Luminescent cryothermometer based on K$_2$YF$_5$ : Er$^{3+}$

The luminescence spectra of the K$_2$YF$_5$ : Er$^{3+}$ (0.5 at.%) were recorded in the spectral regions of the intermultiplet transitions $^4I_{13/2}\to{}^4I_{15/2}$ (region of about 1.5 $\mu$m, falling within the transparency window of a standard optical fiber) and $^4I_{11/2}\to{}^4I_{15/2}$ (region of about 0.98 $\mu$m) with high spectral resolution at different temperatures. Pairs of spectral lines are proposed for the implementation of a Boltzmann luminescent ratiometric cryothermometer in the temperature ranges of about 20, 40, and 60 K with a relative sensitivity of 8 to 3% K$^{-1}$. It is shown that measuring the half-width of a luminescence line with a wavelength of 1.538 $\mu$m (6500 cm$^{-1}$) provides a simple and reliable method for recording temperatures in the range from 20 to 90 K with a relative sensitivity of 3 to 2% K$^{-1}$.