Sharp Fano Resonance within Binary Gold Nanodisk Array through Lattice Coupling Effects

Wenyu Zhao Department of Physics, Harbin Institute of Technology, Harbin, Heilongjiang Province, China Yongyuan Jiang Department of Physics, Harbin Institute of Technology, Harbin, Heilongjiang Province, China Department of Physics, Key Lab of Micro-Optics and Photonic Technology of Heilongjiang Province, Harbin, Heilongjiang Province, China

Fano resonance in plasmonic nanostructures has attracted a tremendous amount of attention in recent years due to its novel optical properties and promising applications such as biosensor, modulator, surface-enhanced Raman scattering, and surface plasmon laser. The origin of such Fano resonance is generally related to the interaction of dark mode and bright mode supported by the coupled nanostructures in strong near-field region. The performance of most Fano-resonance-based devices is, to a great degree, determined by the Q factor and spectral contrast of the resonance such as the detection limit of a biosensor. However, the spectral contrast and Q factor of conventional schemes are often limited to rather small values. This comes from the fact that as the gap distance between the structures decreases, high order resonances start to play a role in radiation resulting in deviation of pure dipole resonance of each constituent and incomplete cancellation of the electric field which strongly precludes the improvement of the spectral contrast and Q factor. Here, we investigate the Fano resonance within a binary gold nanodisk array under the weak near-field coupling where the particles can be regarded as pure dipoles collectively oscillating with the incident light. Two combined arrays hybridize to generate anti-phased lattice collective resonances (dark mode) and in-phased lattice collective resonances (bright mode) through lattice coupling effects. Due to the sharp resonance and strong cancellation of dipole resonances at the Fano dip, Q factor and spectral contrast which follow different evolution tendencies for the variation of particle radii in the arrays can remain at high values simultaneously. The best results in experiment are Q factor of 14 and corresponding spectral contrast of 0.7. For the optimal calculation, Q factor of 85 and spectral contrast of 0.86 can be achieved at the same time, which is very promising for a series of practical applications. We show that by taking advantage of the high Q factor and high spectral contrast, the figure of merit (FOM) of the structure as a refractive sensor can reach as high as 116 which is almost an order higher than conventional plasmonic sensors.

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