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 <italic>Q</italic> factor (<italic>QR</italic>) at the Γ point. By reducing the doping concentration of the quantum wells for a high absorption <italic>Q</italic> factor (<italic>QA</italic>) and tuning the <italic>QR</italic> to make <italic>QR</italic> = <italic>QA</italic> for critical coupling, a narrowband photoresponse with a high peak responsivity is achieved and the background‐limited specific detectivity of 4.55 × 1012 cm Hz1/2 W−1 is obtained for a 2π field of view, surpassing the ideal‐photoconductor limit by 92 times. [ABSTRACT FROM AUTHOR]