Superresolution Imaging With Contact Microspheres: Importance Of Numerical Aperture

Optical nanoscopy with contact microlenses emerged as a simple method which allows overcoming the classical diffraction limit represented by the Abbe's formula. The mechanisms of such imaging are debated in the literature including photonic nanojets, plasmon-polaritons, localized surface plasmon resonances and coherent modal excitations in metallic objects, as well as optical resonances in microspherical lenses. The system containing metallic nano-object coupled to a dielectric microsphere can be considered as a directional antenna. This means that the dependence of the resolution on the numerical aperture (NA) of the microscope objective can be different from that predicted by the Abbe formula. In this work, we tested this hypothesis experimentally and showed that reduction of NA from 0.95 to 0.6 leads to a decrease in resolution from similar to lambda/7 to similar to lambda/4, where lambda is the illumination wavelength.