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

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 Omega(2) = 5.24-11.04 x 10(-20) cm(2), Omega(4) = 2.26-4.47 x 10(-20) cm(2), and Omega(6) = 2.85-6.28 x 10(-20) cm(2) range. Such values are close to those reported in other popular glass systems. The stimulated emission cross-section peak (sigma(p)) values calculated for the glass sample doped with 2.0 mol% resulted to be 0.26 x 10(-20) cm(2) (Nd3+: F-4(3/2) -> I-4(9/2)) and 0.81 x 10(-20) cm(2) (Nd3+: F-4(3/2) -> I-4(11/2)). The Nd3+ emission spectra, recorded upon 585 nm excitation (Nd3+: I-4(9/2) -> (4)G(5/2) + (2)G(7/2)), showed the near-infrared Nd3+ emission bands at 881 nm (Nd3+: F-4(3/2) -> I-4(9/2)), 1063 nm (Nd3+: F-4(3/2) -> I-4(11/2)) and 1341 nm (Nd3+: F-4(3/2) -> I-4(13/2)), being dominated for that coming from the Nd3+: F-4(3/2) -> I-4(11/2) transition. The overall emission reached the optimum intensity at 2.0 mol% of Nd3+, with a maximum quantum efficiency (eta(QE)) of 0.23. From the emission spectra important laser parameters such as gain bandwidth (sigma(EMI)(lambda(p)) x Delta lambda(em)) and optical gain (sigma(EMI)(lambda(p)) x tau(R)), were determined for the sample with the highest eta(QE) value. The sigma(EMI)(lambda(p)) x Delta lambda(em) values resulted to be 11.3 and 32.0 x 10(-27) cm(3) for the Nd3+: F-4(3/2) -> I-4(9/2), (11/2) transitions, respectively. The sigma(EMI)(lambda(p)) x tau(R) parameter values were 11.1 and 35.3 x 10(-25) cm(2)s for Nd3+: F-4(3/2) -> I-4(9/2,11/2) 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.