Few photons probe third-order nonlinear properties of nanomaterials in a plasmonic nanocavity

Quantification of nonlinear optical properties is required for nano-optical devices, but they are challenging to measure on a nanomaterial. Here, we harness enhanced optical fields inside a plasmonic nanocavity to mediate efficient nonlinear interactions with the nanomaterials. We performed reflection Z-scan technique at intensity levels of kWcm^2, reaching down to two photons per pulse, in contrast to GWcm^2 in 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.