Influence of refractive index contrast between microsphere lens and immersion medium on imaging resolution

A microsphere lens could couple the near-field evanescent wave and convert it into propagation wave, which helps microscope to break the diffraction limit and improve its imaging resolution. However, the imaging conditions for microsphere lens is still not clear, and also the conversion process between evanescent wave and propagation wave. Here, we deduced the imaging conditions of microsphere lens through the refractive index contrast between microsphere and immersion medium, and simulated the coupling and scattering process of evanescent wave by microsphere lens. At last, we demonstrated the microsphere assisted super-resolution imaging under different immersion conditions. The experimental results verified that, only within the range predicted by theoretical deduction, the evanescent wave could be converted and the imaging resolution of objective lens could be improved. Our work predicts the super-resolution imaging condition of microsphere lens, and explains its mechanism with evanescent wave conversion, which could guild the design of microsphere lens under different immersion conditions and extend its application in various fields.