Temperature Sensing Using Thermal Population of Low-Lying Energy Levels with (Sm_0.01Gd_0.99)VO₄

A temperature sensing scheme is proposed that is based on the dramatic temperature dependence of the photoluminescence when Sm³⁺ dopants are excited from thermally populated ⁶H_7/2,9/2 levels, rather than the ground level ⁶H_5/2, to the ⁴G_5/2 luminescent level. The scheme has the advantage of eliminating laser heating and background Stokes-type scattering noise. Experimental realization was carried out on a (Sm_0.01Gd_0.99)VO₄ sample by detecting the intensities at 550-580 nm using excitation wavelengths of 601.6 nm (process A) and 644.0 nm (process B) to excite Sm³⁺ to the ⁴G_5/2 level from the ⁶H_7/2 and ⁶H_9/2 levels, which are ca. 1160 and ca. 2270 cm^-1 above the ground ⁶H_5/2 level, respectively. The sensitivities achieved are 1267 K/T² in the temperature range of 183-413 K for process A and 2600 K/T² in 393-603 K for process B. At even higher temperatures (600-800 K), a complementary process C based on the temperature-dependent luminescence decay lifetime resulted in a relative temperature sensitivity increase from 0.52% K^-1 at 640 K to a top value of 3.23% K^-1 at around 750 K. Furthermore, factors affecting the temperature dependence of the luminescence intensities have been successfully explored by taking into account the broadening of the thermally activated energy levels and the quantum efficiency of the luminescent level.