Study of the spectroscopic properties of Thulium doped in Yttrium Ytterbium Gallium Garnet (YYbGG) crystals

Abstract

Rare earth elements have been extensively investigated owing to their unique optical properties, which arise from intra-4f electronic transitions and are strongly influenced by both the dopant ion and the host crystal matrix. Among these materials, thulium-doped yttrium ytterbium gallium garnet (Tm:YYbGG) has attracted attention due to its potential for multifunctional luminescent applications. In this study, Tm:YYbGG crystals synthesised via a nucleation and growth process were systematically investigated to evaluate their spectroscopic and thermometric properties. Under excitation at 800 nm and 980 nm, the material exhibited both downconversion and upconversion luminescence, demonstrating emissions in the visible and near-infrared (NIR) spectral regions. Power-dependent measurements confirmed multiphoton processes under upconversion and down conversion excitation, while temperature-dependent studies were conducted over the range 303–373 K to assess thermometric performance. Six distinct emission bands were observed, including visible emissions at 485 nm (blue), 650 nm (red), and 685 nm (deep red), as well as NIR emissions at 1050 nm, 1450 nm, and 1650 nm. The luminescence intensity ratio (LIR) technique was applied to transitions originating from nonthermally coupled levels to determine temperature sensitivity. The relative sensitivity values were found to range from 0.72 to 1.02 % K−1 under downconversion excitation and from 1.64 to 2.26 % K−1 under upconversion excitation. The enhanced sensitivity observed under upconversion excitation indicates improved thermometric performance. These findings demonstrate that Tm:YYbGG crystals exhibit stable and efficient luminescence behaviour, highlighting their potential as promising candidates for optical temperature sensing applications