Microwave Injection\nInduced Broadband Dual-Comb Operation\nin Terahertz Quantum Cascade Lasers with a Stacked Active Region

Broadband dual-comb sources allow retrieving Fourier\nspectra without\nthe need of moving interferometric components, thereby offering new\npossibilities for real-time and high-resolution spectroscopic measurements.\nIn the terahertz (THz) frequency range, broadband quantum cascade\nlasers (QCLs) are the most powerful chip-scale sources for frequency\ncomb and dual-comb operation. However, the limited gain bandwidth\nand chromatic dispersion of THz QCLs have constrained the comb-based\napplications. Here, we demonstrate a THz QCL structure with a stacked\nactive region which shows a calculated gain bandwidth spanning from\n3.35 to 3.83 THz. The measured lasing bandwidth in free-running is\nbroadened from 250 GHz (single active region) to 470 GHz (stacked\nactive region) which agrees well with the simulation. Although broad\nTHz emission can be obtained at relatively high drive currents, the\nlaser normally demonstrates chaotic emission behavior due to the high\nphase noise introduced by the strong electrical pumping. Herein, by\nemploying a microwave injection technique, we observe a clear transition\nfrom chaos to broadband dual-comb operation in the THz QCL with the\nstacked active region. The measured optical bandwidth is ∼400\nGHz, which reaches 73% of the lasing bandwidth of the THz QCL (550\nGHz under resonant microwave injection). The mechanism of the phase\nnoise compression induced by the external microwave injection can\nbe associated with the modulation of group velocity dispersion and\nphase matching in the THz QCL, which further enhances the four-wave\nmixing locking effect for the broadband dual-comb operation. The THz\nQCLs with a stacked active region under resonant microwave injection\nshow the ability to exploit the full lasing bandwidth to generate\nbroadband THz QCL dual-comb sources for practical applications, e.g.,\nbroadband spectroscopy, ranging, communications, etc.