Photothermal Phase Synchronization on the Fourier Plane for Interferometric Scattering Microscopy

Abstract: We introduce and experimentally demonstrate the concept of phase synchronization on the Fourier plane for enhancing interferometric scattering microscopy. By employing a photothermal phase plate, we realize a synchronized phase difference between all scattering components and the reference beam on Fourier plane of high numerical-aperture microscopes, where the evanescent Fourier components and optical aberration normally produce highly inhomogeneous phase distribution. We show that the point spread function can be substantially improved, exhibiting a tighter focus with 50\% enhancement in interference contrast and a near-perfect circular symmetry. By synchronizing the phase difference to $\pi/2$, we demonstrate the background speckles exhibit an anti-symmetric dependence on axial defocus, enabling the effective suppression of the unavoidable background speckles and thus the detection of 10 nm particles immobilized on the substrate. The concept and technique of seamless dynamic phase control on the Fourier plane constitute a key asset for interferometric scattering microscopy.