Photon-avalanche-like upconversion in NdAl_3(BO_3)_4 nanoparticles excited at 1064 nm

摘要

We report efficient nonresonant ground-state excitation at 1064 nm of trivalent neodymium (Nd~(3+)) ions in stoichiometric neodymium aluminum borate NdAl_3(BO_3)_4 nanoparticles, which are crystalline and, besides the large content of Nd~(3+) ions, present excellent photolumi-nescence properties. Upconversion (UC) emission was observed, and the energy pathways were identified starting from multi-phonon assisted ground-state absorption (~4I_(9/2) → ~4F_(3/2)) and excited-state absorption (4~I_(11/2) → ~4F_(3/2)) with the population of the ~4I_(11/2) level by thermal coupling with the ground state. The excited state ~4I_(11/2) is also populated by relaxations of the Nd~(3+) ions at upper-lying levels. Cross-relaxation between two Nd~(3+) ions (~4F_(3/2), ~4I_(9/2)) → (~4I_(15/2), ~4I_(15/2)) with subsequent phonon emission leads to two ions at the ~4I_(11/2) level, with every iteration triggering a photon-avalanche-like emission, which greatly enhances the efficiency of the UC emissions. Ladder thermal excitation ~4F_(3/2_ → (~4F_(5/2), ~2H_(9/2)) → (~4F_(7/2), ~4S_(3/2)) → ~4F_(9/2) 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 Nd~(3+) ions at the ~4F_(3/2) level allowed the population of (2~G_(3/2), ~4G_(7/2)) from which relaxations to the ~4I_(9/2), ~4I_(11/2), and ~4I_(13/2) 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 Nd~(3+) ions. The present results have potential applications in super-resolution imaging and nanothermometry.