The nonlinear absorption mechanism of CsPbX₃ quantum dots

The nonlinear optical properties of a lead halide perovskite, especially its third-order nonlinear absorption response, enable it to be used in nonlinear optical devices, such as a passive Q-switch, an up-conversion laser, an optical limiter, and an infrared detector. However, there is a lack of systematic research and analysis on the nonlinear absorption mechanism of lead halide perovskites. In this work, CsPbX3 quantum dots with red (CsPbBrI2), green (CsPbBr3) and blue (CsPbClBr2) fluorescence were prepared and systematically tested using a laser with wavelengths of 532 nm, 800 nm and 1064 nm for obtaining a nonlinear absorption response. It is found that both saturation absorption and reverse saturation absorption occur in CsPbX3 quantum dots. When the band gap of the perovskite is smaller than or equal to a single photon energy (Eg = h?), single photon absorption occurs and leads to the Pauli blocking effect, which causes a saturated absorption phenomenon. When the optical band gap of the perovskite is larger than single photon energy and less than or equal to two photon energy (h? < E-g = 2h?), two-photon absorption occurs under the high intensity laser, which causes reverse saturation absorption. The nonlinear absorption performance of CsPbX3 quantum dots is not only affected by the band gap, but also depends on the photon energy of the laser and the stability, crystallinity and uniformity of the quantum dots. CsPbBr3 quantum dots have relatively best nonlinear optical properties among the three quantum dots due to their high stability and size uniformity, as well as suitable band gaps.