Theoretical Foundations of Super-Resolution in Microspherical Nanoscopy

A review of recent developments in the emerging area of imaging through contact dielectric microspheres, which we term “microspherical nanoscopy”, is presented. The focus of this review is on the theoretical studies of the mechanisms of super-resolution imaging, which represents a frontier of modern nanophotonics and microscopy with many fascinating concepts proposed in recent years such as photonic nanojets, far-field superlens, localized plasmonic structured illumination microscopy (LPSIM) , and coupling of emission of nanoscale objects to localized surface plasmon resonances (LSPRs) in underlying short-period plasmonic metasurfaces. In the first part, we review the main results of the classical imaging theory based on rigorous solutions of the Maxwell equations for point-like sources. The theory accurately describes the near-to-far-field conversion and resonant excitation of whispering-gallery modes in such systems. The point spread functions are calculated and their width is compared with the experimental resolution in microspherical imaging. In the second part, we consider various combinations of microspherical nanoscopy with nanoplasmonics and plasmonic metasurfaces used for illumination or coupling with nanoscale objects. These schemes exemplify the most advanced ways of increasing the resolution far beyond the classical diffraction limit.