Optical thermometry with single crystal

Optical thermometry based on upconversion (UC) luminescence of lanthanide is of high sensitivity and rapid response. The basic principle behind UC luminescence is a process which converts low-energy photons (near infrared, NIR) into high-energy photons (visible) through the anti-Stokes mechanism. Firstly, the Yb³⁺ ions absorb low energy photon with wavelength about 980 nm, then Yb³⁺ ions could effectively transfer energy to Ho³⁺ ions due to the energy levels matching between Yb³⁺ ions and Ho³⁺ ions, while the Ho³⁺ ion accepts the energy from two different Yb³⁺ ions, it is able to emit one photon with wavelength of 550 or 650 nm, and the intensity ratio of these two emission bands is called luminescence intensity ratio (LIR). The optical thermometry is realized by the alternation of LIR with temperature changing. This way, the temperature can be deduced by certain LIR.

Researchers led by Prof. Shoujun Ding at Anhui University of Technology (AHUT), China, are interested in optical thermometry with single crystal, the traditional LIR technique relies on detecting the thermally coupled two energy levels such as Er³⁺ (²H_11/2, ³S_3/2) and Tm³⁺ (³F_2,3, ³H₄) which are limited by the energy gaps between these pairs of levels, but in Ho³⁺ ions, the non-thermally coupled energy levels (⁵F₄/⁵S₂ and ⁵F₅) with energy gap of 3000 cm^-1 allow higher sensitivity. The non-contact thermometry technique based on LIR enables the realization of temperature measurement in certain harsh conditions such as intracellular, coalmines and power stations. The single crystal Gd_0.74Y_0.2TaO₄ is selected to hold Yb³⁺ ions and Ho³⁺ ions instead of traditional phosphors, this crystal owns low symmetry and strong crystal field, which is conducive to enhancing the photoluminescence efficiency of Re ions. The maximum S_a and S_r value of this new single crystal achieved 0.0010 and 0.0037 K^-1, respectively. The researchers predict the significant potential of Yb,Ho:GYTO single crystal for applications in optical thermometry. The work entitled “Upconversion luminescence and optical thermometry behaviors of Yb³⁺ and Ho³⁺ co-doped GYTO crystal” was published on Frontiers of Optoelectronics (published on Oct. 31, 2023).