Insulator-Metal-Insulator-Metal-Insulator and Metal-Insulator-Metal Vertical Directional Couplers with Surface Plasmon Polariton Waveguides

We propose two vertical directional couplers (VDCs) based on surface plasmon polariton (SPP) waveguides. One is an SPP insulator-metal-insulator-metal-insulator (IMIMI) VDC. The SPP IMIMI VDC is composed of two 45-μm-long S bends, an L μm-long straight waveguide, a 4-μm-long C gap, and a 500-nm-long M gap. The other is an SPP metal-insulator-metal (MIM) VDC. The SPP MIM VDC is composed of two 10-μm-long S bends, an L μm-long straight waveguide, a 60-nm-long C gap, and a 500-nm-long M gap. Au is used as the metal and a low-loss polymer is used as the insulator in the IMIMI and MIM waveguides. In the SPP IMIMI VDCs, the excited 5.0 μm × 4.5 μm input S-s0 mode propagates along the S bend_1, propagates and couples along the straight waveguide L, and propagates again along the S bend_2. The losses due to the L region of the SPP IMIMI VDC are 1.33 dB when L = 13 μm and 2.26 dB when L = 23 μm and the coupling length is ~10 μm In the SPP MIM VDCs, the excited 1.7 μm × 0.5 μm input G-s0 mode propagates along the S bend_1, propagates and couples along the straight waveguide L, and propagates again along the S bend_2. The losses of L of the SPP MIM VDC are 1.50 dB when L = 5 μm and 11.20 dB when L = 65 μm and the coupling length is ~65 μm. The SPP IMIMI and MIM VDCs can be a potential step in the process of integration from micro-scale to nano-scale.