Quantum-limited linewidth in THz quantum cascade lasers

Quantum cascade lasers (QCLs) are powerful testing grounds for the fundamental physical parameters determined by their quantum nature. The associated wealth of unique physical properties makes the QCL a subject of extensive research, especially across the far-infrared, where the electron-phonon relaxation dynamics, the gain mechanisms and the related intrinsic QCL features need to be extensively investigated. Here, we report a complete overview of the frequency-noise power spectral density of a THz QCL, giving an experimental and analytical evaluation of its intrinsic linewidth and a full investigation of the physics beyond it, offering a new perspective of use of ultra-narrow THz QCL sources as a metrological grade tool for a widespread range of photonic applications.