Point spread function engineering for spiral phase interferometric scattering microscopy enables robust 3D single-particle tracking

TL;DR

This paper introduces a novel spiral phase mask in interferometric scattering microscopy to achieve consistent signal-to-noise ratio and enhance 3D single-particle tracking accuracy, enabling ultrahigh-speed localization of nanoparticles.

Contribution

The study presents a new phase mask engineering technique that stabilizes SNR in iSCAT microscopy for robust 3D tracking, which was previously limited by signal oscillations.

Findings

Enhanced localizability in 3D tracking demonstrated

Achieved ultrahigh-speed (20,000 fps) nanoparticle tracking

Validated through numerical and experimental results

Abstract

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.