NIR-II-to-NIR-II upconversion and covert optical encryption in lanthanide halide double perovskites

NIR‑II‑to‑NIR‑II upconversion under 1550 nm telecommunication-wavelength excitation is promising for advanced photonics, but high concentrations of Er³⁺ sensitizers inevitably cause severe quenching. Here, we report a lanthanide halide double perovskite enabling efficient NIR-II-to-NIR-II upconversion through a dense Er³⁺ excitation network. The highly concentrated Er³⁺ sublattice functions as an energy-harvesting and migration platform, sensitizing Tm³⁺, Nd³⁺, and Ho³⁺ under 1550 nm excitation. A strong NIR-II Tm³⁺ emission is achieved via efficient Er³⁺→Tm³⁺ energy transfer. Introduction of Yb³⁺ as an intermediate energy bridge effectively overcomes energy mismatch and further enhances 1220 nm emission, while cross-relaxation within the Er-Yb system enables excitation-dependent multicolor tunability. The synergy of visible and NIR-II emissions allows multilevel optical encoding, enabling concealed anticounterfeiting and wavelength-selective encryption.