NIR‐II Luminescence in Cr 4+ Activated CaYGaO 4 toward Non‐Invasive Temperature Sensing and Composition Detection

Near‐infrared (NIR) II region (1000–1700 nm) lighting sources possess special potential for temperature sensing and quantitative composition analysis. However, it is a challenge to tune the luminescence toward such long wavelength NIR‐II emission. Herein, Cr 4+ ions occupation is designed and verified in olivine‐type CaYGaO 4 as [CrO 4 ] tetrahedron by site occupancy confinement. Under 620 nm red light excitation, the phosphors display broadband NIR‐II emission centered at 1330 nm with a full width at half maximum (FWHM) of 233 nm, resulting from spin‐allowed 3 T 2 to 3 A 2 transition of Cr 4+ ions in T d symmetry. The optical thermometry application is demonstrated by decoding the spectral shift and decay times varying with temperature, and the maximum relative sensitivities are 5.82% and 0.78% K −1 , respectively. Furthermore, a flexible film containing CaYGaO 4 :Cr 4+ is fabricated to implement the non‐invasive detections of fillet temperature and NH 3 concentration in ammonia. This work represents a step toward developing next‐generation NIR‐II luminescence materials and widens non‐invasive detection applications.