科研之友
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摘要
The imaging properties of high-index immersed microsphere lenses in the diameter range of 5-300 mu m are experimentally studied. Our experimental results show that shifting the focal plane of the objective lens can result in different optical properties. When light beams generated from the objective lens are focused near the low surface of the microsphere lens, interference rings can be observed by a conventional optical microscopy, and its diameter and ring number increase with the diameter of microspheres, which can be similarly described by conventional wave optics. When the focal plane of the objective lens is further turned down about several microns, the Blu-ray disk with sub-wavelength structures can be discerned with a magnification up to 4.5 x and field of view up to 14 mu m. The image contrast and resolution decrease as the microsphere diameter increases. Calculations of the electric field distributions indicate that the "photonic nanojet" induced by the microsphere performs an important role in sub-wavelength imaging. The surrounding medium can be expected to improve the formation of both the sub-wavelength images and the interference rings. Our studies will help the understanding of the imaging mechanisms in microsphere lenses.
