Optoplasmonic-Enhanced Imaging of Monolayer Polystyrene Nanoparticle Arrays by Barium Titanate Glass Microsphere-Assisted Microscopy: Implications for Nanoparticle Characterization

Low-contrast nanoparticles are difficult to detect optically because they weakly interact with photons. In this work, we have addressed this challenge by developing optoplasmonic hybrid structures composed of barium titanate glass (BTG) microspheres and Blu-ray disk (BD) substrates deposited with multilayer metal/dielectric coatings. A 300 nm and a 250 nm hexagonally close-packed polystyrene (PS) nanoparticle monolayer sample self-assembled on both a BD substrate and a metal one (glass slide deposited with the same multilayer coatings) can be discerned by BTG microsphere-assisted microscopy, and the contrast of the virtual images of the sample on the BD substrate is enhanced compared to that of the metal substrate. When the size of BTG microspheres is 20–30 μm, the contrast of the observed virtual images is the best. Moreover, the Talbot images of 300 nm PS nanoparticle arrays assembled on the BD substrate can also be experimentally observed. We can also distinguish 200 nm PS nanoparticles assembled into a one-dimensional stripe structure on the BD substrate. Numerical simulations reveal that the electric field among the gaps of PS nanoparticles and the contacting areas between the nanoparticles and a metal/BD substrate is enhanced, and the enhanced electric field of the sample placed on the BD substrate is stronger than that of the sample placed on the metal substrate. In addition, the enhanced electric field is the strongest when the BTG microsphere size is 20–30 μm in optoplamonic structures. The simulated results fit well with the experimental ones, indicating that the enhanced electric field improves the resolution of microsphere-assisted microscopy in imaging low-contrast PS nanoparticle samples.