Interplay between absorbance, phonon emission and random laser performance in NdxY1.00-xAl3(BO3)4 nanocrystalline powders

Abstract

The NdxY1.00-xAl3(BO3)4 (0.05 ≤ x ≤ 1.00) structures are very attractive for laser and random lasers (RLs). Besides the large quantum efficiency, they allow the observation of self-induced parametric nonlinear phenomena such as second-harmonic and sum-frequency generation. The trivalent neodymium ions (Nd3+) possess several resonant ground-state absorption transitions, which provide laser/RL emission at around 1060 nm. The excitation and emission pathways can be simplified to a 4-level scheme, which explains the success of Nd3+ to development of lasers/RLs. One might wonder whether excitation at the absorption transition with the largest cross-section (749 nm or 808 nm) would result in the highest efficiency. However, the difference between the excitation and laser/RL photon energies, also called quantum defect, increases the phonon occupation number of the crystalline lattice, which can be detrimental to laser action. This intrinsic characteristic increases population losses through phonon transitions in the upper level of the laser/RL electronic transition and increases the population of the lower state by phonon excitations from the ground state. Both effects decrease the population inversion. To minimize the quantum defect, one can excite at the upper state of the laser/RL transition (4I9/2 → 4F3/2 at 884 nm). But, for the Nd3+ the corresponding absorption cross-section is minimal. Here, we perform a thorough investigation to determine the best excitation pathway, focusing on the trade-off between the quantum defect and the absorbance at the excitation wavelength in NdxY1.00-xAl3(BO3)4 crystalline powders. It is a fundamental study that investigate further the excitation and relaxation pathways of Nd3+.