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

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NIR‑II‑to‑NIR‑II upconversion under 1550 nm telecommunication-wavelength excitation is promising for advanced photonics, but high concentrations of Er 3+ sensitizers inevitably cause severe quenching. Here, we report a lanthanide halide double perovskite enabling efficient NIR-II-to-NIR-II upconversion through a dense Er 3+ excitation network. The highly concentrated Er 3+ sublattice functions as an energy-harvesting and migration platform, sensitizing Tm 3+ , Nd 3+ , and Ho 3+ under 1550 nm excitation. A strong NIR-II Tm 3+ emission is achieved via efficient Er 3+ →Tm 3+ energy transfer. Introduction of Yb 3+ 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.