Optimization of InP-based waveguides for high-performance mid- infrared quantum cascade lasers

The maximum output power and operating temperature of mid-infrared quantum cascade lasers (QCLs) is determined to a large extent by the optical and thermal properties of the waveguides. To achieve room-temperature, continuous-wave operation of QCLs, we focused on improving the InP-based waveguides. Due to its low refractive index, InP is the material of choice for both transversal and lateral optical confinement. Moreover, the high thermal conductivity of InP facilitates the extraction of heat from the active region. QCL structures with InP top claddings were grown by solid-source molecular beam epitaxy (MBE). To minimize the optical losses, the doping of each waveguide layer was carefully optimized. Deep-etched ridge-waveguides were defined employing either selective wet etching or dry etching. Using a combination of semiconductor overgrowth, epitaxial- side down mounting, Au electroplating, and high-reflectance coating of the back facet, continuous-wave (cw) operation up to 320 K of a QCL emitting at 8 mm was achieved. In pulsed mode at 300 K the devices exhibit threshold current densities of 1.8 kA/cm2, peak output powers of 260 mW, and maximum slope efficiencies of 346 mW/A. In cw mode at 300 K the threshold current density was measured to be 2.5 kA/cm2, and the slope efficiency was 5 mW/A. A maximum output power of 20 mW in cw mode was achieved at 300 K. The devices operate in cw mode up to 320 K, which is the maximum temperature in our experimental setup.