Optical spectroscopy of Nd3+-doped cadmium-rich borate glasses for near-infrared laser applications

The structural and spectroscopic properties of Nd3+-activated cadmium-rich borate (inverted) glasses are analyzed for near-infrared laser applications. The evaluation of the optimal glass-emitting sample by the Judd-Ofelt (JO) theory revealed JO parameter values of 4.56 x 10-20 cm2 (Omega 2), 2.56 x 10-20 cm2 (Omega 4), and 3.84 x 10-20 cm2 (Omega 6). The Omega 2 value, along with the experimental oscillator strength, suggested that the cadmium-rich borate glass could provide a more asymmetrical Nd3+ environment than other borate glasses like lithium-strontium-zinc, sodium-calcium, and lithium-lead-aluminum. In addition, the quality spectroscopy factor (chi = Omega 4/Omega 6) of 0.67 suggested that the 4F3/2 -> 4I11/2 emission could be more suitable for laser applications. The stimulated emission cross-section (sigma p), theoretical quantum yield (eta Q), gain bandwidth (sigma p x Delta lambda em), and optical gain (sigma p x tau rad) laser parameters were close to those reported in sodium-calcium-borate, zinc-aluminum-barium-borate, and bismuth-borate glasses, while the non-radiative rate (WNR) and emission intensity saturation (IS) resulted to be lower. The emission spectra, under 808 nm laser excitation, displayed the featured neodymium 4F3/2 -> 4I9/2,11/2,13/2 transitions, being the 4F3/2 -> 4I11/2 (1058 nm) transition the more dominant one, in agreement with the chi parameter value. Nd3+ contents higher than 1.4 mol% led to emission quenching due to the increment of the cross-relaxation and/or energy migration rate. Such processes, according to the Inokuti-Hirayama model, were mainly mediated by electric dipole-dipole interactions within Nd-Nd clusters.