Abstract Terahertz waves form one of the most potential technologies to increase the data capacity and transmission rate required for the next generation of communications. Wavelength division multiplexing is indispensable in large-capacity and fast data-transmission communication systems. In this work, a three-channel wavelength division multiplexer working on a terahertz spoof surface plasmon polariton waveguide is designed. The wavelength division multiplexer is composed of three cascaded directional couplers assisted by whispering gallery mode resonators, whereas the plasmonic waveguide is based on periodic metallic pillars with many variable parameters. The tendency of the coupling efficiency between a ring resonator and the straight waveguide can be physically predicted by the coupled mode theory. After further optimization, the three-channel wavelength division multiplexer operates with a low insertion loss of 1.4 dB and a low crosstalk of − 16 dB within 0.65–0.69 THz. Such a device may find important applications in future on-chip terahertz communication systems.
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