Super-resolution by microspheres and fibers - Myth or reality?

In 2011, super-resolution imaging by microspheres emerged as an extraordinary simple and broadband technique. The mechanism of imaging remains a mystery still today. The most likely candidates are the optical near fields, plasmonic resonances, photonic nanojets and whispering gallery modes. The critical problem in this area is the experimental definition of resolution which historically has been based on subjective criteria. In this work, we use the Houston resolution criterion and show that the image treatment techniques widely accepted in diffraction-limited optics can be extended in the super-resolution area, providing a phenomenological tool to determine the super-resolution values. Using this approach, we demonstrated imaging of Au and Al nanoplasmonic arrays by microspheres with the resolution ~λ/7. We also demonstrated super-resolution imaging by microfibers with resolution of ~λ/6 across the fiber and a millimetre-scale field-of-view along the fiber. Finally, we developed a technology of embedding high-index microspheres in PDMS films which can be translated along the sample to achieve a surface scanning functionality. Such thin films can become a standard component in super-resolution imaging of nanostructures and biological objects.