The Role of Host Material in the Design of Ratiometric Optical Density Meters Based on Cr³⁺ Luminescence in Y₃Al_5‐xGa_xO₁₂:Cr³⁺

Abstract The skillful combination of luminescent thermometry and optical heating has enabled the development of luminescent optical density meters. However, developing such sensing materials with predefined sensing performance requires an understanding of which material parameters play a dominant role in the optical response of the system. In this study, a systematic investigation is conducted on the influence of host material composition and crystal field strength affecting the Cr³⁺ ions on the sensing performance of the ratiometric optical density meter based on the ²E→⁴A₂ to ⁴T₂→⁴A₂ luminescence intensity ratio (LIR) in Y₃Al_5‐xGa_xO₁₂:Cr³⁺. As shown, the LIR reveals thermal variation according to the Boltzmann distribution in these materials over the 160 to 500 K range. However, at higher temperatures, monotonic changes in LIR are still observed. This has enabled the development of optical density meters with high sensitivity, operating over a wide range of power densities (<5500 W cm⁻²). The maximum sensitivity is found for Y₃Al₃Ga₂O₁₂:Cr³⁺, reaching 0.086% W⁻¹ cm² at 2 W cm⁻², which is the highest value reported to date among all ratiometric Cr³⁺‐based optical density meters. It is demonstrated that a strong crystal field favorably influences the thermal sensitivity of the analyzed systems, thereby achieving high sensitivity in the low optical power density range (<1000 W cm⁻²). However, it also limits the operating range of the sensing material. Conversely, to extend the operating range of the optical density meter, materials with intermediate or low crystal field strengths are preferred.