Bright Er 3+ ‐Activated Multi‐Band Luminescence in Yb 3+ ‐Rich Eulytite Phosphate for Bio‐Imaging and Dynamic Authentication

ABSTRACT The integration of the second nearinfrared window (NIR‐II, 1000–1700 nm) with visible light (400–700 nm) offers a powerful route for bioimaging, optical communication, and advanced security. However, realizing bright and polymorphic multiband emission within a single material remains challenging. Herein, Er 3+ doped eulytitetype Ba 3 Yb(PO 4 ) 3 phosphors have been rationally designed and synthesized to harness the rich energy level architecture of Er 3+ ions, yielding simultaneous bright blue (450 nm), green (525 and 550 nm), and NIR‐II (1550 nm) emissions. Tailored Yb 3+ →Er 3+ energy transfer delivers an exceptional NIR‐II quantum yield of 80%, enabling 12 mm tissue penetration in bovine tissue, roughly twice that of conventional NIR‐II probes. The Ba 3 Yb(PO 4 ) 3 : Er 3+ /polydimethylsiloxane (PDMS) elastomer film exhibits both mechano‐ and radioluminescence, featuring an atypical blue emission centered at ∼ 450 nm. This behavior is attributed to a strong triboelectric field arising from the high dielectric constant of the Yb 3+ rich eulytite host (ε ≈ 13‐14) under ultralow excitation, which suppresses the 4 F 5/2 → 2 H 11/2 pathway and promotes the 4 F 5/2 → 4 I 15/2 transition. These breakthroughs in bright, stimuli‐responsive, multi‐band emissions define a new paradigm for the design of versatile optical materials, offering broad opportunities in non‐contact sensing, dynamic authentication, and next‐generation photonic communication.