Multimodal luminescence in Ca2Ga2GeO7:Er3+,Yb3+ phosphors for optical thermometry and anti-counterfeiting applications

Abstract

Lanthanide-activated luminescent materials capable of simultaneous spectral conversion and optical sensing are highly desirable for next-generation photonic security and sensing technologies; however, many existing systems suffer from limited multimodal response, moderate thermometric sensitivity, or insufficient excitation-selective optical encoding capability. In this work, we demonstrate that the dual-mode up- and downconversion luminescence in Er³⁺/Yb³⁺-activated Ca₂Ga₂GeO₇ (CGGO) phosphors synthesized via a solid-state route effectively addresses these limitations. Under 980 nm excitation, the material exhibits intense green and red upconversion (UC) emissions through a two-photon process, with Yb³⁺ co-doping leading to a ∼12-fold and ∼24-fold enhancement in green and red UC, respectively, while also showing broadband downconversion (DC) and radioluminescence, enabling a multimodal photonic response. Temperature-dependent upconversion studies (100–650 K) reveal strong thermally governed population redistribution between coupled Er³⁺ energy levels, yielding a maximum relative sensitivity of 0.65% K⁻¹ and excellent fitting based on a modified Boltzmann distribution (R² = 0.999). Additionally, excitation-dependent dual-color emission allows advanced optical encryption in which printed patterns remain invisible under ambient light but display distinct DC and UC signatures under UV and NIR excitation, providing high contrast and enhanced anti-counterfeiting capability. The demonstrated combination of dual-mode luminescence, high-sensitivity optical thermometry, and excitation-selective optical encoding establishes CGGO:Er³⁺,Yb³⁺ as a versatile photonic platform for non-contact temperature sensing, secure information encryption, and next-generation anti-counterfeiting technologies.