Label-free incoherent super-resolution optical microscopy

Abstract The photo-kinetics of fluorescent molecules has enabled the circumvention of far-field optical diffraction-limit. Despite its enormous potential, the necessity to label the sample may adversely influence the delicate biology under investigation. Thus, continued development efforts are needed to surpass the far-field label-free diffraction barrier. The coherence of the detected light in label-free mode hinders the application of existing super-resolution methods based on incoherent fluorescence imaging. In this article, we present the physics and propose a methodology to circumvent this challenge by exploiting the photoluminescence of silicon nitride waveguides for near-field illumination of unlabeled samples. The technique is abbreviated EPSLON, Evanescently decaying Photoluminescence Scattering enables Label-free Optical Nanoscopy. We demonstrate that such an illumination has properties that mimics the photo-kinetics of nano-sized fluorescent molecules. This allows to develop a label-free incoherent system that is linear in intensity, and stable with time thereby permitting the application of techniques like structured illumination microscopy (SIM) and intensity-fluctuation based optical nanoscopy (IFON) in label-free mode to circumvent the diffraction limit. We experimentally demonstrate label-free super-resolution imaging of nanobeads (polystyrene and gold), extra-cellular vesicles and human placenta tissue. We believe EPSLON is a step forward within the nascent field of label-free super-resolution microscopy that holds the key to investigate delicate biological systems in its natural state without the need for exogenous labels.