Upconversion luminescence and optical thermometry behaviors of Yb3+ and Ho3+ co-doped NaBi(MoO4)2 single crystal

Upconversion (UC) fluorescence intensity ratio (FIR) technology has emerged as a promising approach for optical thermometry due to its ability to provide non-contact, high-precision temperature measurements. Unlike conventional UC phosphor-based systems, single-crystal UC materials offer enhanced responsiveness and sensitivity for thermal detection, thanks to their structural stability and superior thermal conductivity. In this study, we successfully synthesized an Yb 3+ , Ho 3 ⁺:NaBi(MoO 4 ) 2 (NBM) single crystal with exceptional optical quality using the Czochralski (Cz) growth technique. Comprehensive structural characterization was carried out using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The results confirmed that Yb 3+ , Ho 3+ :NBM crystallizes in a tetragonal structure with the space group I4 1 /a. The emission of different visible light colors under various excitation wavelengths was found to be due to the multiphonon relaxation process. Systematic analysis of power-dependent up-conversion emission spectra conclusively showed that the green and red up-conversion emissions arise from two-photon excitation processes. Moreover, temperature-dependent spectral evolution in the range of 300–500 K was quantitatively analyzed, allowing the calculation of both absolute sensitivity ( S ₐ) and relative sensitivity ( S ᵣ) parameters to evaluate the material's optical thermometric performance. These findings highlight the promising potential of Yb 3+ , Ho 3+ :NBM single crystal for developing high-performance optical thermometry devices.