2D Perovskites with Giant Excitonic Optical Nonlinearities for High‐Performance Sub‐Bandgap Photodetection

Two‐dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub‐bandgap photodetection by employing excitonic effects in 2D Ruddlesden–Popper‐type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two‐photon absorption (D‐2PA) coefficients as giant as 0.2–0.64 cm MW−1. 2D RPP‐based sub‐bandgap photodetectors show excellent detection performance in the near‐infrared (NIR): a two‐photon‐generated current responsivity up to 1.2 × 104 cm2 W−2 s−1, two orders of magnitude greater than InAsSbP‐pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered‐RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D‐2PA coefficients (β[001]/β[011] = 2.4, 70% larger than the ratios reported for zinc‐blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C4H9NH3)2(CH3NH3)Pb2I7 can be utilized for three‐photon photodetection in the NIR region. Single‐crystalline 2D Ruddlesden–Popper‐type halide perovskites (RPPs) are demonstrated to exhibit extremely large two‐photon absorption coefficients in the near‐IR due to near‐resonance with 2D excitons. Efficient polarization‐resolved sub‐bandgap photodetection is realized at room temperature by utilizing 2D‐RPP two‐photon detectors, which opens avenues for future exploration of nonlinear optics in this class of hybrid quantum materials.