Exploring nonlinear nanophotonics with laser scanning microscopy (Invited Paper)

Nonlinear optical interactions are of fundamental significance for advanced photonic applications, but often the nonlinearity magnitude is insufficient. In the past decade, we apply laser scanning confocal microscopy, which is a routine tool in biology but unusual with nanomaterials, to inspect single metallic and semiconductor nanostructures. Via the combination of Mie resonance and coupled photothermal/thermo-optic effects, we discovered 1000- to 100000-fold enhanced nonlinear optical indices over bulk materials. The potential applications include all-optical switch and label-free super-resolution microscopy, based on suppression of scattering, saturation (sub-linearity) and reverse saturation (super-linearity). More recently, we uncovered novel light-matter interactions, such as optical bistability in nano-silicon with record-low Q-factor and footprint, as well as displacement resonance. The latter features that linear scattering efficiency is maximal when the focus is misaligned, thus showcasing a significant reduction of nonlinear response threshold, sign flip in all-optical switching, and spatial resolution enhancement.