Reducing metal-induced optical loss in graphene-on-silicon-nitride electro-absorption modulators

Metal-induced absorption limits the performance of graphene-on-silicon-nitride (Si₃N₄) modulators due to the evanescent coupling of the optical mode into lossy metallic gate-electrodes placed near the waveguide core. This work presents a systematic study of electrode geometry and proximity on the optical performance of dual-layer graphene modulators fabricated on a CMOS-compatible Si₃N₄ platform. Three-dimensional electromagnetic simulations and experimental measurements reveal that replacing continuous bar electrodes with segmented via-type contacts reduces metal-induced attenuation from 0.36 dB/µm to 0.09 dB/µm while maintaining a graphene absorption of ≈ 0.16 dB/µm. The optimized via configuration yields a smooth transmission spectrum, reduced metal-induced insertion loss, and a modulation slope of approximately 1 dB/V at 1.31 µm, extracted from the linear region of the loss-voltage curve near the device operating point. These results establish practical design rules linking electrode geometry and electrode spacing to optical loss, providing a clear route toward CMOS-compatible graphene modulators with reduced metal-induced loss for photonic integrated circuit platforms.