  Interferometric scattering (iSCAT) microscopy has undergone significant
development in recent years. It is a promising technique for imaging and
tracking nanoscopic label-free objects with nanometer localization precision.
The current iSCAT-based photometry technique allows quantitative estimation for
the size of a nanoparticle by measuring iSCAT contrast and has been
successfully applied to nano-objects smaller than the Rayleigh scattering
limit. Here we provide an alternative method that overcomes such size
limitations. We take into account the axial variation of iSCAT contrast and
utilize a vectorial point spread function model to uncover the position of a
scattering dipole and, consequently, the size of the scatterer, which is not
limited to the Rayleigh limit. We found that our technique accurately measures
the size of spherical dielectric nanoparticles in a purely optical and
non-contact way. We also tested fluorescent nanodiamonds (fND) and obtained a
reasonable estimate for the size of fND particles. Together with fluorescence
measurement from fND, we observed a correlation between the fluorescent signal
and the size of fND. Our results showed that the axial pattern of iSCAT
contrast provides sufficient information for the size of spherical particles.
Our method enables us to measure the size of nanoparticles from tens of
nanometers and beyond the Rayleigh limit with nanometer precision, making a
versatile all-optical nanometric technique.
