Bias-dependent intersubband electroluminescence linewidth of quantum cascade lasers

The electroluminescence linewidth is a significant performance indicator of quantum cascade lasers (QCLs) as it determines peak gain, degree of monochromaticity, and wavelength tunability. In this work, we have modeled the intersubband electroluminescence linewidth of QCLs considering the intersubband and intrasubband electron-LO phonon scattering and electron-interface roughness scattering of the two lasing levels. We have studied the change in the electroluminescence linewidth as the applied bias changes. We find that, in QCLs those employ vertical radiative transition in real space, the linewidth decreases as the applied bias increases since the wavefunctions of the two lasing levels overlap more in active region quantum wells with increased bias. By contrast, we find that, in QCLs those employ diagonal radiative transition in real space, the linewidth increases since the wavefunctions of the two lasing levels overlap less in active region quantum wells with increased bias. We also find that the interface roughness is the dominant scattering mechanism to contribute to linewidth. The results that we have found agree with the experimental observations.