Bias dependence of gain spectrum and output emission characteristics of two phonon resonance design quantum cascade lasers

Quantum cascade lasers (QCLs) are the most promising light sources for optical spectroscopy in the molecular finger-print mid-infrared region. To this purpose, QCLs are required to be wavelength selective and the change of emission wavelength needs to be known beforehand as the operating conditions, such as the applied bias changes. In this paper, we study the change in the gain spectrum and the output emission characteristics of QCLs as the applied bias changes. We find that the gain spectrum becomes narrower and the wavelength of the peak gain is blue shifted as the applied bias increases. As a result, the emission wavelengths at particular biases change sharply as the lasing hops between neighboring modes. For the QCL that we study, we find that the emission wavelength tunes from 8.862 μm to 8.538 μm as the applied bias changes from ~ 44 kV/cm to 64 kV/cm.