Super-resolution imaging of plasmonic nanostructures by microsphere-assisted microscopy

We fabricate both triangularly and circularly shaped Au, Ag, and Cr nanoparticle arrays and observe the imaging properties of these plasmonic nanostructures by BaTiO₃ glass (BTG) microsphere-assisted microscopy. We experimentally find that the resolution of triangularly shaped Ag nanoparticle arrays is higher than that of Au and Cr ones, and a gap resolution of ∼λ/7.7 is demonstrated for the circularly shaped Au, Ag, and Cr nanostructures. Numerical simulations show that when a fully immersed BTG microsphere is dispersed on the surface of a plasmonic nanostructure sample, an enhanced electric field is generated in the vicinity of the sample, especially at the gap of the microsphere and the sample, due to the focusing effect of the microsphere and the excitation of localized surface plasmon resonance in the plasmonic nanostructure. The enhanced electric field in Ag nanostructures is significantly stronger than that in Au and Cr ones. Besides, the microsphere collects, amplifies, and propagates the enhanced near-field information to the far field, resulting in the improvement of imaging resolution.