Compact Plasmonic Structure Induced Mode Excitation and Fano Resonance

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.