Enabling Efficient NIR‐III Emission Under 808 nm Excitation Through Tm 3+ to Ho 3+ Energy Transfer

Near‐infrared‐III (NIR‐III, 1700–2500 nm) phosphors are promising for deep‐tissue imaging and long‐wavelength photonic applications but are limited by weak excitation compatibility, low efficiency, and severe thermal quenching. Here, we report a Y 2 GeO 5 :Tm 3+ ,Ho 3+ phosphor system that extends emission beyond 2000 nm using rare‐earth energy‐transfer engineering and enhances performance through a glass‐in‐phosphor (GiP) design. Under 808 nm excitation, Tm 3+ acts as the sensitizer, generating NIR‐III emission near 1835 nm and transferring energy to Ho 3+ , thereby enhancing the 2026 nm emission. The optimized composition achieved a total external quantum efficiency of 4.57%, confirming the effectiveness of the co‐doping strategy. Temperature‐dependent spectroscopy revealed a thermally assisted population of the Tm 3+ emitting level at moderate temperatures, followed by quenching at elevated temperatures. To address power‐dependent instability, we incorporated the optimized phosphor into a GiP composite on an AlN substrate, which delivered higher radiant flux than the powder counterpart under high‐power laser excitation while maintaining stable NIR‐III emission. This work presents a practical and systematic strategy for broadening the emission spectrum of oxide phosphors from the NIR‐II to the NIR‐III region by integrating activator‐level energy matching with composite‐enhanced thermal stability.