Microwave Injection Induced Broadband Dual-Comb Operation in Terahertz Quantum Cascade Lasers with a Stacked Active Region

Broadband dual-comb sources allow retrieving Fourier spectra without the need of moving interferometric components, thereby offering new possibilities for real-time and high-resolution spectroscopic measurements. In the terahertz (THz) frequency range, broadband quantum cascade lasers (QCLs) are the most powerful chip-scale sources for frequency comb and dual-comb operation. However, the limited gain bandwidth and chromatic dispersion of THz QCLs have constrained the comb-based applications. Here, we demonstrate a THz QCL structure with a stacked active region which shows a calculated gain bandwidth spanning from 3.35 to 3.83 THz. The measured lasing bandwidth in free-running is broadened from 250 GHz (single active region) to 470 GHz (stacked active region) which agrees well with the simulation. Although broad THz emission can be obtained at relatively high drive currents, the laser normally demonstrates chaotic emission behavior due to the high phase noise introduced by the strong electrical pumping. Herein, by employing a microwave injection technique, we observe a clear transition from chaos to broadband dual-comb operation in the THz QCL with the stacked active region. The measured optical bandwidth is ∼400 GHz, which reaches 73% of the lasing bandwidth of the THz QCL (550 GHz under resonant microwave injection). The mechanism of the phase noise compression induced by the external microwave injection can be associated with the modulation of group velocity dispersion and phase matching in the THz QCL, which further enhances the four-wave mixing locking effect for the broadband dual-comb operation. The THz QCLs with a stacked active region under resonant microwave injection show the ability to exploit the full lasing bandwidth to generate broadband THz QCL dual-comb sources for practical applications, e.g., broadband spectroscopy, ranging, communications, etc.