Composition and concentration dependence of spectroscopic properties of Nd3+-doped tellurite and metaborate glasses

The spectroscopic properties of tellurite glasses of composition (in mol%) TNKNd: (70 − x)TeO2–15Nb2O5–15K2O–xNd2O3 (x = 0.1, 1.0, 1.5, 2.0 and 2.5) and TNLNd10: 69TeO2–15Nb2O5–15Li2O–1.0Nd2O3 and lithium metaborate glass of composition LBNNd10: 89LiBO2–10Nb2O5–1.0Nd2O3 have been investigated using absorption and emission spectra and decay curve analysis. An energy level analysis has been carried out considering the experimental energy positions of the absorption and emission bands, using the free-ion Hamiltonian model. The spectral intensities have been calculated by using the Judd–Ofelt theory and in turn the radiative properties such as radiative transition probabilities, emission cross-sections, branching ratios and radiative lifetimes have been estimated. The decay curves at the lower concentrations are exponential while they show a non-exponential behavior at higher concentrations (⩾1.0 mol%) due to energy transfer processes. The effective lifetimes for the 4F3/2 level are found to decrease with increase in Nd2O3 concentration for all the glasses under investigation. The non-exponential decay curves have been well-fitted to the Yokota–Tanimoto model with S = 6, indicating that the nature of energy transfer is of dipole–dipole type and energy migration also plays an important role. The results obtained have been compared with Nd3+-doped phosphate, fluorophosphate, lead borate, tellurite, germanate and silicate glasses and Nd3+-doped YAG ceramic and Ca2Nb2O7 crystals.

Research highlights
► The optical properties of different concentrations of Nd3+-doped tellurite (modified with Nb2O5/K2O/Li2O) and 1.0 mol% Nd3+-doped borate (modified with Nb2O5) glasses have been investigated.
► The higher value of the Ω2 parameter for 1.0 mol% Nd3+-doped tellurite glass indicates its stronger Nd3+-ligand covalent bond and/or higher asymmetry around Nd3+ ions.
► The 1.0 mol% Nd3+-doped tellurite glass possess a smaller spectroscopic quality factor which indicates that the 4F3/2 → 4I11/2 laser transition is relatively stronger in this glass than the 1.0 mol% Nd3+-doped borate glass.
► In the present tellurite glasses, the cross-relaxation energy transfer between Nd3+ ions is of dipole–dipole nature and the energy transfer through diffusion also plays a prominent role, both the energy transfer processes increase with increase in Nd3+ ions concentration.