Imaging properties of microsphere-assisted microscopy under transmitted illumination

Microsphere-assisted microscopy (MAM) is a label-free super-resolution imaging technique based on traditional optical microscopy, but the current research usually chooses reflected illumination (RI). In this work, the effects of microsphere size, refractive index, and immersion environment on the imaging properties of MAM in imaging a 300-nm-period grating sample under transmitted illumination (TI) are systematically investigated. The experimental results reveal that under TI, microspheres with a smaller size exhibit higher resolution, and microspheres with a higher refractive index show higher magnification in imaging. Under three immersion conditions (in the air, partial immersion, and full immersion in an SU-8 film), lower refractive index SiO₂ microspheres (partial immersion) and higher refractive index BaTiO₃ glass microspheres (full immersion) can obtain higher contrast and resolution in imaging. Despite the difference in illumination modes, the imaging properties of MAM under TI and RI are similar. Specifically, the relative refractive index between the microsphere and the surrounding environment affects the magnification, while the contrast is strongly influenced by the interfacial reflection and the relative refractive index between the microsphere and the surrounding medium. The magnification and resolution of MAM are also related to photonic nanojet (PNJ) of the microsphere. When the full width at half-maximum of the PNJ is narrow and the PNJ is close to the surface of the microsphere, it is beneficial for improving resolution. Our findings can help to reveal the underlying mechanisms of MAM and promote its practical applications.