Point Spread\nFunction Engineering for Spiral Phase\nInterferometric Scattering Microscopy Enables Robust 3D Single-Particle\nTracking and Characterization

Interferometric scattering (iSCAT) microscopy is currently\namong\nthe most powerful techniques available for achieving high-sensitivity\nsingle-particle localization. This capability is realized through\nhomodyne detection, where interference with a reference wave offers\nthe promise of exceptionally precise three-dimensional (3D) localization.\nHowever, the practical application of iSCAT to 3D tracking has been\nhampered by rapid oscillations in the signal-to-noise ratio (SNR)\nas particles move along the axial direction. In this study, we introduce\na novel strategy based on back pupil plane engineering, wherein a\nspiral phase mask is used to redistribute the phase of the scattered\nfield of the particle uniformly across phase space, thus ensuring\nconsistent SNR as the particle moves throughout the focal volume.\nOur findings demonstrate that this modified spiral phase iSCAT exhibits\ngreatly enhanced localizability characteristics. Additionally, the\nuniform phase distribution enables reliable characterization of the\nparticle’s optical properties regardless of its position. We\nsubstantiate our theoretical results with numerical and experimental\ndemonstrations, showcasing the practical application of this approach\nfor high-precision, ultrahigh-speed (20,000 frames per second) 3D\ntracking and polarizability measurement of freely diffusing nanoparticles\nas small as 20 nm.