LAUSR

DOI: 10.1002/adom.201900447 to their plasmonic counterparts, alldielectric nanoparticles do not suffer from nonradiative losses, and they allow novel functionalities. Owing to a strong localfield confinement in the volume of such nanostructures, as well as high stability under strong laser radiation, Mie-resonant nanoparticles are very attractive for nonlinear nanoscale photonics.[10–13] As an example, enhanced nonlinear optical harmonic generation has been predicted and demonstrated for a number of alldielectric systems, ranging from single resonant nanoparticles[14–18] and nanoparticle arrays[19–21] to more special structures supporting subradiant anapole modes[22–25] and bound states in the continuum.[26] When resonant nanoparticles are placed close to each other to form subwavelength clusters (also known as oligomers), novel effects such as the mode hybridization[27] and formation of electric and magnetic hot spots[28] emerge. These effects result from the near-field coupling between the constituent nanoparticles spawning an assortment of collective modes. The collective modes and their interference can result in pronounced asymmetric spectral features in the linear response of the nanoparticle oligomers, often associated with the Fano resonances.[29–34] Nonlinear response of oligomers and its link to the structure symmetry have been studied as well.[35–38] Although the study of nonlinear effects with isolated dielectric resonant nanoparticles received a considerable attention in nonlinear nanophotonics, their clusters and oligomers have not been utilized yet to tailor anisotropic properties of their nonlinear response. In this paper, we study all-dielectric nanoparticle clusters composed of one, three, or four identical silicon nanodisks and reveal a strongly anisotropic nonlinear response, while their linear scattering remains isotropic. The pronounced orientation angle-dependence of the generated third-harmonic (TH) signal discloses the intrinsic symmetries of the nanoparticle oligomers, which are otherwise not accessible through the linear scattering data. We provide numerical simulations of the nonlinear response, which confirm the observed experimental signatures of the C3 and C4 point-group symmetries of the oligomers. Notably, the symmetry analysis of the effective nonlinear susceptibility tensors reveals that in the C3 case only out-of-plane nonlinear dipoles are responsible for the effective anisotropy. We believe these results can provide a viable path 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.