Antithermal-Quenching\nNear-Infrared Emitting Lu<sub>2</sub>SrAl<sub>4</sub>SiO<sub>12</sub>:Cr<sup>3+</sup> Garnet-Type\nPhosphor for High-Resolution Nonvisual Imaging

Near-infrared (NIR) light allows fast and nondestructive\ndetection\nwith deep penetration into biological tissues and is widely used in\nfood inspection, biomedical imaging, night vision security, and other\nfields. Cr<sup>3+</sup>-doped NIR first region (NIR-I) phosphors have\nmany interesting features that have attracted a lot of attention recently.\nHowever, practical issues, such as low photoluminescence quantum efficiency\nand poor thermal stability, need to be addressed. We synthesized Cr<sup>3+</sup>-doped Lu<sub>2</sub>SrAl<sub>4</sub>SiO<sub>12</sub> garnet-type\nphosphors using a high-temperature solid-state reaction method. Under\nblue-light (@ 430 nm) excitation, the phosphors exhibited broadband\nNIR-I emission in the range of 600–1000 nm, with an emission\npeak at 710 nm. This system is unique, as the emission had multipeak\nsharp-line superposition and the Cr<sup>3+</sup> ions were situated\nin a relatively strong crystal field environment, as opposed to a\nweak crystal field environment for other matrix. The optimal doping\ncontent of Cr<sup>3+</sup> ions was 2 mol %, and its internal quantum\nefficiency was ∼76.8%. Surprisingly, this NIR phosphor showed\nan antithermal quenching effect, and the integrated luminescence intensity\nof NIR-I emission measured at 573 K was 206.3% of that measured at\n303 K. We found that the antithermal quenching of NIR-I luminescence\nwas caused by the extremely low thermal expansion coefficient and\nrigid structure of the Lu<sub>2</sub>SrAl<sub>4</sub>SiO<sub>12</sub> matrix in the temperature range, as well as the weakening of electron–phonon\ncoupling with increasing temperature. The optimized phosphor was packaged\nwith a blue chip into a NIR phosphor-converted light-emitting diode\ndevice. The light source device showed an output power of 119.02 mW\nand an electro-optical conversion efficiency of 11.02% under a driving\ncurrent of 300 mA. The application potential of this NIR phosphor\nwas demonstrated in the field of high-resolution nondestructive imaging.