Luminescence and temperature properties of Y₂O₃: Ho³⁺/Yb³⁺ millimeter crystals obtained by laser annealing

FIR temperature measurement based on thermally coupled levels of rare earth ions has been widely studied, but its sensitivity is limited by the thermal coupling levels’ spacing. In this letter, a transition from non-thermally coupled levels of Ho³⁺ to thermally coupled levels is reported for the first time. It can effectively improve the sensitivity of temperature measurement based on thermal coupling levels. The Ho³⁺/Yb³⁺ co-doped Y₂O₃ millimeter crystals have been prepared by laser annealing. The fluorescence properties of laser-annealed Ho³⁺/Yb³⁺ co-doped Y₂O₃ millimeter crystals were investigated as a function of temperature. It was found that the I_667nm/I_549nm show non-thermal coupling below 453K, but changes to thermal coupling above 453K. The maximum relative sensitivity of I_667nm/I_549nm in thermal coupling is greater than the relative sensitivity of any known thermal coupling energy levels of Ho³⁺. It provides a new way to improve the sensitivity of thermally coupled FIR temperature measurement.