Multiphonon Relaxation Inhibition and Photon Cutting Multiplication in Cs ₂ NaYCl ₆ : Ho ³⁺ /Er ³⁺ for Information Encryption and NIR Bioimaging

The perovskite‐based multifunctional integrated light‐emitting platform has garnered significant attention due to the ultralow phonon energy and flexible lattice plasticity. Achieving both high‐efficiency multiband light emission and excellent thermal stability for extreme conditions remains a critical challenge in system design and performance regulation. Herein, Ho 3+ and Er 3+ co‐doped Cs 2 NaYCl 6 crystals suppress multiphonon relaxation, enabling precise control of the cross‐relaxation between ions, thereby achieving mutual sensitization and multicolor visible fluorescence. Simultaneously, a parallel near‐infrared (NIR) channel is activated through quantum cutting, where a high‐energy photon is converted into two NIR photons via successive resonant energy transfer from Ho 3+ to Er 3+ , achieving a high internal quantum yield of 147.6%. Notably, the material exhibits high thermal sensitivity and exceptional anti‐thermal quenching, with a luminescence intensity of 145.0% at 433 K relative to that at 313 K, showcasing tremendous potential for high‐temperature optical sensing. The dynamic multicolor luminescence and highly efficient NIR emission are successfully integrated through a dual‐channel luminescence mechanism, which establishes a multidimensional platform for optical encryption and bioimaging based on multifunctional luminescent system and high thermal stability, providing an innovative strategy for developing high‐security anti‐counterfeiting and deep tissue imaging.