Plasmonic electro-optic modulators integrated with a silicon rib waveguide

We propose, model, and design plasmonic electro-optic modulators integrated with a silicon-on-insulator (SOI) rib waveguide. The devices are based on a pair of metal-insulator-semiconductor-metal (MISM) stacks operating as metal-oxide-semiconductor (MOS) capacitors proximity-coupled laterally to the SOI rib. The SOI rib is tapered laterally to adiabatically evolve the TM₀ mode therein into a coupled mode exhibiting field enhancement and strong localization to the MISM stacks. The semiconductor used in the MISM stacks consists of a thin highly doped indium tin oxide (ITO) layer to enable a low drive voltage followed by a thicker and lower doped ITO layer to reduce the on-state loss. The ITO is driven into accumulation, such that epsilon-near-zero is reached, which further enhances the fields in the MISM stacks and induces high loss in the off-state via the carrier refraction effect. An example design, 3.75 µm long including input and output tapers, yields an on-state insertion loss of 5.8 dB, an extinction ratio of 5 dB, and an RC-limited electrical bandwidth of 214 GHz.