Near-infrared-to-visible photon upconversion process induced by exchange interactions inYb3+-dopedRbMnCl3

Crystals of ${\mathrm{RbMnCl}}_3$ doped with ${\mathrm{Yb}}^{3+}$ exhibit a strong orange luminescence under near-infrared ${\mathrm{Yb}}^{3+}$ excitation at 10 K. The broad emission band extending from 17 200 ${\mathrm{cm}}^{\mathrm{-}1}$ to 13 000 ${\mathrm{cm}}^{\mathrm{-}1}$ is identified as a ${}^4{\overrightarrow{T_1}}^6{A}_1$ transition on ${\mathrm{Mn}}^{2+}.$ Of the two inequivalent ${\mathrm{Yb}}^{3+}$ sites identified by site-selective high-resolution spectroscopy, only one is able to induce efficient upconversion. Excitation with 10-ns pulses indicates that no energy transfer is involved, and the upconversion process consists of a sequence of ground-state absorption and excited-state absorption steps. One- and two-color excitation spectra confirm the occurrence of these two steps. The efficiency of the upconversion process at 10 K and for an excitation power of 200 mW focused by a 53-mm lens is 2 percent. An upconversion mechanism based on exchange interactions between the ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ ions is proposed to account for the observed behavior.