Epitaxial-Side Mounting of Terahertz Quantum- Cascade Lasers for Improved Heat Management

First results on epitaxial-side (epi-down) mounting of terahertz quantum-cascade lasers (QCLs) on sapphire submounts using indium solder are presented. The single-plasmon ridge waveguide lasers emit in the range 3.1–3.3 THz. An epi-down mounting scheme for stressless assembly is realized, which provides enhanced heat dissipation at the corresponding operating temperature and high stability against thermal cycling. The thermal conductance of the devices is improved by 30%, and the maximum continuous-wave operating temperature is increased by 10 K for epi-down mounted lasers achieving output power levels ${>}{\rm 3}~{\rm mW}$ at 55 K.