Few Photons Probe Third‐Order Nonlinear Properties of Nanomaterials in a Plasmonic Nanocavity

Abstract Quantification of nonlinear optical properties is important for nano‐optical devices, yet such measurements remain challenging at the single‐particle level. Here, enhanced optical fields are harnessed inside a plasmonic nanocavity to mediate efficient nonlinear interactions with an individual particle. Reflection Z‐scan technique was performed on individual nanocavities, reaching down to two photons per pulse, thus demonstrating a significantly higher efficiency beyond conventional methods. The few photons are sufficient to extract the nonlinear refractive index and nonlinear absorption coefficient of different nanomaterials, including perovskite and Au nano‐objects and a molecular monolayer. This work is of great interest for investigating nonlinear optical interactions on the nanoscale and characterizing nanomaterials, including fragile biomolecules.