Compact Plasmonic Structure Induced Mode Excitation and Fano Resonance

Abstract

Metal-insulator-metal (MIM) waveguide has deep sub-wavelength field confinements, which makes it an important component in many aspects. In MIM structure, both of the symmetric and anti-symmetric modes could be supported. However, the anti-symmetric mode was hardly used in the SPP-based devices due to the critical excitation condition. Here, we demonstrate anti-symmetric mode excitation and Fano resonance in a compact MIM-based plasmonic structure. By changing the position of the output channel, the symmetric mode is suppressed and only anti-symmetric mode is excited. Then, we tune the position of the output channel; anti-symmetric and symmetric mode are both achieved. Furthermore, Fano resonance is realized due to the coupling between anti-symmetric mode and symmetric mode. In addition, we analyze the effects of the parameters of the structure on the transmission spectra, and a plasmonic refractive index sensor with sensitivity about 800 nm/RIU and 1100 nm/RIU based on different waveguide modes is also realized. The proposed structure provides a novel method to achieve anti-symmetric mode excitation, and it has important applications in nanophotonic devices such as filter, sensor, and photoswitch, and has important significance in achieving all-optical on-chip integration.

Appendices

Appendix 1 Symmetric and anti-symmetric waveguide modes

As we know, both symmetric and anti-symmetric waveguide modes can be supported in MIM structure. According to ref. [21], symmetric and anti-symmetric waveguide modes have quite different field distributions. The symmetric waveguide mode has no node for the field distributions (Hz) in the air gap, while the anti-symmetric one has a node. To show these two different waveguide modes directly, we plot the Hz distributions at λ = 1152 nm and λ = 865 nm for the symmetric and anti-symmetric waveguide modes, which are shown in Fig. 5a and b, respectively. To display the phenomenon intuitively, the field distributions of Hz along the black dashed line are shown in Fig. 5c and d. Obviously, at λ = 1152 nm and λ = 865 nm, the symmetric and anti-symmetric modes are excited, respectively. For symmetric mode at λ = 1152 nm, the effective index is about n_eff ≈ 1.046535 + 0.0004306i, propagation distance of SPPs is about L_SPPs ≈ 212.9 μm. For anti-symmetric mode at λ = 865 nm, the effective index is about n_eff ≈ 0.668646 + 0.001087i, and the propagation distance of SPPs is about L_SPPs ≈ 63.3 μm. These results are consistent with the results shown in Fig. 1 in ref. [21].

Fig. 5

Field distributions of Hz at a λ = 1152 nm, b λ = 865 nm for L = H = 500 nm. Field distributions of Hz along the black dashed line in a, b at c λ = 1152 nm and d λ = 865 nm

Appendix 2 SPP power flows in the cavity for different waveguide modes

Different coupling methods make the phase of the SPPs coupled to the output channel change, which in turn causes the transmission spectrum to change. Different transmission peaks/valleys correspond to different excitation modes of the resonator. Each excitation mode reflects the different transmission behavior of SPPs, which is important for understanding the relationship between the resonant wavelength of SPPs and the resonator parameters. Figure 6. show the field distributions of |Hz|² and the SPP power flows (blue cone) at the resonant wavelength with L = H = 500 nm for different structures proposed in the manuscript. It is clear that the propagation behaviors of SPPs in the cavity are different in each other, even if the excited modes are the same, e.g., Fig. 6b, c, and e.

Fig. 6

a–h Field distributions of |Hz|² and the SPP power flows (blue cone) at the resonant wavelength with L = H = 500 nm