Sensitivity enhancement through overlapping simultaneously excited Fano resonance modes of metallic-photonic-crystal sensors

Fano coupling between Rayleigh anomaly and localized surface plasmon resonance for sensor applications

Fano coupling between Rayleigh anomaly and localized surface plasmon resonance has been observed in diffractive grating structures consisting of aluminum nanolines deposited on the top surface of photoresist with each nanoline composed of tightly aggregated aluminum nanoparticles. Localized surface plasmon resonance is excited both in the nanoparticles and in the nanolines by differently polarized light. The surface propagation mode excited by the first- and second-order Rayleigh diffraction anomaly is strongly scattered and diffracted by the plasmonic aluminum grating structures, producing light rays in the same direction as the reflected light beam with the same spectral feature as the Rayleigh anomaly. The narrow-band diffracted and scattered light appears as sharp dips in the broad-band reflective optical extinction spectrum of plasmon resonance, which is recognized as a kind of Fano coupling. This kind of coupled mode is utilized successfully in refractive-index-sensor devices with excellent sensitivity.

Fano resonance properties of gold nanocrescent arrays

The Fano resonance induced by symmetry breaking could improve the sensitivity of localized surface plasmon resonance sensors. In this work, the spectra of gold nanocrescent arrays are measured and confirmed by simulation results through the finite element method (FEM). The Fano resonance presented in the spectra could be modulated by the symmetry breaking with different waist widths, which are understood through plasmonic hybridization theory with the help of surface charge distribution. Our results indicate the Fano lineshape is generated by the coherent coupling of the quadrupole plasmon mode Q_H of nanohole and the antibonding plasmon mode D(AB) of nanocrescent. Finally, the high figure of merit (FoM=1.6-3.5) of the Q mode in the visible region illustrates this nanocrescent Fano sensor is of great value in the biological and chemical scientific fields.