ThisDemultiplexer chapter presents a novel functional device based on three-dimensional (3-D) metal-insulator-metal (MIM) waveguides. The performance of the demultiplexer is demonstrated for wavelengths of 1.31 and 1.55  \(\upmu \) m. The demultiplexer is composed of slit- and stub-type filters based on metal-insulator-metal waveguides. To achieve a narrower bandwidth for the slit-type filter, a dielectric material is inserted in the slit section and the metal thickness is intentionally set to 0.2  \(\upmu \) m. Then, a demultiplexer with two filters in parallel is considered. Numerical results show that the separation between the two output ports is 18.9 dB at 1.31  \(\upmu \) m and only 9.8 dB at 1.55  \(\upmu \) m. To improve this separation, stub sections forming attenuation poles are introduced into the waveguide. As a result, it is found that a separation of more than 20 dB is obtained at both wavelengths. All calculations are performed using a GPU-accelerated finite-difference time-domain method based on the trapezoidal recursive convolution method.