Light-hole SiGe quantum cascade lasers in the THz and mid-IR

The proposed project aims at the first demonstration of a quantum cascade laser in the silicon-germanium system, where the pursued novel approach is based on a radiative transition between light-hole states. Such a practical silicon-based laser source is of high interest for compact and robust sensing and detection systems for various applications. Due to the prospect of CMOS compatibility, silicon photonics is highly attractive as a platform for next-generation data transfer solutions and spectroscopy and detection systems. However, up to now the monolithic integration of all components required for such compact optoelectronic systems has been prevented by the lack of an essential building block: That of a convenient silicon-based laser source. The proposed project targets the realization of such a laser. A novel optically active material based on nanometer-thin layers of silicon-germanium compounds with varying composition will be designed and fabricated, forming a so-called quantum cascade structure. In other material systems, lasers based on the quantum cascade concept are realized with great success. However, all of the quantum cascade lasers demonstrated up to now employ radiative transitions between electron states in the conduction band. This is not possible for silicon-germanium structures, as no sufficiently deep potential wells (quantum wells) for electrons can be formed. However, silicon-germanium quantum cascades can be realized for holes, which suffer from the disadvantage of generally being heavier than electrons and experience different masses depending on the occupied band. The charge carrier mass strongly affects the performance of the optical material, and up to now silicon-germanium quantum cascade structures have been based on disadvantageous heavy-holes. Only very recently, the author of this proposal has demonstrated the first quantum cascade structure based on a transition between light-hole states, which are closer to electrons in their properties and allow more efficient emission of radiation and charge transport. In the proposed follow-up project, next-generation structures with gain sufficiently high for lasing will be developed. The realized material will be integrated into waveguide cavities, and lasing will be demonstrated. The first silicon-germanium quantum cascade laser demonstration will be carried out in the terahertz regime, and in parallel active material for the mid-infrared region will be designed and realized. Once such a silicon- germanium laser is demonstrated, it can be integrated with silicon-based optoelectronic circuits. Potential applications of integrated optoelectronic platforms include medical diagnosis (lab-on-a-chip) and breath analysis, environmental and air pollution control, threat reduction, industrial process control and water purity monitoring.

In the course of the project, the first silicon-germanium quantum cascade structures were integrated into metal-metal waveguide cavities, allowing to drive the material at high current densities as well as providing a cavity for terahertz radiation. The achieved progress is a crucial step towards the realization of a practical silicon-based laser source, which is of high interest for compact and robust sensing and detection systems for various applications. Due to the prospect of CMOS compatibility, silicon photonics is highly attractive as a platform for next-generation data transfer solutions and spectroscopy and detection systems. However, up to now the monolithic integration of all components required for such compact optoelectronic systems has been prevented by the lack of an essential building block: That of a convenient silicon-based laser source. The achievements of the presented project are an important step towards realizing such a laser. A novel material based on nanometer-thin layers of silicon-germanium compounds with varying composition has been designed and fabricated, forming a so-called quantum cascade structure. In other material systems, lasers based on the quantum cascade concept are realized with great success. However, all of the quantum cascade lasers demonstrated up to now employ radiative transitions between electron states in the conduction band or interband transitions. This is not possible for silicon-germanium structures, as no sufficiently deep potential wells (quantum wells) for electrons can be formed. However, silicon-germanium quantum cascades can be realized for holes, which suffer from the disadvantage of generally being heavier than electrons and experience different masses depending on the occupied band. The charge carrier mass strongly affects the performance of the optical material, and prior to our effort silicon-germanium quantum cascade structures have been based on disadvantageous heavy-holes only. In contrast, the approach pursued in this project uses transitions between light-hole states, which are closer to electrons in their properties and allow more efficient emission of radiation and charge transport. Once the first silicon-based quantum cascade laser is demonstrated, it can be integrated with silicon-based optoelectronic circuits. Potential applications of integrated optoelectronic platforms include medical diagnosis (lab-on-a-chip) and breath analysis, environmental and air pollution control, threat reduction, industrial process control and water purity monitoring.