Broadband Gain and Loss Characterisation of Quantum Cascade Laser based on Quantum Wells and Dashes using Synchrotron Infrared Radiation

Full text: We use the high-brilliance infrared beam of a synchrotron light source (Swiss Light Source SLS) to investigate broadband transmission spectra (0.08 - 1eV) of quantum cascade (QC) structures with single-mode waveguides at the diffraction limit. The nature of intersubband transitions, which interact only with light polarised along the growth direction, is exploited to decouple waveguide and coupling losses from true intersubband effects, giving access to key parameters of laser devices, such as spectral gain and loss. We present an overview of our characterisation method of QC devices and show recent results obtained for broadband multi-section QC laser devices, emitting around 5 and 8 μm, respectively. We measure gain over more than ∼350 cm-1 at roll-over conditions, which however would only be achieved with an imaginary infinitively long device having zero mirror losses. In addition, we discuss the prospect of improving QC laser performance by embedded quantum dashes. In current devices, nonetheless of strong electroluminescence, we only see broadband losses monotonically increasing with current. (author)