Continuous-wave terahertz quantum cascade lasers based on quasi-flatband BIC and 2D dual-patch arrays.

We present a terahertz quantum cascade laser (THz-QCL) design incorporating photonic structures based on quasi-flatband bound states in the continuum (BICs) enabled by a two-dimensional dual-patch rhombic lattice. The photonic band structure design achieves low group velocity near the Γ-point, which enhances quasi-BIC robustness against quality factor degradation caused by finite device dimensions and fabrication imperfections. The device demonstrated pulsed operation up to 123 K and continuous-wave (CW) operation up to 89 K. Controlled patch asymmetry enabled tunable radiation efficiency, achieving an optimized CW output power of 8.0 mW at 20 K with a Gaussian beam profile. The distributed array design improved thermal dissipation, resulting in a narrow far-field divergence of 7° × 9°. Our work demonstrates the potential of quasi-flatband BICs to improve THz-QCL performance in operating temperatures, output power and beam quality for practical applications.