Upconversion luminescence and optical thermometry behaviors of Yb³⁺ and Ho³⁺ co-doped GYTO crystal

Optical thermometry based on the upconversion (UC) luminescence intensity ratio (LIR) has attracted considerable attention because of its feasibility for achievement of accurate non-contact temperature measurement. Compared with traditional UC phosphors, optical thermometry based on UC single crystals can achieve faster response and higher sensitivity due to the stability and high thermal conductivity of the single crystals. In this study, a high-quality 5 at% Yb³⁺ and 1 at% Ho³⁺ co-doped Gd_0.74Y_0.2TaO₄ single crystal was grown by the Czochralski (Cz) method, and the structure of the as-grown crystal was characterized. Importantly, the UC luminescent properties and optical thermometry behaviors of this crystal were revealed. Under 980 nm wavelength excitation, green and red UC luminescence lines at 550 and 650 nm and corresponding to the ⁵F₄/⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively, were observed. The green and red UC emissions involved a two-photon mechanism, as evidenced by the analysis of power-dependent UC emission spectra. The temperature-dependent UC emission spectra were measured in the temperature range of 330-660 K to assess the optical temperature sensing behavior. At 660 K, the maximum relative sensing sensitivity (S_r) was determined to be 0.0037 K^-1. These results highlight the significant potential of Yb,Ho:GYTO single crystal for optical temperature sensors.