Strong red upconversion luminescence of CaAl2O4: Yb3+/Er3+ for optical thermometry

A CaAl2O4 upconversion luminescent material co-doped with Yb3+ and Er3+ ions was successfully synthesized via a high-temperature solid-state method. The CaAl2O4: Yb3+/Er3+ phosphor, when excited by a 980 nm near- infrared laser, shows weak green emissions at 542 nm and 554 nm (from 2H11/2 and 4S3/2 to 4I15/2), and stronger red emissions at 653 nm and 670 nm (from 4F9/2 to 4I15/2). The optimal doping concentrations for Yb3+ and Er3+ were determined to be 0.07 and 0.02, respectively, with a red-to-green emission intensity ratio reaching as 11.9. In this material, the dominant excitation pathway for the upconversion process involves energy transfer through Yb3+ (2F5/2) + Er3+ (4I13/2) -> Yb3+ (2F7/2) + Er3+ (4F9/2). In addition, CaAl2O4: 0.07Yb3+/0.02Er3+ phosphor has good optical temperature measurement at 298-673 K with a maximum relative sensitivity of 0.64 % K-1 (673 K).