Third-harmonic generation driven by nonlinear anisotropy in asymmetric silicon nanodisks

Nonlinear asymmetric generation of light in metasurfaces offers a powerful route to compact devices for optical isolation and directional control of harmonic emission without external magnetic bias. In this work, we experimentally demonstrate strongly asymmetric third-harmonic generation in a nonlinear metasurface, which is based on a single-material design and fabricated on a silicon-on-insulator platform (SOI). The metasurface consists of asymmetric silicon nanodisks whose broken mirror symmetry induces nonlinear anisotropy and a direction-dependent cubic nonlinear response. Inverting the pump propagation direction results in a 24-fold modulation of the third-harmonic signal. This pronounced asymmetry arises from the combined effect of the nanodisk structural asymmetry and interference within the finite-thick SiO2 layer of the SOI stack. Tuning the oxide-layer thickness enables control over the magnitude and spectral position of the asymmetric nonlinear response, making such metasurfaces attractive for applications in compact lasers and optical-signal processing devices.