Operating silicon-based lasers with high quantum efficiency above room temperature

Mid-infrared lasers are useful for trace gas analyses in applications as varied as chemical processing, health diagnosis, and environmental measurements. Recent studies on lead-salt, mid-infrared (MIR) tunable, vertical external-cavity surface-emitting lasers (VECSELs) on silicon inspired physicists from the Shizuoka University in Japan to develop a very compact, edge emitting, diode laser with no external cavity.

Reporting their findings in Applied Physics Letters, the researchers describe how they grew high quality double hetero-structure PbSrS/PbS films directly on silicon by hot wall epitaxy. Coauthor Akihiro Ishida explains that they used a focused ion beam process to fabricate the short-cavity edge-emitting laser structure. The laser had a cavity size of 25µm × 3µm and high-reflection edge mirrors consisting of multiple semi-conductor/air structures. A YAG laser measurement system was then used to assess the optical emission spectra of the laser at various temperatures.

Maximum peak output power was 40mW at 260K and indicated high quantum efficiency, with the laser’s threshold excitation power comparable to conventional PbS-based lasers. The laser was operated up to 313K with emission around 3µm in wavelength under pulsed laser excitation. Referred to in the paper, this is the highest operation temperature reported in PbS lasers, which is important for laser development as it allows temperature control by Peltier cooling to be used, a necessary step for achieving a compact tunable laser system.

Ishida explains that realizing low electrical resistivity in the IV-VI semiconductor to silicon interface is crucial in the development of widely tunable, compact MIR laser diodes, as well as the development of a low loss top electrode ohmic contact. Simple and effective fabrication methods for short-cavity edge-emitting laser diodes still need to be developed, but Ishida anticipates that continuous-wave diode lasers with a narrow single-mode line will be developed in the near future and that this will allow short-cavity lasers to become much more useful.

Source: “PbSrS/PbS mid-infrared short-cavity edge-emitting laser on Si substrate,” by Akihiro Ishida and Seisuke Nakashima, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4996259 .