Rafael Valiente, Oliver S. Wenger and Hans U. Güdel
Near-infrared-to-visible photon upconversion process induced by exchange interactions in Yb³⁺-doped RbMnCl₃
Phys. Rev. B 63, 165102/1-11 (2001)                 

Abstract: Crystals of RbMnCl₃ doped with Yb³⁺ exhibit a strong orange luminescence under near-infrared Yb³⁺ excitation at 10 K. The broad band extending from 17200 cm^–1 to 13000 cm^–1 is identified as a ⁴T₁⁶A₁ transition on Mn²⁺. Of the two inequivalent Yb³⁺ 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 Yb³⁺ and Mn²⁺ ions is proposed to account for the observed behavior.