Ultrawideband bandstop filter based on Fano resonance and rectangular resonators

In this study, the effect of length of the stub on the formation of the Fano resonance in structures which possess a metal-insulator-metal (MIM) waveguide coupled to rectangular cavities by the stub is investigated theoretically and numerically. The resulting Fano resonance is used to design an ultrawideband bandstop filter that can filter all wavelengths between two telecommunication windows of $\lambda = 850\;{\rm nm}$ and $\lambda = 1310\;{\rm nm}$. The structure is based on two rectangular cavities coupled to the MIM waveguide by stubs that are located at an adjusted distance from each other; the interference superposition of reflected and transmitted waves from each other will make this filtering phenomenon. The center wavelength of the bandstop of the structure is highly adjustable by changing the dimensions of the structure. The theoretical and the numerical results are, respectively, based on the transmission line model and the finite-difference time-domain method. The theoretical results comply well with the numerical ones. To analyze the Fano resonance, temporal coupled mode theory is also exploited. The proposed structure has significant applications in highly integrated optical circuits.