  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.
