Microsphere-mediated optical contrast tuning for designing imaging systems with adjustable resolution gain

Upon illumination, a dielectric microsphere (μS) can generate a photonic nanojet (PNJ), which plays a role in the super-resolution imaging of a sample placed in the μS's immediate proximity. Recent microscopy implementations pioneered this concept but, despite the experimental characterization and theoretical modeling of the PNJ, the key physical factors that enable optimization of such imaging systems are still debated. Here, we systematically analyzed the parameters that govern the resolution increase in the case of large-diameter (>20 µm) μS-assisted incoherent microscopy by studying both the illumination and the detection light paths. We determined the enhanced-resolution zone created by the μS, in which the detection system has a net resolution gain that we calculated theoretically and subsequently confirmed experimentally. Our results quantitatively describe the resolution enhancement mediated by the optical contrast between the μS and its surrounding medium, and provide concrete means for designing μS-enhanced imaging systems for several application requirements.