Spectral properties and anti-Stokes luminescence of TeO$_{2}$–BaF$_{2}$ : Ho$^{3+}$, Ho$^{3+}$/Yb$^{3+}$ ceramics and glass excited by 1.9-$\mu$m radiation of a Tm:LiYF$_4$ laser

We demonstrate the up-conversion of Tm:LiYF$_4$ infrared (IR) laser radiation with 1908-nm wavelength into visible light with a spectral maximum at 650 nm by ceramics with a composition of (100-$x$)TeO$_{2}$–$x$BaF$_{2}$–1 wt% HoF$_{3}$–$y$YbF$_{3}$, where $x$ = 20, 30, or 40 mol % and $y$ = 0 or 0.5 wt %. The samples of 60TeO$_{2}$–40BaF$_{2}$–1 wt% HoF$_{3}$–0.5 wt% YbF$_{3}$ exhibited anti-Stokes luminescence at a threshold radiation power density of 1.0–1.5 W cm$^{-2}$.