Microsphere-assisted microscopy

Light microscopy is one of the most powerful techniques for nondestructive real-time imaging of specimens at a resolution beyond the reach of human eyes. However, the spatial resolution of any conventional microscope is fundamentally limited by the diffraction of light waves at the lens aperture. Microsphere-assisted microscopy (MAM) has emerged in the past decade as an interestingly simple yet efficient method to improve imaging resolution. In MAM, a micrometer-scale dielectric sphere is placed in the immediate vicinity of the specimen to enhance the imaging resolution and magnification. MAM is highly versatile and can be combined with various systems including wide-field, confocal, and fluorescent microscopes to name a few. The exact resolution enhancement mechanism in MAM is not yet clearly understood and is under intense investigation; however, fundamentally, it can be linked to the increase in the system's effective numerical aperture and evanescence wave collection, and possibly to contributions from photonic nanojet effect, resonance, and coherent effects. Various claims have been made in the literature about the resolution gain in MAM, most of which are due to using arbitrary criteria for quantifying the resolution and possible contributions from specimen-specific parameters in imaging metallic nanostructures. In this Tutorial, we discuss the progress in MAM with special scrutiny of the imaging resolution.