“…Due to not only supporting modes with deep sub-wavelength scales and high group velocity over a very wide range of frequencies but also offering very high optical confinement and acceptable propagation length [5], metal-insulator-metal (MIM) structures, such as optical filters [6,7,8,9], optical switches [10], demultiplexers [11,12], and sensors [13,14,15,16], are widely used. Plasmonic filters based on MIM waveguide structures, such as asymmetric nanodisk filter and sensor [17,18,19], side-coupled cavity sensor [20], notch resonator filter and sensor [21], and circular ring filter and sensor [22,23], are one of the most important optical devices, have attracted tremendous attention, and have been investigated widely in recent years. All of the above-mentioned devices are promising candidates for highly integrated optical circuits.…”