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

Abstract

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 × 10^–20 cm² (Ω₂), 2.56 × 10^–20 cm² (Ω₄), and 3.84 × 10^–20 cm² (Ω₆). The Ω₂ 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 (χ = Ω₄/Ω₆) of 0.67 suggested that the ⁴F_3/2 → ⁴I_11/2 emission could be more suitable for laser applications. The stimulated emission cross-section (σ_p), theoretical quantum yield (η_Q), gain bandwidth (σ_p × Δλ_em), and optical gain (σ_p × τ_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 (W_NR) and emission intensity saturation (I_S) resulted to be lower. The emission spectra, under 808 nm laser excitation, displayed the featured neodymium ⁴F_3/2 → ⁴I_9/2,_11/2,_13/2 transitions, being the ⁴F_3/2 → ⁴I_11/2 (1058 nm) transition the more dominant one, in agreement with the χ 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.