Photon-avalanche-like upconversion in NdAl3(BO3)4 nanoparticles excited at 1064 nm

Abstract

We report efficient nonresonant ground-state excitation at 1064 nm of trivalent neodymium (Nd3+) ions in stoichiometric neodymium aluminum borate NdAl3(BO3)4 nanoparticles, which are crystalline and, besides the large content of Nd3+ ions, present excellent photoluminescence properties. Upconversion (UC) emission was observed, and the energy pathways were identified starting from multi-phonon assisted ground-state absorption (4I9/2 → 4F3/2) and excited-state absorption (4I11/2 → 4F3/2) with the population of the 4I11/2 level by thermal coupling with the ground state. The excited state 4I11/2 is also populated by relaxations of the Nd3+ ions at upper-lying levels. Cross-relaxation between two Nd3+ ions (4F3/2, 4I9/2) → (4I15/2, 4I15/2) with subsequent phonon emission leads to two ions at the 4I11/2 level, with every iteration triggering a photon-avalanche-like emission, which greatly enhances the efficiency of the UC emissions. Ladder thermal excitation 4F3/2 → (4F5/2, 2H9/2) → (4F7/2, 4S3/2) → 4F9/2 was achieved, and the ground-state relaxation from these levels provided emission at 880 nm, 810 nm, 750 nm, and 690 nm, respectively. Energy transfer UC (Auger) between Nd3+ ions at the 4F3/2 level allowed the population of (2G3/2, 4G7/2) from which relaxations to the 4I9/2, 4I11/2, and 4I13/2 states provided emissions around 536 nm, 600 nm, and 660 nm, respectively. Associated with the nonradiative relaxations, we observed the heating of the nanoparticles (22 °C–240 °C) with subsequent thermal enhancement of the frequency UC emissions due to population redistribution between the coupled energy levels of the Nd3+ ions. The present results have potential applications in super-resolution imaging and nanothermometry.