biostudies-literatureUnknown14(21)Liu HPhotonic 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 lasNanophotonics (Berlin, Germany)3433-3441https://www.ebi.ac.uk/biostudies/studies/S-EPMC12552860biostudies-literatureHigh Q-contrast terahertz quantum cascade laser via bandgap-confined bound state in the continuum.PMC12552860Liu HLinfield EHWang QCui JDavies AGWang QJLi LfalseHigh 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 las2025-01-01T00:00:00Z2025 Oct2026-06-05T05:11:21.774Z2026-05-13T14:29:19.929ZS-EPMC125528604114209510.1515/nanoph-2024-0757