Giant Three-Photonand Four-Photon Absorption in HybridMAPbBr₃ and FAPbBr₃ Perovskite Nanocrystals

We have investigated the peak intensity and wavelength-dependent optical nonlinear (NLO) behavior of the nanocrystalline (NC) bromide perovskites (MAPbBr₃ and FAPbBr₃) dispersed in <i>n</i>-hexane via the single-beam femtosecond Z-scan technique. The average edge dimensions of the bromide perovskite NCs estimated from the transmission electron microscope were 12.1 ± 1.1 and 14.3 ± 0.8 nm for the MAPbBr₃ and FAPbBr₃ NCs. At the same time, their excitonic peaks appear at 508 and 516 nm with bright photoluminescence emissions at 519 and 529 nm, respectively. Our intensity-dependent Z-scan studies (with ∼50 fs pulses) on FAPbBr₃ NCs have demonstrated an interesting reverse saturable absorption (RSA) in SA behavior with two-photon absorption coefficients (β) of ∼10^–11 cm/W. On the other hand, the MAPbBr₃ NCs exhibited such RSA in SA at lower peak intensities (32, 43, and 54 GW/cm²) and a pure RSA with strong three-photon absorption coefficients (γ) at higher intensities (65, 76, and 87 GW/cm²). In contrast, the wavelength-dependent studies on the NCs with input wavelengths of 1000, 1100, and 1200 nm (∼70 fs, 1 kHz pulses) at a peak intensity of ∼65 GW/cm² exhibited only RSA behavior with strong four-photon absorption coefficients (δ) of ∼ 10^–32 cm⁵/W³. All other characteristic NLO coefficients involving these hybrid NCs, including intensity-dependent refractive indices (<i>n</i>₂), two- and three-photon absorption cross sections (σ₂ and σ₃, respectively), further unveil their promising utility in optoelectronic and related NLO applications.