Energy transfer mechanisms and non-radiative losses in Nd3+ doped phosphate laser glass: Dopant concentration effect study

Abstract

The aim of the present work is to show a simple quantitative theoretical study on the relaxation mechanisms of the 4F3/2 levels of the Nd3+ ions in the widely used meta-phosphate laser glass system (P2O5–Al2O3–BaO–K2O) for the studied concentration range of the Nd3+ ions from 0.10 × 1020 to 8.15 × 1020 ions/cm3. Auzel's diffusion-limited model for energy transfer has been adopted in the present study for estimating zero concentration level lifetime, τ0 of 4F3/2 levels in the Nd3+ ions present in the studied glass matrix. Primarily, the relaxation mechanisms are dominated by radiative decay (krad) and energy transfer to hydroxyl groups (kOH) present in the glass network structure up to the dopant, Nd3+ concentration 0.78 × 1020 ions/cm3. Later on, with increasing dopant ion concentration, fluorescence from trivalent Nd3+ ions is unfortunately quenched by the interaction between Nd3+ ions, also known as concentration quenching. The concentration quenching of the fluorescence lifetime has been analysed using proposed Inokuti-Hirayama (I–H), Yokota-Tanimoto (Y-T) and Brushtein (B) models and found to be migration-assisted cross-relaxation mechanisms at or beyond 1.47 × 1020 ions/cm3 dopant concentration. The diffusion coefficient, D derived from the Y-T model and the energy migration rate, Wm attained from the B model has demonstrated diffusion-based energy migration initially at about >1.47 × 1020 ions/cm3 Nd3+ concentration and after that, it has switched to a hopping mechanism with further increasing ion concentration, >2.97 × 1020 ions/cm3. Critical quenching concentration, Q and critical radius, R0 for the studied glass composition have also been determined and correlated to the experimental results. No evidence for ion clustering has been found within the studied dopant ion concentration range (0.10 × 1020 - 8.15 × 1020) ions/cm3 with respect to the estimated R0. Finally, the contribution of the individual relaxation process to the experimentally recorded emission spectra and measured lifetime, τexp has been investigated in order to identify the most suitable Nd3+ ion concentration for developing large-aperture high-energy/high-peak-power lasers and optical amplifiers. Therefore, the present work is expected to be significant in aiding to formulate an effective glass composition for construction of compact high-power lasers and optical amplifiers.