Design of silicon traveling-wave Mach-Zehnder modulators with transparent electrodes

High-speed silicon traveling-wave Mach-Zehnder modulators (MZMs) are key components to support optical fiber communication. However, one major challenge with all-silicon MZMs is to achieve efficient high-speed electro-optic (EO) modulation. The reported 3 dB bandwidth of silicon MZMs are generally below 70 GHz, with half-wave voltage (<i>V</i> _π) around 5 V or larger, which can not support future 200 Gbaud data transmission. Here we break the voltage-bandwidth trade-off limit in silicon MZMs by replacing the doped silicon slab with CMOS compatible transparent electrodes. Benefit from the measured high conductivity, low extinction coefficient, and low refractive index of indium tin oxide (ITO) materials, the microwave dielectric loss of the traveling wave electrode can be greatly reduced. The bandwidth would potentially increase from 40 GHz to 168 GHz, while <i>V</i> _π and optical insertion loss remains almost unchanged. According to Si/ITO interface contact states, three operating mode were found, corresponding to Si/ITO ohmic contact, schottky contact and hybrid schottky/ohmic contact, respectively. We comprehensively analysis the Si/ITO interface characteristic, establish a complete high frequency equivalent circuit model. Our proposed transparent electrodes will open a new window for the high-speed silicon photonics platform.