Comprehensive assessment of terahertz quantum-cascade lasers performance characteristics

Terahertz (THz) quantum-cascade lasers (QCLs) are attracting an ever-increasing interest for both scientific and industrial applications in key areas, such as high-resolution spectroscopy of atomic and molecular absorption lines. Advancements in the active-region and resonator designs, hence, play a pivotal role in determining the future of this technology, especially regarding the wall-plug efficiency and the operating temperature, which are still the main factors limiting their widespread adoption. A sound characterization approach is, therefore, the foundation of the coming improvements to these semiconductor lasers. To overcome the overreliance on simulation tools for the determination of fundamental device characteristics, we report a comprehensive characterization approach to measure all relevant electrical, optical, and thermal parameters of THz QCLs in a consistent manner. Based on the lattice temperature dependence of the QCL output power, the thermal conductivity of the QCLs is extracted. We then retrieve light–current density–lattice temperature maps to decouple the influence of the bias and lattice temperature on the device performance. Applying this method to two sets of QCLs with different active-region designs allowed us to determine the internal quantum efficiency (∼12%), waveguide losses (8–20 cm−1), and transparency current density. A transparency current density greater than 60% of the threshold current density is observed for the two active regions, which demonstrates leakage currents to be the dominant factor limiting THz QCLs efficiency even at low temperatures and for optimized designs employing tall barriers of nominally pure AlAs.