FDTD analysis of optical integration between SiGe waveguides and Ge-rich SiGe heterostructures on graded buffer for quantum-confined Stark effect optical modulators

Abstract

Ge-rich SiGe heterostructures with superior crystal and photonic quality have been enabled via a thick linear graded SiGe buffer platform on Si. Nevertheless, due to a very low refractive index contrast between the Ge-rich SiGe heterostructures and the graded buffer, it is challenging to have efficient optical integration with other photonic components. In this paper, a coupling scheme based on a top adiabatic coupling between Ge-rich SiGe heterostructures on graded buffer and a SiGe waveguide in SiO2 cladding is proposed to address a low-loss coupling of the Ge-rich SiGe heterostructures with a high-index-contrast waveguide structure. 3D-FDTD simulations demonstrate that efficient optical coupling between the top SiGe waveguide (high-index-contrast platform) and Ge-rich SiGe heterostructures on a thick graded buffer (low-index-contrast platform) can be adiabatically achieved with practical fabrication tolerance. In addition, the proposed coupling structure can be potentially applied with Ge/SiGe MQWs, enabling SiGe waveguide integrated Ge/SiGe MQWs optical modulator with a viable integrated extinction ratio and insertion loss performance.