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

TL;DR

This paper demonstrates that enhanced optical fields inside a plasmonic nanocavity enable measurement of third-order nonlinear optical properties of nanomaterials using very low photon numbers, facilitating nanoscale characterization.

Contribution

The study introduces a reflection Z-scan technique at extremely low light intensities, allowing efficient measurement of nonlinear properties of nanomaterials with only a few photons per pulse.

Findings

Nonlinear refractive index and absorption coefficients measured for various nanomaterials.

Effective nonlinear interactions achieved at photon levels of two photons per pulse.

Method enables characterization of fragile biomolecules and nanostructures.

Abstract

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.