Luminescent thermometry (LT) is a
technique that enables contactless temperature determination based
on temperature dependent luminescence of phosphors. Among different
LTs that have been described in the literature so far, the luminescence
intensity ratio (LIR) based thermometers have shown the highest application
potential. Nevertheless, to determine accurate temperature, ratiometric
method encounters technical restrictions related to the need for spectral
separation of temperature dependent emission bands. An alternative
ratiometric approach can exploit the intensity of a single emission
band being excited in two ways related to ground state absorption
(GSA) and excited state absorption (ESA). In this work, this approach,
i.e., luminescent thermometry involving ESA process in LaPO4:Nd3+ nanocrystals, was demonstrated. Thermal energy delivered
to the system was responsible for partial population of Nd3+:4I11/2 level, which enabled non-GSA-absorption
of 1060 nm excitation line and resulted in appearance of strongly
temperature dependent emission band at 890 nm. The further temperature
increase favored population of higher laying levels, resulting in
observation of 810 and 750 nm emission. On the other hand, the intensity
of the emission band at 890 nm being excited in a resonant GSA way
via the 808 nm line was strong and barely dependent on temperature,
thus serving as a reference. Therefore, three luminescence intensity
ratio (LIR
i
) equations were defined to
determine temperature in a contactless way. The subsequent LIRs were
calculated as the ratios of emission intensities at 890, 810, and
750 nm being excited in a non-GSA-resonant ESA-resonant way normalized
to the band at 890 nm excited with 808 nm line (through GSA). The
highest relative sensitivities were unprecedentedly high and reached S
1 = 7.19%/°C at 30 °C, S
2 = 3.04%/°C at 100 °C, and S
3 = 4.35%/°C at 180 °C for the subsequent LIR
i
ratios.