Single-protein optical holography
Light scattering by nanoscale objects is a fundamental physical property defined by their scattering cross-section and thus polarisability. Over the past decade, a number of studies have demonstrated single molecule sensitivity, by imaging the interference between coherent scattering from the object of interest with a reference field. This approach has enabled mass measurements of single biomolecules in solution owing to the linear scaling of the image contrast with the molecular polarisability. Nevertheless, all implementations to date based on a common-path interferometer cannot separate and independently tune the reference and scattered light field, prohibiting access to the rich toolbox available to holographic imaging. Here, we demonstrate comparable sensitivity using a non-common path geometry based on a dark-field scattering microscope, similar to a Mach-Zehnder interferometer. We separate the scattering and reference light into four parallel, inherently phase stable detection channels, delivering a five orders of magnitude boost in sensitivity in terms of scattering cross-section over the state-of-the-art, demonstrating detection and mass measurement of single proteins below 100 kDa. Amplitude and phase measurement yields direct information on sample identity and the first experimental determination of the polarisability of single biomolecules.