The optical properties of the Nd³⁺ ion in NdGaO₃ and LaGaO₃:Nd: Temperature and concentration dependence

The energies of most of the f-f crystal-field levels of NdGaO₃ and neodymium-doped LaGaO₃ crystals are given. The oscillator strengths of the J to J' Nd³⁺ absorption bands and of some zero-phonon transitions between individual Stark crystalline states have been determined at temperatures between 9 and 300 K for both compounds. Within the frame of the Judd-Ofelt theory we have calculated the Nd³⁺ intensity parameters from the 300 K spectra. A comparison between the stoichiometric and the diluted material reveals a very large concentration enhancement of vibronic bands and an unusual and strong temperature dependence of some zero-phonon oscillator strengths in NdGaO₃. These phenomena point towards the existence of an enhanced electron-phonon interaction presumably related to the excited configuration, superexchange interaction between adjacent Nd ³⁺ ions in the stoichiometric crystal. In LaGaO₃:Nd crystals we detected ⁴F_3/2 emission with a quantum efficiency close to unity and a temperature-independent lifetime of approximately=250 mu s. This emission is completely quenched in NdGaO ₃.