Optical thermometry using phonon-assisted luminescence thermal enhancement of Tm³⁺-doped upconversion under multi-wavelength excitation

Thermally enhanced upconversion luminescence demonstrates distinct advantages for optical thermometry in complex scenarios. In this work, NaYS₂:Tm³⁺ phosphors are synthesized via a solid–gas reaction method, followed by systematic investigation of the upconversion luminescence characteristic. The abundant energy level structure of Tm³⁺ enables its dual functionality as both sensitizer and activator, achieving self-sensitized upconversion luminescence. Under both 808 and 1208 nm excitation, significant thermal enhancement of emission is observed with elevating temperature, which can be attributed to activated phonon-assisted thermal population effect. The temperature-dependent luminescence of NaYS₂:Tm³⁺ is thoroughly analyzed by LIR technique, focusing on the thermally coupled energy levels of ¹G₄₍₁₎/¹G₄₍₂₎ and ³F₂/³F₃. Remarkably, the developed Tm³⁺ single-doped thermometer based on the luminescence thermal enhancement exhibits excellent temperature sensing performance under multi-wavelength excitation of 808 and 1208 nm, exhibiting promising potential for thermometry with wide temperature range and high sensitivity. These findings not only provide a viable strategy for achieving high performance thermometry but also deepen the understanding of thermal enhancement mechanisms in upconversion materials.