On-Chip Dual-Comb Source Based on Terahertz Quantum Cascade Lasers Under Microwave Double Injection

High-power and broadband dual-comb sources in the terahertz frequency regime are much in demand for fast and high-resolution spectroscopies. The terahertz quantum cascade laser (QCL) is a powerful radiation source in the frequency range between 1 and 5 THz, which is an ideal candidate for frequency-comb and dual-comb generations in this frequency regime. In this work, we report on broadband on-chip dual-comb sources based on terahertz QCLs emitting at approximately 4.2 THz with a single plasmon waveguide configuration. Due to the fast intersubband transitions in QCLs, the dual-comb lines can be detected by employing the laser self-detection scheme. A finite-element analysis shows that a weak optical coupling between the two laser combs is sufficient for generating dual-comb lines via the nonlinear multiheterodyne frequency beatings. The noise-equivalent power of the QCL detector is also evaluated for different optical powers; furthermore, a microwave double injection is employed to stabilize the comb frequency and broaden the optical comb bandwidth. We find that even with weak radio-frequency powers, the optical comb bandwidth can be significantly broadened from 86 to 166 GHz. The double injection also allows us to investigate the carrier offset noise of QCL combs.