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Abstract Active nanostructured optical components show promise as potential building blocks for novel light‐based computing and data processing architectures. However, nanoscale all‐optical switches that have low activation powers and high‐contrast ultrafast switching have been elusive so far. Here, pump–probe measurements performed on amorphous‐Ge‐based micro‐resonator metasurfaces that exhibit strong resonant modes in the mid‐infrared are reported. Relative change is observed in transmittance of ΔT/T ≈ 1 with picosecond (down to τ ≈ 0.5 ps) free carrier relaxation rates, obtained with very low pump fluences of 50 μJ cm⁻². These observations are attributed to efficient free carrier promotion, affecting light transmittance via high quality‐factor optical resonances, followed by an increased electron–phonon scattering of free carriers due to the amorphous crystal structure of Ge. Full‐wave simulations based on a permittivity model that describes free‐carrier damping through crystal structure disorder find excellent agreement with the experimental data. These findings offer an efficient and robust platform for all‐optical switching at the nanoscale. 

Abstract The field of Mie‐resonant nanophotonics has attracted a lot of attention recently due to many promising applications in linear and nonlinear metaoptics. Optically induced magnetic resonances define novel characteristics of isolated high‐index dielectric nanoparticles and their oligomers. Here, the orientation‐dependent nonlinear frequency generation from dielectric oligomers with different symmetries, being all characterized by isotropic linear response, is demonstrated. The rotational dependence of the third‐harmonic signal emitted by the nanoparticle oligomers in accord with their point‐group symmetry (e.g., C3 or C4) is observed experimentally, while their linear scattering remains isotropic. The experimental data are in a good agreement with numerical simulations and the symmetry analysis of the nonlinear susceptibility tensor. The results open a new avenue for tailoring nonlinear properties of nanoscale structures.