mid-ir quantum cascade Laser Integrated on Ge-based pHoTonics circuits – LIGHTUP

The groundbreaking concept of the project is the epitaxial growth of QCL devices on the Ge-based photonics circuit, to offer large-volume, wafer-level complete mid-IR photonic platform. Both the QCL active region and the Ge-based photonics circuit will be specifically designed to optimize both the coupling strategy and the material properties, i.e., to preserve low optical losses and large optical gain.

Results have been obtained on different aspects : (i) towards the integration of III-Sb QCLs on SiGe mid IR platform, (ii) on the development of photonics building blocs in SiGe mid-IR platfom, (iii) on identifying and solving issues related to the integration of III-Sb optoelectronic devices with passive photonic circuits, and are summarized hereafter :

Towards the integration of III-Sb QCL on SiGe mid IR platform :

- The growth of III-Sbs on the Ge-based virtual substrates has been investigated in details in the project, building on the pre-existing knowhow on growing III-Sbs on Si substrates

- The conditions to reproducibly achieve antiphase-domain free III-Sb layers on Ge substrates have been established. The conditions to reproducibly achieve antiphase-domain free III-Sb layers on Ge/Si templates have been established.

- The growth conditions of III-Sbs on Ge/SiGe/Si templates has made much progress. Work is underway to get fully reproducible results.

- A III-Sb QCL has been grown on a Ge substrates and demonstrated performances similar to those of QCLs grown on their native InAs substrate. This was the first-ever QCL grown on a Ge substrate.

- In parallel a butt coupling strategy has been successfully implemented. Coupling losses of 10 dB have been obtained in the first attempt, but allows to understand the limiting factors for future works.

Development of photonics building blocs in SiGe mid-IR platform :

- High quality factor resonators (Q<105) operating around 8 µm wavelength have been demonstrated for the first time .

- Electro optical modulators have been demonstrated, based on free carrier plasma dispersion effect. Different diode configuration have been investigated (Schottky, PIN), and electro optical bandwidth around 1GHz has been obtained.

- Photodetection in the mid-IR has been demonstrated using the electro optical devices. The physical mechanism at the origini of the photodetection has been investigated in details, and seems related with residual defects during SiGe growth.

Issues related to the integration of III-Sb devices with passive photonic devices

- In parallel, work has also been performed with GaSb-based diode lasers (DLs), the model optoelectronic device in the III-Sb technology, to advance the integration steps.

- The impact of dislocations, an unavoidable defect in lattice-mismatched devices, has been investigated on GaSb-based diode lasers. It has been shown that dislocations do not introduce optical losses, but non-radiative recombination channels.

- A strategy to butt-couple GaSb-diode lasers to SiN waveguides (WGs) by direct epitaxy of the DLs on a Si PIC has been elaborated. Light transmission through the WGs has been demonstrated. Coupling losses of 10 dB have been measured, and their origin understood. Strategies to improve the coupling efficiency have been proposed and demonstrated.

The LIGHT UP projects allowed both to develop the mid-IR SiGe photonics platform and its integration with compact electrically driven III-Sb QCL. Light coupling between both has been successfully achieved in the butt coupling approach, and strong progess has been achieved in the monolithic integration strategy. The main perspective of this work is the combination of III-Sb QCL with SiGe photonics circuits providing advanced optical function such as frequency comb generation.

Silicon photonics is expected to have a major impact for the development of mid-infrared photonics by leveraging the reliable and high-volume fabrication technologies already developed for microelectronic integrated circuits. A key point for the development of real applications for mid-IR photonics is the coupling of the photonic circuits with the mid-IR light source. In this context, LIGHT UP project addresses the integration of an an InAs/AlSb – based Quantum Cascade Laser (QCL) on a mid-InfraRed Germanium (Ge)-based photonics integrated circuit. The groundbreaking concept of the project is the epitaxial growth of QCL devices on the Ge-based photonics circuit, to offer large-volume, wafer-level complete mid-IR photonic platform. Both the QCL active region and the Ge-based photonics circuit will be specifically designed to optimize both the coupling strategy and the material properties, i.e., to preserve low optical losses and large optical gain. As a final demonstrator, the demonstration of a Ge-based PIC with a monolithically integrated QCL working at 7.4 µm is targeted. The choice of this wavelength is related to potential application to alkane sensing, that can be easily tested and can show rapidly the impact of the proposed work. The results achieved within the LIGHT UP project will give answers on the possibilities and opportunity of a new and ambitious monolithically integrated III-Sb on Ge-based mid-IR platform. In a long term vision, a large number of unique and innovative devices such as an ultra-compact, wideband and tunable light source coupled with on-chip optical modulator, photodetectors, or with integrated spectrometers will then be envisioned. Such a platform is expected to be have a major impact for many applications, including real-time environmental monitoring of pollutants, early medical diagnostic, or free space telecommunications, among others.