Optical spectroscopy of the dimer system Cs3Yb2Br9

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Abstract
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 4.0×10−12 corresponds to the square of the NIR absorption oscillator strength of 1.9×10−6, 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.