Passive Plasmonic Demultiplexers Using Multimode Interference

Two passive plasmonic demultiplexers with multimode interferometer (MMI) based rectangular and bow-tie designs are demonstrated for 1.31- and 1.55-μm-band wavelength division multiplexing system operation in plasmonic circuits. These demultiplexers are designed using the finite-difference time-domain (FDTD) method and fabricated using complementary metal-oxide-semiconductor compatible processes. SiO2 films deposited on metal films are etched to form mesa structures by focused ion beam milling, and single-mode waveguides, MMIs, and bow-tie structures are then formed on the metal films. In the plasmonic circuits, 1.31- or 1.55-μm-wavelength surface plasmon polariton (SPP) signals can be distributed to each of the different demultiplexer outputs without any change in signal propagation speed. The SPP intensity distributions that are measured using a scanning near-field optical microscope agree well with the distributions that were calculated using the FDTD method. The insertion loss and crosstalk are then calculated for each demultiplexer. In the rectangular-type demultiplexer, the calculated values were less than 9.5 dB and -2.4 dB, and in the bow-tie-type demultiplexer, the corresponding values were 8.9 dB and less than -2.3 dB, respectively. These results demonstrate the applicability of the developed plasmonic demultiplexers to plasmonic circuits when monolithically integrated on a silicon substrate.