Quantum-cascade lasers without injector regions

We present the status of quantum-cascade lasers without injector regions, based on a four- and five-level staircase, respectively. First lasers were realized at a wavelength of ~ 10 mm. By applying an optimized design, we achieved high performance injectorless quantum-cascade lasers emitting at ~ 6.7 mm. On the basis of this design, we investigated the influence of doping density and the number of periods of active sections. A sample with a doping sheet density of 2.5x10 10 cm -2 and 60 periods of active sections shows record low threshold current densities of 0.75 kA/cm 2 at 300 K. Recently, we have further extended the wavelength range even down to ~ 4.2 mm.