Reduced distortion in high-index microsphere imaging by partial immersion.

We experimentally demonstrate that pincushion distortion is obvious when using a BaTiO glass (BTG) microsphere fully immersed in ethanol or SU-8 2002 resist to image a Blu-ray disk with sub-diffraction features. The distortion is related to the layer thickness of the immersing medium. For a BTG microsphere partially immersed in the SU-8 resist where the SU-8 thickness is around 4/5 the diameter of the microsphere, its distortion decreases dramatically, but it can still clearly resolve the Blu-ray disk. For such a partially immersed microsphere, the calculated position of the photonic nanojet is outside the microsphere and close to the object, indicating the microsphere has a super-resolution imaging property, and the distortion simulated by ZEMAX is decreased.