Subwavelength surface plasmon polariton optical filters based on metal–insulator–metal slot nanocavites are proposed and analyzed by using coupled mode theory and a finite element method. Simulation results reveal that a single slot cavity coupled with two access waveguides possesses a bandpass-filtering characteristic with its performance affected by its geometric parameters. To further improve the filtering performance, we explore coupled slot cavities as high-order plasmonic filters. When the slot cavities are side-coupled, the bandpass filtering spectrum is dependent on the positions of the access waveguides. The two slot cavities can also be set orthogonal, leading to strong mutual coupling. With careful tuning of the relative length between the two cavities, improved filtering spectrum can be obtained. Given the subwavelength footprint of the proposed plasmonic filters, they can be used in an ultradense plasmonic integrated circuit for optical signal processing.
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