Unconventional magnification behaviour in microsphere-assisted microscopy

Microsphere-assisted microscopy is an original sub-diffraction-limit imaging technique allowing to reach a few hundred nanometres of lateral resolution in air only by placing a microbead in a classical optical microscope. This work aims to highlight the magnification process in microsphere-assisted microscopy which behaves differently from the case in optical microscopy. As a matter of fact, the lateral magnification of an optical microscope does not change according to focus plane positions. Experiments on the super-resolution imaging technique, performed in air through soda-lime-glass microspheres and attested by simulations, demonstrate a significant increase in the magnification factor along the microsphere imaging depth, i.e. at different object-focal-plane position of the objective. Moreover, it is shown that the magnification range, as well as its slope, depend on the size of the microsphere. Additionally, the influence of the spectral width of the illumination light source on the magnification range is highlighted.