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. 2018 Sep 10;11(9):1675.
doi: 10.3390/ma11091675.

Independently Tunable Fano Resonances Based on the Coupled Hetero-Cavities in a Plasmonic MIM System

Qiong Wang  1   2 , Zhengbiao Ouyang  3   4 , Mi Lin  5   6 , Qiang Liu  7   8
Affiliations

Independently Tunable Fano Resonances Based on the Coupled Hetero-Cavities in a Plasmonic MIM System

Qiong Wang et al. Materials (Basel). .

Abstract

In this paper, based on coupled hetero-cavities, multiple Fano resonances are produced and tuned in a plasmonic metal-insulator-metal (MIM) system. The structure comprises a rectangular cavity, a side-coupled waveguide, and an upper-coupled circular cavity with a metal-strip core, used to modulate Fano resonances. Three Fano resonances can be realized, which originate from interference of the cavity modes between the rectangular cavity and the metal-strip-core circular cavity. Due to the different cavity-cavity coupling mechanisms, the three Fano resonances can be divided into two groups, and each group of Fano resonances can be well tuned independently by changing the different cavity parameters, which can allow great flexibility to control multiple Fano resonances in practice. Furthermore, through carefully adjusting the direction angle of the metal-strip core in the circular cavity, the position and lineshape of the Fano resonances can be easily tuned. Notably, reversal asymmetry takes place for one of the Fano resonances. The influence of the direction angle on the figure of merit (FOM) value is also investigated. A maximum FOM of 3436 is obtained. The proposed structure has high transmission, sharp Fano lineshape, and high sensitivity to change in the background refractive index. This research provides effective guidance to tune multiple Fano resonances, which has important applications in nanosensors, filters, modulators, and other related plasmonic devices.

Keywords: coupled cavities; finite element method; surface plasmon polaritons; tunable fano resonances.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
(a) Schematic illustration of a two-dimensional plasmonic metal-insulator-metal (MIM) nanosystem consisting of a rectangular cavity coupled with a waveguide, and an upper-coupled circular cavity with an angle-tunable metal strip located at the center. (b) In the metal-strip-core circular cavity, the rotation direction of the metal strip is defined by an angle φ between the y-axis and the long axis of the metal strip.
Figure 2
Figure 2
(a) The transmission of a metal-strip-core cavity coupled with a waveguide. The Hz magnetic field distributions of the transmission dips at (b) TD1, λ = 960 nm; (c) TD2, λ = 737 nm. (d) The transmission of a rectangular cavity connected with a waveguide. The Hz magnetic field distributions of the transmission peaks at (e) TP1, λ = 1118 nm; (f) TP2, λ = 707 nm. (g) The transmission of a coupled system consisting of a waveguide, a rectangular cavity, and a metal-strip-core circular cavity. The Hz magnetic field distributions of the Fano peaks at (h) FR1, λ = 960 nm; (i) FR2, λ = 745 nm; (j) FR3, λ = 711 nm.
Figure 3
Figure 3
Transmission changes for (a) different radii of the circular cavity, Rcir = 155 nm, 160 nm, 165 nm, 170 nm, 175 nm; (b) different widths of the rectangular cavity, Wrec = 285 nm, 290 nm, 295 nm, 300 nm, 305 nm.
Figure 4
Figure 4
(a) Transmissions from changing the direction angle φ of the metal strip in the circular cavity with φ = 0°, 10°, 30°, 50°, 70°, and 90°. (b) The q values are calculated for φ changing from 0° to 90°. The Hz magnetic field distributions for the Fano peak of FR2 at (c) φ = 0° and (d) φ = 90°.
Figure 5
Figure 5
The figure of merit (FOM) values of the Fano resonances FR1, FR2, and FR3 related to the direction angle φ.
See this image and copyright information in PMC

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