Free-carrier electro-refraction modulation based on a silicon slot waveguide with ITO

Recently, silicon-waveguide-based hybrid modulators with high-performance electro-optic materials have been proposed to overcome the intrinsic limitations of silicon materials. Indium-tin-oxide (ITO) is one of the important candidates for such applications due to its unique features including the ENZ effect and electrically tunable permittivity. In this paper, we propose an ultra-compact integrated phase modulator which consists of a silicon slot waveguide with a thin ITO film in the slot region. In the near-infrared regime, bias-voltage-dependent free-carrier accumulation at the dielectric-ITO interface induces an epsilon-near-zero (ENZ) effect, and contributes to the strong phase modulation of the guided electromagnetic wave. With a voltage swing of 2 V, the device experiences a large variation of the effective modal index, resulting in a π radian phase shift within the device length of <5 μm at 210 THz according to our computer simulations. A high modulation efficiency of V(π)L(π)~0.0071 V·cm and a large device bandwidth of ~70 GHz suggest a potential for an ultra-compact optoelectronic component in the integrated silicon photonics platform.