonlinear light generation from plasmonic metasurfaces. The polarization state of four‐wave mixing from the ultrathin metasurfaces, comprising gold meta‐atoms with four‐fold rotational symmetry, can be controlled by manipulating the spin of the excitation beams. The mutual orientation of the meta‐atoms in the metasurface influences the intensity of four‐wave mixing via the geometric phase effects. These findings provide novel solutions for designing metasurfaces for spin‐controlled nonlinear optical processes with inbuilt all‐optical switching.

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. Here, we demonstrate a nanoscale platform for highly efficient HHG driven by intense 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 study the controllable transition between perturbative and non-perturbative regimes of light-matter interactions at the nanoscale.

Nonlinear microscopy is widely used to characterize thick, optically heterogeneous biological samples. While quantitative image analysis requires accurately describing the contrast mechanisms at play, the majority of established numerical models neglect the influence of field distortion caused by sample heterogeneity near focus. In this work, we show experimentally and numerically that finite-difference time-domain (FDTD) methods are applicable to model focused fields interactions in the presence of heterogeneities, typical of nonlinear microscopy. We analyze the ubiquitous geometry of a vertical interface between index-mismatched media (water, glass, and lipids) and consider the cases of two-photon-excited fluorescence (2PEF), third-harmonic generation (THG) and polarized THG contrasts. We show that FDTD simulations can accurately reproduce experimental images obtained on model samples and in live adult zebrafish, in contrast with previous models neglecting field distortions caused by index mismatch at the micrometer scale. Accounting for these effects appears to be particularly critical when interpreting coherent and polarization-resolved microscopy data.

Fourier transform infrared (FTIR) spectroscopy is a popular technique for the analysis of biological samples, yet its application in characterizing live cells is limited due to the strong attenuation of...