Ultracompact and ultrabroadband mode division multiplexer based on an Au nanocube array assisted directional coupler

An ultracompact and ultrabroadband two-mode (de)multiplexer based on an asymmetric directional coupler for mode division multiplexing is proposed. The device structure consists of a pair of silicon waveguides with an array of plasmonic Au nanocubes sandwiched in the coupling region. The coupling region length of the directional coupler is decreased to 1 µm for coupling of the fundamental transverse magnetic (TM) mode to the first order mode by excitation of the surface plasmon polaritons. This is the shortest length reported for multiplexing of the TM modes until now, to the best of our knowledge. The proposed mode (de)multiplexer has a low loss of 0.72 dB and low crosstalk of -28.3dB at the communication wavelength of 1.55 µm. Also, the 3D finite-difference time-domain simulation results show that a broad bandwidth of 190 nm is realized with crosstalk less than -10dB and the insertion loss lower than 1.29 dB. Furthermore, impact of the fabrication tolerances on the performance of the proposed (de)multiplexer is studied in detail.