Widely repetition-rate-tunable pulse emission in THz quantum cascade lasers

It is well known from basic laser science that the repetition rate of a laser depends on the physical dimensions of the optical cavity. In the case of a Fabry-resonator the free spectral region can controlled either by changing the cavity length or the group refractive index; such changes are normally limited to a few percent. Here, we overcome this fundamental limit by demonstrating a THz quantum cascade laser with a continuously tunable repetition rate from 4 up to 16 GHz. We leverage on the recently demonstrated broadband planarized double metal waveguide taking advantage of the possibility to modulate the entire laser cavity. The microwave driving signal induces a spatiotemporal gain modulation along the entire laser cavity, generating intracavity mode-locked pulses. The pulse length varies depending on the modulation frequency, reaching minimum values below 5 ps when the modulation frequency matches integer multiples of the cavity repetition rate. We performed as well extensive numerical simulations of the modulated laser cavity. Our dynamic model is based on the semiclassical Maxwell-density matrix formalism wh