Up conversion of telecom photons in Tm³⁺:CeO₂ Thin Films at Cryogenic Temperatures

We report both photoluminescence and up-conversion in Tm³⁺-doped CeO₂ thin films grown on Si and YSZ substrates. Under ~795 nm excitation, the spectra reveal multiple infrared emissions at 1205, 1210, 1406, 1497, and 1542 nm, corresponding to intra-4f transitions among the ³H₄,³H₅, ³F₄, and ³H₆ manifolds. Conversely, when excited near 1205–1210 nm or 1140–1170 nm, strong anti-Stokes up-converted emission emerges around ~795 nm and ~808 nm, demonstrating upconversion of photons. The upconversion is particularly strong when the excitation laser is resonant with Tm³⁺ transitions, indicating the role of thulium dopants in the process. These results highlight Tm³⁺:CeO₂ as a promising wide-bandgap host system for integrated photonic devices.