Time- and frequency-resolved dynamics of resonant two-photon terahertz quantum cascade lasers

Two-photon terahertz (THz) quantum cascade lasers (QCLs) can immensely improve the conventional applications of THz frequency sources and open windows to new applications due to their ability to generate quantum-entangled twin photon beams. The rich intrinsic non-linearity in a two-photon THz QCL arises from cascaded photon transitions, leading to time-resolved and frequency-resolved dynamics that are less predictable. This work investigates the time- and frequency-resolved dynamics of a resonant two-photon THz QCL by numerically solving the coupled Maxwell-Bloch equations. Our findings indicate that the output intensity is significantly higher, and the emission spectra are much broader in a two-photon THz QCL than in a conventional one-photon THz QCL. Additionally, a pronounced Risken-Nummedal-Graham-Haken (RNGH) instability onsets for a resonant two-photon QCL at a smaller pumping current. We also observe an increase in the number of lasing modes, as well as a higher power per line in a resonant two-photon QCL. Furthermore, Rabi splitting is observed in the emission spectra at high pumping currents.