We propose a simple and effective method to improve the super-resolution imaging properties of a microsphere by combining plasmon coupling with the microsphere lens. Plasmon coupling can be excited when random silver nanoparticles are deposited on a self-assembled hexagonally closed-packed (hcp) SiO2 microsphere array. The electric field enhancement is found to be strongly influenced by plasmon coupling, and the enhanced near-field information of the samples can significantly affect the resolution quality. Using BaTiO3 glass (BTG) microsphere-assisted optical microscopy, we can clearly resolve a 200-nm-diameter hcp microsphere array when nanoparticle-nanoparticle plasmonic interactions and nanoparticle-thin film plasmonic interactions are excited. On the contrary, when there are no plasmon interactions excited, the 200-nm-diameter silver-coated hcp microsphere array is completely unresolved.
We add a 20-nm-thick SiO film between a fully immersed BaTiO3 glass (BTG) microsphere and a sample with subdiffraction features, and study its imaging properties. Compared with a fully immersed BTG microsphere without the SiO film, the magnification becomes larger, and the initial image position is farther away from the sample. The imaging rules no longer satisfy the imaging rules used in geometrical optics. We propose that the thin SiO film can enhance evanescent waves and the enhanced evanescent waves affect the magnification and the image position in BTG microsphere imaging. Our studies will help to understand the imaging rules of microspheres more comprehensively.
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