Abstract
We report on the design and simulation of a waveguide-integrated Ge/SiGe quantum-confined Stark effect (QCSE) optical modulator based on the use of a Ge-rich SiGe relaxed buffer on a graded buffer as an optical waveguide. Despite the promising potential of this waveguide platform, efficient and wideband optical integration with a Ge-based active device has not been properly addressed so far. In this paper, via 3D finite-difference time domain simulation, we demonstrate that a simple 2D taper is sufficient to enable adiabatic optical coupling from the fundamental mode of the input SiGe waveguide to the fundamental mode of the Ge/SiGe multiple quantum well (MQW) modulator without the excitation of higher-order modes in Ge/SiGe MQWs. The 2D taper shows good fabrication tolerance considering critical variations in its dimensions. Significantly, wideband optical modulation performance in terms of extinction ratio and insertion loss is presented over the whole low-loss spectral range of the Ge/SiGe MQWs at different electrical bias values, device lengths, and numbers of quantum wells in order to comprehensively report its potential for Si-based optical modulators.
Highlights
Quantum-confined Stark effect (QCSE) from germanium /silicon–germanium multiple quantum wells (Ge/SiGe MQWs) has been regarded as one of the strongest physical mechanisms to obtain a group-IV optical modulator, which could be fabricated using top-down optical lithography and dry etching techniques
We report on the design and simulation of a waveguide-integrated Ge/SiGe quantum-confined Stark effect (QCSE) optical modulator based on the use of a Ge-rich SiGe relaxed buffer on a graded buffer as an optical waveguide
Via 3D finite-difference time domain simulation, we demonstrate that a simple 2D taper is sufficient to enable adiabatic optical coupling from the fundamental mode of the input SiGe waveguide to the fundamental mode of the Ge/SiGe multiple quantum well (MQW) modulator without the excitation of higher-order modes in Ge/SiGe MQWs
Summary
Quantum-confined Stark effect (QCSE) from germanium /silicon–germanium multiple quantum wells (Ge/SiGe MQWs) has been regarded as one of the strongest physical mechanisms to obtain a group-IV optical modulator, which could be fabricated using top-down optical lithography and dry etching techniques. Zang et al. have theoretically shown from finite-difference time domain (FDTD) simulations that for the optical integration with the SOI waveguide, using a 2D taper (tapering in the direction horizontal to the plane of the wafer) might not be sufficient to suppress the excitation of the higher-order modes inside the Ge/SiGe MQW optical modulator, and a 3D taper. We have investigated the wideband optical modulation performance of the SiGe waveguide-integrated Ge/SiGe QCSE optical modulator over the whole low-loss spectral range of the Ge/SiGe MQWs under investigation with different values of device’s length and numbers of QW layers, in order to comprehensively report its potential for compact Si-based optical modulator. It should be noted that in MQWs, electric fields higher than the breakdown field of the bulk material can be possibly applied; due to the quantum confinement effect, the hole and electron wavefunctions are displaced from the band edges resulting in increased ionization energy.
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