  Interferometric scattering (iSCAT) microscopy is currently among the most
powerful techniques available for achieving high-sensitivity single-particle
localization. This capability is realized through homodyne detection, where
interference with a reference wave offers the promise of exceptionally precise
three-dimensional (3D) localization. However, the practical application of
iSCAT to 3D tracking has to date been hampered by rapid oscillations in the
signal-to-noise ratio (SNR) as particles move along the axial direction. In
this study, we introduce a novel strategy based on back pupil plane
engineering, wherein we use a spiral phase mask to re-distribute the phase of
the scattered field of the particle uniformly across phase space, thus ensuring
consistent SNR as the particle moves throughout the focal volume. Our findings
demonstrate that this modified spiral phase iSCAT exhibits greatly enhanced
localizability characteristics. We substantiate our theoretical results with
numerical and experimental demonstrations, showcasing the practical application
of this approach for high-precision, ultrahigh-speed (20,000 frames per second)
3D tracking, and characterization of freely diffusing nanoparticles.
