Enhanced Interferometric Imaging by Rotating Coherent Scattering Microscopy

Label-free microscopy on the nanoscale requires high-sensitivity imaging. The challenge of visualizing very small objects, such as nanoparticles, arises from their weak interaction with light. As a result, a combination of high signal-to-noise ratio imaging and background rejection is needed for detection and quantification. Here, we combine concepts from interferometric scattering (iSCAT) microscopy and rotating coherent scattering (ROCS) microscopy to optimize both background rejection and high-sensitivity imaging. Total internal reflection produces a background light intensity more than 2 orders of magnitude stronger than in iSCAT. Despite this, we successfully image 20 nm gold nanoparticles using our combined approach while achieving a signal-to-noise ratio (SNR) comparable to iSCAT at incident power densities as low as 0.04 kW/cm². We experimentally characterize the effect of different incident polarizations and achieve maximal optical contrast using s-polarized illumination. We further demonstrate that ROCS-based illumination at or near total internal reflection yields an approximate 4-fold contrast enhancement and 2-fold background suppression, producing substantially improved SNR compared to iSCAT for the same illumination power entering the microscope objective and integration time. We attribute this to the increased spatial resolution, enhanced incident power density, and rotational averaging of surface-generated speckle. These advantages highlight the potential to achieve and exceed the sensitivity levels attained by related interferometric imaging techniques.