A bright spot is observable in the center of Bull’s eye plasmonic pattern with a fluorescence microscope due to the plasmonic nanoantenna effect. In this effect, a propagating wave of surface plasmon resonance concentrates in the center. This study focused on the relationship between the center structure of Bull’s eye pattern and the nanoantenna effect in four fabricated Bull’s eye-type plasmonic chips with centers of different sizes (full- or half-pitch diameter) and shapes (convex or concave). The fluorescence intensity of the fluorescent nanoparticles adsorbed to these plasmonic chips was measured with an upright–inverted microscope to evaluate the plasmonic chip enhancement factor composed of the product of the excitation and emission enhancement and individual factors. When the emission enhancement factor was investigated under nonresonance excitation conditions, by the disappearance of a bright spot, excitation enhancement was found to contribute to the plasmonic nanoantenna effect. The concave Bull’s eye structure with a half-pitch diameter demonstrates the highest nanoantenna effect due to the formation of a larger constructive wave in the superposition of the diffraction wave of incident light under resonance conditions. In addition, the electromagnetic field intensity simulated by discrete dipole approximation agrees with the microscopy results. Overall, the results indicate that the plasmonic nanoantenna effect could be controlled depending on the resonance condition and center structure.

The fluorescence enhancement is induced by Grating-coupled surface plasmon resonance (GC-SPR) of a plasmonic chip in which a wavelength-sized periodic structure is coated with a thin metal film. In the fluorescence microscopic images of nanoparticles adsorbed on the plasmonic chip observed with transmitted light, the bright spot based on the nanoantenna effect was observed at the center of a Bull's eye pattern composed of concentric circles in a plasmonic chip. In this study, resonance wavelength was controlled by SiO2 film thickness deposited on a thin Ag film and the excitation and emission enhancement was individually evaluated by an upright-inverted microscope to clarify the cause of the nanoantenna effect. As the observation result, nanoantenna effect was larger in SiO2 80 nm than SiO2 20 nm.