Ultracompact directional couplers (DCs) based on dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) are analyzed using the effective index method (EIM), with the coupling, both in the parallel interaction region and in- and out-coupling regions, being taken into account. Near-field characterization of fabricated DCs performed with a scanning near-field optical microscope verifies the applicability of the EIM in the analysis and design of DLSPPW-based wavelength-selective DCs. The design approach applicable to a large variety of integrated optical waveguides is developed, enabling the realization of DCs in which optical signals at two different wavelengths are coupled into two separate output channels. The developed approach ensures minimization of the crosstalk and overall DC length via simultaneous adjustment of the waveguide separation and length of the interaction region. As an example, the design of a DLSPPW-based DC for complete separation of telecommunication signals at the wavelengths $\lambda=1400$ nm and $\lambda=1600$ nm between two output channels separated by 6 $\mu$m is worked out, resulting in the total device length of 52.3 $\mu$m. In addition, the design of an ultracompact DLSPPW-based DC waveguide crossing that ensures a very low crosstalk over a large wavelength band in the telecommunication range is considered.
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