Undamped Effective Rabi Frequency Triggers Harmonic Frequency Combs in ultrafast Semiconductor Laser

The phenomenon of the spontaneous generation of Optical Frequency Combs (OFCs) in ultrafast semiconductor lasers, i.e. associated with a carriers decay time (τe) smaller or comparable with the photon life time (τp) [1], has been studied for more than a decade. Attention has been focused on unipolar lasers as Quantum Cascade Lasers for applications in the Terahertz and mid-infrared spectral regions in the fields of e.g. high resolution molecular spectroscopy and long-range free space optical communications [2,3]. OFCs are the results of CW instability leading to multimode competition and eventually to mode-locking through an efficient cascaded four wave mixing. Recently the unconventional unidirectional QCLs ring configuration attracted lot of interest since it is suitable for photonic integration and allows for the description of OFCs formation in terms of the paradigmatic complex Ginzburg-Landau equation [4]. On the other hand not only dense OFCs have been reported, but also Harmonic Frequency Combs (HFCs) where the separation between adjacent optical lines is an integer multiple of the FSR (also denoted as HFC order). So far, the origin and properties of HFCs remain unclear.