Luminescent thermometry and up-conversion in LiNbO$_3$: Er crystals

This paper presents the results of studying the temperature dependence of up-conversion “green” photoluminescence (PL) in a congruent LiNbO$_3$: Er$^{3+}$ crystal excited by a laser diode operating at a wavelength of 808 nm. The study covers the temperature range from 77 to 413 K. Our results show that the temperature-dependent intensity ratio of green PL emitted by thermally bound states of the erbium ion ($^2$H$_{11/2}$ and $^4$S$_{3/2}$) can be used as a thermometric sensor. It is noteworthy that LiNbO$_3$: Er$^{3+}$ exhibits absolute temperature sensitivity (0.0044–0.0055 K$^{-1}$), which is one of the highest for materials activated by rare-earth ions, in the temperature range of 20–50$^\circ$C, which is relevant for physiological measurements. This observation suggests that materials based on LiNbO$_3$: Er$^{3+}$ have great prospects for optical (contactless) temperature measurements in biological media.