Seven-photon absorption from Na⁺/Bi³⁺-alloyed Cs₂AgInCl₆ perovskites

Nonlinear multi-phonon (2-7) absorption in the Na⁺/Bi³⁺-alloyed Cs₂AgInCl₆ lead-free double perovskites with ∼100% photoluminescence quantum yield and superior stability is observed for the first time, which can be pumped by a femtosecond laser in a wide spectral range (800-2600 nm). First-principles calculations verify that the parity-forbidden transition from the valence band maximum and conduction band minimum (at the Γ point) is not broken by Na⁺/Bi³⁺ doping, and strong optical band-to-band absorption occurs at the L&X points. Time-resolved emission spectra evidence that single-photon and multi-photon pumping leads to the same self-trapped exciton transition and high-order nonlinear absorption will not induce a remarkable thermal effect. Finally, we demonstrate that the Cs₂Na_0.4Ag_0.6In_0.99Bi_0.01Cl₆ DP shows great potential for next-generation wavelength-selective and highly sensitive multiphoton imaging applications.