Nanoantenna Effect at the Center of the Bull’s Eye Pattern by Controlling the Refractive Indices and Layer Thicknesses of Dielectric Media on a Silver Surface

Abstract

The light that is illuminated on a silver-film-coating substrate with a periodic structure, i.e., a plasmonic chip, can couple to plasmon polaritons and enhance the electric field on the surface of the chip. Fluorescent molecules fixed to the plasmonic periodic pattern are excited by an enhanced electric field, enhancing their fluorescence. Particularly, a bright fluorescence point appears at the center of a concentric circle pattern called a Bull’s eye pattern. This nanoantenna effect has been studied in various types of concentric circles and has been comprehended by a constructive wave superposed with diffraction light on the grooves of a plasmonic pattern. Here, the antenna effect of fluorescent nanoparticles immobilized on the chip surface was studied based on the controlling factors of the surface plasmon resonance wavelengths, such as the pitch of a pattern, the refractive index, and the layer thickness of the dielectric media on the silver film, and it was improved by their factors. The pitches of the plasmonic patterns were set at 400 and 480 nm, and the nanoantenna rate (Ap) of the 480 nm pitch was higher than that of the 400 nm pitch when a 20 nm thick SiO2 layer was used. By changing the refractive index of the dielectric media on the silver film from 1.45 (silica layer) to 2.10 (zinc oxide layer), Ap increased at a 400 nm pitch. These results were well explained by a constructive wave that was formed by the superposition of the diffraction waves on the grooves at the center of the pattern. The most enhanced antenna effect was found to be obtained by controlling the pitch of a plasmonic chip as the resonance wavelength is adjusted to the excitation wavelength. Conversely, the distance from the silver surface was controlled using silica layer thicknesses of 20 and 80 nm, and Ap increased remarkably at 480 nm pitch for the 80 nm thick silica layer. This result was supported by the electric field intensities at the center and edge calculated by discrete-dipole approximation, revealing that the distance factor can contribute to the electric field intensities of propagated waves. The nanoantenna effect could be enhanced by the pitch and dielectric media prepared on the silver film