Multiphoton-Excited Upconversion Luminescence and Amplified Spontaneous Emission from Te⁴⁺-Doped Cs₂SnCl₆ Nanocrystals

High-order nonlinear multiphoton absorption (MPA) is technologically important for a variety of photonic and biological applications owing to its superior advantages over linear absorption and low-order MPA such as greater spatial confinement, larger penetration depth, reduced autofluorescence, and enhanced imaging resolution. However, practical implementation beyond three-photon processes remains notoriously difficult due to the sharp reduction of absorption cross sections with increasing nonlinearity and inherent material instability under high-density irradiation. Herein, we address these challenges through rationally designed Te⁴⁺-doped Cs₂SnCl₆ nanocrystals (NCs), which demonstrate wideband nonlinear responsiveness across 800-2600 nm, allowing achievement of two- to seven-photon absorption (PA) with cross sections outperforming conventional nonlinear optical materials. Particularly, the engineered NCs enable 3PA-excited amplified spontaneous emission (ASE) with an ultralow excitation threshold of 0.22 μJ cm^-2 under a 1300 nm femtosecond-pulsed laser excitation, representing 1-4 orders of magnitude improvement compared to existing nonlinear ASE systems. This work presents the excellent 7PA properties in metal halide NCs, positioning lead-free metal halide NCs promising as efficient light-emitting materials for extreme nonlinear nanophotonics.