Ultra‐Linear Power Dependency of Photon Release in Cs ₂ NaInCl ₆ : Ho ³⁺ /Yb ³⁺ for Multifunctional Integration of Flexible Temperature Sensing and NIR Bioimaging

Abstract The construction of perovskite‐based luminescent systems for multifunctional integrated platforms via reciprocal energy transfer (ET) cascades emerges as a central focus, but the material design and photophysical control remain a significant challenge. Herein, a strong power‐dependent quantum yield behavior is observed in Cs ₂ NaInCl ₆ : Ho ³⁺ ‐Yb ³⁺ (CNIC: Ho‐Yb) phosphors with ultra‐low phonon energy, and quantified multiphoton upconversion (UC) efficiencies exhibit ultra‐linear enhancement with the increasing excitation power, ensuring practical applicability for UC‐based temperature sensing. Moreover, an efficient quantum cutting process is unveiled in the CNIC: Ho‐Yb, where a single high‐energy photon of Ho ³⁺ is converted into multiple NIR emissions of Yb ³⁺ via energy redistribution under 453 nm laser excitation, offering significant potential for enhancing near‐infrared (NIR) imaging capability in biological tissues. To address the inherent structural and functional limitations of powder‐based materials, the CNIC: Ho‐Yb/polyacrylonitrile (CNIC: Ho‐Yb/PAN) nanofibers are synthesized by electrospinning, and the functional integration platform based on the efficient bidirectional ET is built by coupling 453 and 980 nm lasers to enable selective applications in flexible thermal monitoring and NIR bioimaging. These findings enable next‐generation flexible thermometry platforms with higher sensitivity, signal fidelity, and device‐level applicability, while also paving the way for non‐invasive and high‐contrast deep‐tissue imaging in biomedical diagnostics.