Spectroscopy of Nd3+ luminescence centres in Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses

The electron paramagnetic resonance (EPR), optical absorption, luminescence (emission and excitation) spectra as well as luminescence kinetics of the Nd3+ centres in a series of borate glasses with Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd compositions containing 0.5 and 1.0 mol% Nd2O3 have been investigated and analysed. Using EPR and optical spectroscopy data it was shown that the Nd impurity is incorporated into the network of investigated glasses as Nd3+ ((4)f(3), I-4(9/2)) ions, exclusively. All observed f - f electronic transitions of the Nd3+ centres in optical absorption and luminescence spectra of the investigated glasses were identified. Local structure of the Nd3+ luminescence centres in the borate glasses network is proposed. Theoretical oscillator strengths (f(theor)) and phenomenological intensity parameters (Omega(2), Omega(4), Omega(6)) for observed Nd3+ absorption transitions containing 1.0 mol% Nd2O3 have been determined using standard Judd-Ofelt theory. The spectroscopic parameters of relevance to laser applications such as the radiative transitions rate (W-r), luminescence branching ratio (beta), radiative lifetime (tau(rad)), and emission cross-section (sigma(em)) for Nd3+ centres in the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses have been calculated. Luminescence decay curves of the Nd3+ centres from the F-4(3/2) emitting level for all investigated glasses were satisfactory described by single exponential decay. The obtained experimental lifetimes were compared with those calculated and quantum efficiency (eta) of the F-4(3/2) -> I-4(11/2) transition for Nd3+ centres in the investigated glasses were estimated. Based on the obtained results it can be concluded that the further efforts should be made to improve the Nd-doped borate glasses parameters relevant for neodymium solid state lasers with LED pumping.