Background-free 3D nanometric localisation and sub-nm asymmetry detection of single plasmonic nanoparticles by four-wave mixing interferometry with optical vortices

TL;DR

This paper introduces a novel four-wave mixing interferometry method using optical vortices for nanometric 3D localization and asymmetry detection of single plasmonic nanoparticles, surpassing traditional fluorescence-based techniques.

Contribution

The authors develop a background-free, single-point measurement technique that achieves high-precision 3D localization and asymmetry detection of non-fluorescing nanoparticles, enabling faster and more accurate tracking.

Findings

Achieves <20 nm lateral and 1 nm axial localization accuracy

Detects particle asymmetries as small as 0.5% aspect ratio

Works effectively within biological environments

Abstract

Single nanoparticle tracking using optical microscopy is a powerful technique with many applications in biology, chemistry and material sciences. Despite significant advances, localising objects with nanometric position accuracy in a scattering environment remains challenging. Applied methods to achieve contrast are dominantly fluorescence based, with fundamental limits in the emitted photon fluxes arising from the excited-state lifetime as well as photobleaching. Furthermore, every localisation method reported to date requires signal acquisition from multiple spatial points, with consequent speed limitations. Here, we show a new four-wave mixing interferometry technique, whereby the position of a single non-fluorescing gold nanoparticle is determined with better than 20 nm accuracy in plane and 1 nm axially from rapid single-point acquisition measurements by exploiting optical vortices. The technique is also uniquely sensitive to particle asymmetries of only 0.5% aspect ratio, corresponding to a single atomic layer of gold, as well as particle orientation, and the detection is background-free even inside biological cells. This method opens new ways of of unraveling single-particle trafficking within complex 3D architectures.