OPTICAL SPECTROSCOPY OF THE DIMER SYSTEM CS3YB2BR9

Single crystals of Cs3Yb2Br9 were grown using the Bridgman technique. Well-resolved absorption, luminescence, excitation, and Raman measurements down to 5 K are reported and analyzed. The crystal-field splitting of Yb3+ in Cs3Yb2Br9 was determined and can be rationalized in terms of a trigonally distorted YbBr63- octahedron. Under near-infrared (NIR) cw-laser excitation of the 2F7/2 --> 2F5/2 transitions, Cs3Yb2Br9 shows up-conversion luminescence in the green spectral region. Conversely, NIR 2F5/2 --> 2F7/2 luminescence can be excited at twice the energy of the 2F7/2 --> 2F5/2 transitions in the green spectral region. These effects are shown to result from cooperative processes within the Yb2Br93- dimers. The cooperative dimer-absorption oscillator strength of 4.0 X 10(-12) corresponds to the square of the NIR absorption oscillator strength of 1.9 X 10(-6), as expected for weak coupling within the Yb2Br93-dimer. By a numerical deconvolution the cooperative dimer luminescence spectrum can be quantitatively correlated with the NIR luminescence spectrum in which reabsorption effects are important.