Abstract
In this paper, a type of tunable plasmonic refractive index nanosensor based on Fano resonance is proposed and investigated. The sensor comprises a metal-insulator-metal (MIM) nanocavity with a center-deviated metal core and two side-coupled waveguides. By carefully adjusting the deviation angle and distance of the metal core in the cavity, Fano resonances can be obtained and modulated. The Fano resonances can be considered as results induced by the symmetry-breaking or geometric effect that affects the field distribution intensity at the coupling region between the right waveguide and the cavity. Such a field-distribution pattern change can be regarded as being caused by the interference between the waveguide modes and the cavity modes. The investigations demonstrate that the spectral positions and modulation depths of Fano resonances are highly sensitive to the deviation parameters. Furthermore, the figure of merit (FOM) value is calculated for different deviation angle. The result shows that this kind of tunable sensor has compact structure, high transmission, sharp Fano lineshape, and high sensitivity to the change in background refractive index. This work provides an effective method for flexibly tuning Fano resonance, which has wide applications in designing on-chip plasmonic nanosensors or other relevant devices, such as information modulators, optical filters, and ultra-fast switches.
Highlights
In the past decade, the surface plasmon resonances of noble metal nanostructures have attracted considerable research interest due to their unusual abilities to manipulate light within subwavelength dimensions and to produce extremely strong local electromagnetic fields near the metal surfaces [1,2,3]
Fano resonances can be considered to be results induced by the symmetry breaking or geometric effect that affects the field distribution intensity at the coupling region between the right waveguide and the cavity
The black squares denote the transmission scatters and the red lines denote the fitted lines. This shows that |q| reaches its maximum values at θ dev = 85◦, meaning that the asymmetry is high for this case
Summary
The surface plasmon resonances of noble metal nanostructures have attracted considerable research interest due to their unusual abilities to manipulate light within subwavelength dimensions and to produce extremely strong local electromagnetic fields near the metal surfaces [1,2,3]. The plasmonic structures described by quasi-lorentzian profiles usually exhibit a symmetric and wide lineshape in transmission spectra [4,5,6] In this paper, a kind of simple nanostructure with symmetry breaking along a wide range of directional changes is considered to adjust Fano resonances. This may result in more flexibility for adjusting Fano resonances. This kind of sensor has a compact structure that is appropriate for integration with other on-chip devices, which plays an important role in nano-integrated plasmonic systems
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