（1.National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu 611731, China) （2.Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore)
Due to the existence of optical diffraction limit, the traditional optical microscope can not distinguish the tiny features smaller than the diffraction limit. And, its application scope is limited. Many researchers have conducted a lot of studies on how to break the diffraction limit and improve the efficiency of optical microscope. Among them, the optical glass microsphere can assist the traditional optical microscope to distinguish the tiny features smaller than the optical diffraction limit. In this work, high refractive index (n~2.2) BaTiO3 glass microspheres (BTG, diameter=28 μm) immersed inside liquids with different refractive indices are used to achieve nano-imaging. It provides a route to improve the property of optical microscope. A blue disk sample with a feature size down to 100 nm can be clearly observed under an incident light with wavelength of 405 nm. The nano-imaging quality and contrast can be optimized by tuning the microsphere-medium refractive index contrast. When the BTG microsphere is immersed inside the liquid with the refractive index of 1.46, the contrast of virtual image is the best. This nano-imaging of high refractive index microsphere can be widely applied in nano-photonics, biomedicine and fluid fields.