A theoretical and experimental study on optimal immersion method for microsphere-assisted microscopic imaging

Abstract With assistance of a microsphere immersed into a liquid medium, imaging resolution of microscope can exceed the Abbe diffraction limit, making the technique promising for super-resolution microscopic imaging. To achieve optimum assistance to the microscopic imaging, we have studied the microsphere immersion method in both theory and experiment. The impact of liquid immersion depth on the super-resolution imaging quality has been theoretically studied at first in both viewpoints of photonic nanojet and angular spectrum theory of backward propagation of waves. The results show that intensity, full width at half maximum and focal length of the focusing beam/spot are optimal at the semi-immersion state, resulting in considerable promotion of imaging contrast, resolution and magnification. To verify the theoretical results, experiments have been done in case that 10-μm-diameter SiO2 microspheres were semi-immersed into ethanol liquid. The results show that the semi-immersed SiO2 microspheres can assist, in the largest extent, the imaging with a contrast improved by 73.3% and a magnification increased by a fold of 1.9. In comparison with previous preliminary experimental study on super-resolution imaging of immersed microspheres, present paper performs a study in both theory and experiment, and proves that the semi-immersion state is optimum.