High Q‐contrast terahertz quantum cascade laser via bandgap‐confined bound state in the continuum

Photonic bound states in the continuum (BICs) are optical modes that remain highly localized despite co-existing with radiating waves in the continuum, attracting considerable attention for both fundamental studies and technological innovations. Conventional single-mode BIC lasers predominantly focus on maximizing the Q-factor of a specific mode, often overlooking the critical role of Q-contrast - the difference in Q-factors between the highest-Q BIC mode and competing modes - which is crucial for achieving stable single-mode lasing. In this study, we present a compact, high Q-contrast BIC laser, enabled by strategically optimizing the alignment of the TM₁ band of the core domain with the shell domain to confine the high Q-factor mode within the core. Using a quantum cascade laser chip operating in the terahertz (THz) regime, this design achieves a Q-contrast ratio of approximately 2.3, resulting in stable single mode lasing across the dynamic region with a side-mode suppression ratio of ∼20 dB. These findings underscore the pivotal role of Q-contrast in photonic lasers, with promising implications for applications in THz lasers, sensors, harmonic signal generators and modulators.