Design and material optimization are used to both decrease the threshold current density and increase the output power for quantum cascade lasers. Waveguides are designed to try and minimize free-carrier and surface-plasmon absorption. Excellent material characterization is also presented, showing excellent control over layer thickness, interface quality, and doping level. Experiments are done to both optimize the injector doping level and to maximize the output power from a single aperture. At 300 K, a threshold current density as low as 1.8 kA/cm(2) is reported, along with peak powers of approximately 2.5 W. Strain-balanced lasers are also demonstrated lambda similar to 5 mum, exhibiting threshold current densities < 300 A/cm(2) at 80 K. These values represent the state-of-the-art for mid-infrared lasers with lambda > 4 mum. (C) 2002 Elsevier Science Ltd. All rights reserved.