Cooperative luminescence of Yb3+ ion pairs in Li6Y(BO3)3:Yb single crystals

Cooperative luminescence (CL) of Yb3+ pairs was investigated in Yb-doped Li6Y(BO3)3 crystals. It was reliably distinguished from other types of up-conversion luminescence related to the energy transfer from Yb3+ ions to other rare-earth ions unintentionally introduced into the crystal. The CL was observed in the region of 19,000–21,000 cm−1 (526-476 nm) under excitation at about 10,285 cm−1 (972.3 nm) by continuous wave or pulsed lasers. The emission band is nearly featureless at room temperature but develops a pronounced line spectrum at low temperatures. The width of the absorption line corresponding to the 2F7/2 → 2F5/2 transition of the Yb3+ ions at 10,285 cm−1 demonstrates an extreme dependence on temperature, increasing by a factor of 250 from 6 to 300 K. The dependence of the CL intensity on laser irradiation power in the continuous wave mode is interpreted by an analytical model based on the intensity saturation effect strongly related to the peak amplitude of the absorption line. The model updates earlier descriptions of this dependence given in terms of power functions with “anomalous” exponents differing from the value of 2 expected for the luminescence of pair centres and accounts for the observed sharp temperature dependence of the saturation intensity. The decay kinetics of CL was shown to be approximately twice as fast as that of a single-ion luminescence, as expected. However, the decay times were found to be decreasing with increasing Yb concentration, which is explained by excitation escape from Yb3+ pairs due to energy transfer along Yb chains.