C. Sirtori

In quantum cascade laser frequency combs, the intensity distribution of the optical spectrum can be split into two well-separated lobes of longitudinal modes that, even when far apart, have a common phase relation and preserve equal frequency separation. The temporal dynamics of two lasers emitting at 4.4 and 8.1 µm operating in this bilobed regime are here investigated. The laser intensity shows a peculiar temporal behavior associated with the spectral features whereby, every half a round-trip, the total emitted power switches from one lobe to the other, with a perfect temporal anti-correlation. The anti-correlation between the lobes is also observed in the intensity noise figure of the emission. This coherent phenomenon arises from gain nonlinearities induced by spatial hole burning and the extremely fast gain dynamics typical of quantum cascade lasers.

The dynamics of electrons submitted to voltage pulses in a thin semiconductor layer is investigated using a kinetic approach based on the solution of the electron Boltzmann equation using particle-in-cell/Monte Carlo collision simulations. The results showed that due to the fairly high plasma density, oscillations emerge from a highly nonlinear interaction between the space-charge field and the electrons. The voltage pulse excites electron waves with dynamics and phase-space trajectories that depend on the doping level. High-amplitude oscillations take place during the relaxation phase and are subsequently damped over time-scales in the range 100–400 fs and decrease with the doping level. The power spectra of these oscillations show a high-energy band and a low-energy peak that were attributed to bounded plasma resonances and to a sheath effect. The high-energy THz domain reduces to sharp and well-defined peaks for the high doping case. The radiative power that would be emitted by the thin semiconductor layer strongly depends on the competition between damping and radiative decay in the electron dynamics. Simulations showed that higher doping level favor enhanced magnitude and much slower damping for the high-frequency current, which would strongly enhance the emitted level of THz radiation.