Giant Three-Photon and Four-Photon Absorption in Hybrid MAPbBr₃ 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 n-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 (n2), two- and three-photon absorption cross sections (σ2 and σ3, respectively), further unveil their promising utility in optoelectronic and related NLO applications.