Multimodal Luminescence in Ca2Ga2GeO7:Er3+,Yb3+ Phosphors for Optical Thermometry and Anti-Counterfeiting Applications

Abstract

Lanthanide-activated multifunctional luminescent materials capable of simultaneous spectral conversion and optical sensing are highly desirable for next-generation photonic security and sensing technologies. Herein, we report dual-mode up- and down-conversion luminescence in Er3+/Yb3+ -activated Ca2Ga2GeO7 (CGGO) phosphors synthesized via a solid-state route. Under 980 nm excitation, intense green and red upconversion (UC) emissions are achieved through a two-photon process, exhibiting ~12-fold and ~24-fold enhancement in green and red emissions, respectively, upon Yb3+ co-doping. The material further demonstrates broadband down-conversion (DC) and radioluminescence (RL), highlighting its multimodal photonic response. Temperature-dependent upconversion studies (100–650 K) reveal pronounced thermally governed population redistribution between coupled Er3+ energy levels, enabling reliable optical thermometry with a maximum relative sensitivity of 0.65 % K-1 and excellent Boltzmann fitting (R2 = 0.99). In addition, excitation-dependent dual-color emission enables advanced optical encryption, where printed patterns remain invisible under ambient light but display distinct emissions under UV and NIR excitation, providing high contrast and enhanced anti-counterfeiting capability. The demonstrated combination of dual-mode luminescence, high thermometric sensitivity, 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.