Label-Free Single-Molecule Imaging with Numerical-Aperture-Shaped Interferometric Scattering Microscopy Daniel Cole Gavin Young Alexander Weigel Aleksandar Sebesta Philipp Kukura 10.1021/acsphotonics.6b00912.s003 https://acs.figshare.com/articles/media/Label-Free_Single-Molecule_Imaging_with_Numerical-Aperture-Shaped_Interferometric_Scattering_Microscopy/4596457 Our ability to optically interrogate nanoscopic objects is controlled by the difference between their extinction cross sections and the diffraction-limited area to which light can be confined in the far field. We show that a partially transmissive spatial mask placed near the back focal plane of a high numerical aperture microscope objective enhances the extinction contrast of a scatterer near an interface by approximately <i>T</i><sup>–1/2</sup>, where <i>T</i> is the transmissivity of the mask. Numerical-aperture-based differentiation of background from scattered light represents a general approach to increasing extinction contrast and enables routine label-free imaging down to the single-molecule level. 2017-01-18 00:00:00 Numerical-Aperture-Shaped Interferometric Scattering Microscopy Label-Free Single-Molecule Imaging aperture microscope objective extinction contrast