Passive directional couplers in Si and a-Si:H: a comparative study of conventional and shallow-rib strip configurations for 0.5 × 0.22 µm waveguides

This work investigates directional coupling in standard 0.5 × 0.22 μm silicon (Si) and hydrogenated amorphous silicon (a- Si:H) single-mode waveguides under fundamental TE excitation, with the aim of clarifying the physical differences between conventional symmetric directional couplers and slab-assisted shallow-rib strip configurations. A conventional symmetric directional coupler, formed by two identical parallel waveguides, is first analyzed as the reference structure for the Partial (3 dB) and Cross states using a coupling separation of 𝑠 = 0.1 μm, in order to establish the baseline behavior of coupling length, field confinement, and power transfer governed by direct lateral evanescent interaction. The SRS directional coupler is then introduced as a fabrication-aware alternative and analyzed at a larger separation of 𝑠 = 0.5 μm, where the residual slab region partially mediates the interaction between the waveguides. The study is carried out for buried-channel Si and a-Si:H waveguides with SiO₂ cladding, using the Effective Index Method, three-dimensional Beam Propagation Method-3D and FullWAVE electromagnetic simulations. The results show that the shallow-rib strip waveguides configuration preserves the characteristic sinusoidal power exchange of directional coupling while enabling efficient transfer at substantially larger lateral spacing than the conventional symmetric coupler. These findings indicate that the shallow-rib strip waveguides approach provides a practical fabrication-aware strategy for directional couplers in integrated photonic platforms.