Nanoantenna effect dependent on the center structure of Bull’s eye-type plasmonic chip

Abstract

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.

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

“…The fabrication method for the plasmonic chip has been described elsewhere in detail. , Briefly, the periodic patterns of the mold comprised concentric circles with two pitch types, 400 and 480 nm, called the Bull’s eye, and five and six patterns (diameter = 100 μm) were arrayed in the mold plane, respectively. The replica was fabricated by ultraviolet (UV) nanoimprinting using a UV-curable resin, PAK-02A (Toyo Gosei).…”

“…The surface of the plasmonic chip was modified with 1 wt % aminopropyltriethoxysilane (APTES, Sigma-Aldrich). , After 1 h of incubation at 23 °C, the surface was rinsed with milli-Q water.…”

“…The center-structure dependence of the antenna effect for fluorescent nanoparticles was investigated using the plasmonic chip exhibiting a Bull's eye pattern. 18 The fluorescence intensities at the center and edge of each Bull's eye pattern, as well as at the flat silver area without patterns, were evaluated under the three conditions, namely, excitation enhancement, emission enhancement, and both enhancements by selecting the excitation/emission wavelengths and incident/detection angles. After each enhancement factor was evaluated for a center and an edge, the rate of the nanoantenna enhancement was obtained as the ratio of the enhancement factor at a center to that at an edge.…”

“…The Darkred nanospheres were densely adsorbed onto the APTES surface of the plasmonic chip via the electrostatic interaction with the carboxylic acid of the nanospheres. 18 The amount of nanospheres adsorbed was precisely not homogeneous between on the Bull's eye's pattern and on the flat area because of the difference in the surface area, but the difference based on the surface area was calculated as less than 9% considering the periodic structure with the inclination of 20 degree. 23 Therefore, the small difference in the amount of nanospheres adsorbed is considered to not contribute to the fluorescence intensities measured for evaluation of enhancement factors.…”

Design and Simulation of Broadband Horn Nanoantennas for Nanophotonic Applications

“…The first plasmonic nanostructures were used as optical nanostructures, and since then they have been widely used to redirect light scattering and emission, as they have excellent directional light scattering properties [20]. Among the numerous applications of nanoantennas in the literature, their integration in optical chip applications [23] and [24] stands out; wireless optical nanolink [25] and [26]; biosensors [27] and [28]; nanocircuits [29], [30]; anti-reflective coating [31], solar energy harvesting [32], at the same time spectral and spatial separation of light [33] and [34]; and hydrogen detection [35].…”