Broadband white radiation in Yb$^{3+}$- and Er$^{3+}$-doped nanocrystalline powders of yttrium orthophosphates irradiated by 972-nm laser radiation

The up-conversion luminescence of Er$^{3+}$ from the ${}^2H_{11/2}$, ${}^4S_{3/2}$ and ${}^4F_{9/2}$ levels in nanocrystals of Y$_{0.95(1-x)}$Yb$_{0.95x}$Er$_{0.05}$PO$_4$ ($x=0$, $0.3$, $0.5$, $0.7$, $1$) orthophosphates activated with Er$^{3+}$ ions has been studied under the excitation of Yb$^{3+}$ ions to the $^2$F$_{5/2}$ level by $972$-nm cw laser radiation. Broadband radiation in the wavelength range of $370$–$900$ nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of $972$ nm at a pulse repetition frequency of $10$ Hz and a duration of a pulse of $15$ ns. Experimental data indicating that this radiation is thermal in nature have been presented.