Room temperature CW operation of quantum cascade lasers has recently been demonstrated. However, this important performance milestone still remains a technological challenge. QCLs are characterized by high electrical power consumption (3 - 5 W) and low wall plug efficiencies (1 - 4%). This leads to considerable self-heating that can block CW operation. In order to overcome this self-heating device fabrication has to be optimized for high thermal extraction. In this paper we will demonstrate the factors that influence CW operation in quantum cascade lasers (QCLs). We will compare the performance of different device processing design to achieve maximum thermal dissipation and reduced power consumption.
The intersubband light emission of GaAs/GaAlAs quantum cascade lasers is measured under pulsed magnetic fields parallel to the current, up to 60 T. A giant modulation of the laser intensity is observed with complete suppression of the laser emission when the energy spacing between intersubband Landau quantized states matches the GaAs optical phonon energy. When the level separation is not equal to the phonon energy, the laser output increases as a result of quenched phonon emission from the upper subband electrons. In this situation, the laser threshold current was found reduced by a factor of two.
Quantum cascade lasers are coherent light sources in the mid-IR spectral region. They are based on resonant tunneling and optical transitions between discrete energy levels in the conduction band arising form size quantization in semiconductor heterostructures. QCLs have been demonstrated on GaInAs/AlInAs/InP and GaAs/AlGaAs outperforming existing semiconductor laser technologies in the mid-IR spectral range. The present paper reports the realization of a QCL based on GaAs/AlGaAs material designed with an emission wavelength of 9.3 micrometers . Specific properties inherent to this material system and their influence on laser operation are discussed in detail. The paper concludes with the presentation of a new waveguide concept, which offers considerable performance improvements.