Achieving Ultra-High Background-Limited Detectivity by a Brillouin Zone Folding Induced Quasi-Bound State in the Continuum

During infrared detection, the thermal radiation from the background generates substantial photon noise and thus severely limit the capability of an infrared detector to identify a target. Going beyond this limitation has been a long-standing challenge in the development of infrared detectors. This paper proposes to break this limitation by creating a narrow photoresponse band with a high peak responsivity to reject the background radiation and enhance the responsivity to the target with characteristic emission lines. This scheme is numerically demonstrated in a dimerized grating integrated quantum well infrared photodetector, based on critical coupling with a Brillouin zone folding induced quasi-bound state in the continuum (BIC). The asymmetric deformation of the grating structure folds the photonic band and generates a quasi-BIC with a tunable high radiation Q factor (Q_R) at the Γ point. By reducing the doping concentration of the quantum wells for a high absorption Q factor (Q_A) and tuning the Q_R to make Q_R = Q_A for critical coupling, a narrowband photoresponse with a high peak responsivity is achieved and the background-limited specific detectivity of 4.55 × 10¹² cm Hz^1/2 W⁻¹ is obtained for a 2π field of view, surpassing the ideal-photoconductor limit by 92 times.