Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase matching conditions requiring thickness in the order of hundreds of wavelengths and disadvantaged by the short optical interaction depth of nanometer-scale materials and weak photon-photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant SHG with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain-and electrical-tunable; a record >0.2 % absolute SHG conversion efficiency and an effective NL susceptibility 𝜒 !"" ($) in the order of 10 −9 m V -1 are demonstrated at average pump intensity of 8 kW/cm 2 . Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient NL optical components.

High–refractive index nanostructured dielectrics have the ability to locally enhance electromagnetic fields with low losses while presenting high third-order nonlinearities. In this work, we exploit these characteristics to achieve efficient ultrafast all-optical modulation in a crystalline gallium phosphide (GaP) nanoantenna through the optical Kerr effect (OKE) and two-photon absorption (TPA) in the visible/near-infrared range. We show that an individual GaP nanodisk can yield differential reflectivity modulations of up to ~40%, with characteristic modulation times between 14 and 66 fs, when probed at the anapole excitation (AE). Numerical simulations reveal that the AE represents a unique condition where both the OKE and TPA contribute with the same modulation sign, maximizing the response. These findings highly outperform previous reports on sub–100-fs all-optical switching from resonant nanoscale dielectrics, which have demonstrated modulation depths no larger than 0.5%, placing GaP nanoantennas as a promising choice for ultrafast all-optical modulation at the nanometer scale.

A system of amorphous gallium phosphide nanopatches is shown to be a flexible, cheap and efficient platform for ultrafast and nonlinear nanophotonics.