Vertical Directional Couplers With Ultra-Short Coupling Length Based on Hybrid Plasmonic Waveguides

We propose and analyze a series of vertical directional couplers based on SOI-compatible hybrid plasmonic waveguides. We investigate two configurations: metal-insulator-metal (MIM) and insulator-metal-insulator (IMI). Both slab and three-dimensional analyses show that MIM directional couplers have better coupling performance such as sub-micron coupling length and lower loss. To ensure the normalized power loss lower than 5%, the coupling length can be chosen as short as 0.492 μm (about one third of the wavelength 1.55 μm). The coupler size can be traded for even lower power loss. In one example, a less-compact design yields 3% power loss, while maintains sub-micron coupling length. We further verified our design with beam propagation analysis. The analysis independently verifies the compact MIM design does show guided mode coupling length along vertical direction can be as short as 0.492 μm.