Background-free interferometric dynamic light scattering

Heterodyne detection of dynamic light scattering (DLS) leverages the interference between light scattered by a sample and a reference field, and it is especially suited to amplify weak scattering signals. Unfortunately, the reference field used for amplification results in a strong baseline, which can mask small fluctuations and saturate the photodetector at higher gains. We introduce a novel DLS sensing architecture that integrates balanced detection into a dual-path configuration where coherence-gated (CG), common-path, fiber interferometers are introduced for interferometric amplification and background removal. In the proposed design of balanced CG-DLS, two independent reference fields arise intrinsically from the Fresnel reflection at the tip of an optical fiber in separate arms; one acts as local oscillator for the scattered field and the other is used for baseline removal, overall enabling the detection of interferometrically amplified scattering in a background-free manner. We provide comprehensive descriptions of the instrument’s assembly and its optimization using general-purpose optical components as well as the signal processing, characterization, and information retrieval through experiments with model colloidal suspensions. Ultimately, we demonstrate the capability of balanced CG-DLS to perform particle sizing at ultra-low concentrations, down to 1 × 10⁻⁷ vol/vol, where around ten nanoparticles are present in the measurement volume.