Spectroscopic analysis of Nd3+-doped cadmium-vanadate invert glasses for near-infrared laser applications

Abstract

Nd3+ activated CdO-V2O5 invert glasses were synthesized by the well-stablished melt quenching technique, using starting proportions of 95.0 and 5.0 mol% of CdO and V2O5, respectively. The XRD patterns revealed that the glass system remains amorphous up to 4.0 mol% of Nd3+. Such fact was mainly attributed to the low V2O5 content, which avoided the segregation of additional phases. The minimal addition of 0.1 mol% of Nd3+ reduced the direct and indirect glass bandgap energies from 2.79 to 2.66 eV, and from 2.48 to 2.13 eV, respectively. For higher Nd3+ contents, the bandgap energy was recovered, reaching values of 2.78 and 2.40 eV for direct and indirect allowed transitions, respectively. This fact was associated with a Burstein-Moss like effect, observed in semiconductors highly doped. The tail of the absorption edge revealed that the Urbach energy systematically grows with the addition of Nd3+, because of the creation of localized states into the bandgap. The Judd-Ofelt (JO) parameters obtained by least-square method from the experimental and theoretical oscillator strengths, were found in the Ω2 = 5.24-11.04 × 10−20 cm2, Ω4 = 2.26-4.47 × 10−20 cm2, and Ω6 = 2.85-6.28 × 10−20 cm2 range. Such values are close to those reported in other popular glass systems. The stimulated emission cross-section peak (σp) values calculated for the glass sample doped with 2.0 mol% resulted to be 0.26 × 10−20 cm2 (Nd3+: 4F3/2 → 4I9/2) and 0.81 × 10−20 cm2 (Nd3+: 4F3/2 → 4I11/2). The Nd3+ emission spectra, recorded upon 585 nm excitation (Nd3+: 4I9/2 → 4G5/2 + 2G7/2), showed the near-infrared Nd3+ emission bands at 881 nm (Nd3+: 4F3/2 → 4I9/2), 1063 nm (Nd3+: 4F3/2 → 4I11/2) and 1341 nm (Nd3+: 4F3/2 → 4I13/2), being dominated for that coming from the Nd3+: 4F3/2 → 4I11/2 transition. The overall emission reached the optimum intensity at 2.0 mol% of Nd3+, with a maximum quantum efficiency (ηQE) of 0.23. From the emission spectra important laser parameters such as gain bandwidth (σEMI(λp) × Δλem) and optical gain (σEMI(λp) × τR), were determined for the sample with the highest ηQE value. The σEMI(λp) × Δλem values resulted to be 11.3 and 32.0 × 10−27 cm3 for the Nd3+: 4F3/2 → 4I9/2, 11/2 transitions, respectively. The σEMI(λp) × τR parameter values were 11.1 and 35.3 × 10−25 cm2s for Nd3+: 4F3/2 → 4I9/2,11/2 transitions, respectively. The Inokuti-Hirayama model suggested that the Nd3+ cross-relaxation process might be dominated by an electric dipole-dipole interaction, inside Nd3+-Nd3+ clusters.