Thermal-enhanced near-infrared upconversion luminescence of Er3+ for high-sensitive optical temperature sensing

In this paper, we report the thermal-enhanced near-infrared upconversion luminescence (UCL) from thermal-coupled (TC) energy levels of Er 3+ and a new optical temperature sensing strategy in Er 3+ -doped phosphors. The near-infrared UCL of Er 3+ in the wavelength range of 750–900 nm due to the transitions from the 2 H 11/2 and 4 S 3/2  TC energy levels to the 4 I 13/2 metastable energy level showed a thermally enhanced luminescence behavior at elevated temperatures that is related to the enhanced temperature-dependent non-radiative relaxation. The luminescence intensity ratio (LIR) between the near-infrared ( 2 H 11/2 / 4 S 3/2  →  4 I 13/2 ) and the green ( 2 H 11/2 / 4 S 3/2  →  4 I 15/2 ) UCL of Er 3+ was studied as a function of temperature and demonstrated good optical thermometry behavior based on the classic Boltzmann distribution . The absolute sensitivity based on the LIR between near-infrared and green UCL of Er 3+ was as high as 0.061 K −1 , showing a nearly two orders of magnitude improvement compared to that based on the traditional LIR between two green UCL of Er 3+ . It is anticipated that the near-infrared UCL from TC energy levels of Er 3+ can be used to construct new luminescent thermometry devices with high sensitivity, low temperature uncertainty, and good repeatability.