Abstract
<title>Abstract</title> <p>Fluorescence microscopy is the cornerstone of biological imaging. Enforced by super-resolution techniques circumventing the Abbe limit it can now resolve nanometer scale labelled structures. However, reliance on exogenous labels can perturb native biological function, while photobleaching and finite photon emission rates restrict measurement duration. These constraints motivate growing interest in label-free optical microscopy, which tracks nano-objects by probing their intrinsic optical properties. Interferometric scattering microscopy (iSCAT), first introduced by Sandoghdar and co-workers, has emerged as a powerful technique for detecting nanoscale objects with exceptional stability, decoupled from fluorophore photophysics. Nevertheless, conventional iSCAT performance is fundamentally confined by the balance between intrinsic reflectivity-scattering ratio and their relative phase. Optimization of the reflectivity-scattering ratio was previously addressed by introducing a fixed obstruction of the reflectivity. Here, we present an interferometric detection scheme that enables continuous independent tuning of both the field amplitudes and their relative phases, allowing the interferometer to be optimized for each measurement while simultaneously providing access to both the scattering amplitude and phase through quadrature detection and yielding significantly enhanced interferometric contrast. We demonstrate the power of this unified framework through direct, label-free detection of proteins with molecular masses as low as 5.7kDa.</p>