Efeitos da concentração de Nd3+ na conversão ascendente de energia devido a um mecanismo do tipo avalanche de fótons em partículas cristalinas de NdxY1.00-xAl3(BO3)4 sob excitação não ressonante em 1064 nm

Abstract

Trivalent rare earth ions have proven to be extremely efficient in the absorption and emission of various wavelengths. This is because the optically active electrons are not the outermost ones, thus weakening the bond of ions when doped into a host material + (neodymium) in NdxY1.00-xAl3(BO3)4 particles under non-resonant (~1064 nm) Anti Stokes excitation from the ground state to the 4F3/2 level was demonstrated. Even with non-resonant excitation, broad intensities were observed in the photoluminescent spectrum, spanning from visible to infrared. This is explained by the avalanche photon excitation (AF) mechanism, which efficiently favored the population of the 4F3/2 level along with phonon annihilation to initially populate the 4F3/2 level. In the interaction between an excited ion (4F3/2) and another in the ground state (4I9/2), the excited ion undergoes cross-relaxation by interacting with a ground-state ion, causing the first to relax from the 4F3/2 level to 4 I15/2 and the second to be excited from 4 I9/2 to 4 I15/2, both in a non-radiative manner. Due to the proximity to lower levels, these ions non-radiatively relax to the 4 I13/2 and 4 I11/2 levels. Once at the 4 I11/2 level, ions can absorb radiation from the excitation beam resonant with the 4 I11/2 4F3/2 transition. Thus, an ion initially excited to the 4F3/2 level with low probability at room temperature, in a non-resonant manner, leads two ions to the 4F3/2 state. Following the sequence of events: ground state absorption assisted by phonons, cross-relaxation, non-radiative relaxations, and resonant excited state absorption, the two ions at the 4F3/2 level can transfer energy to two ground state neighbors and bring four ions to the 4F3/2 level. The repetition of this set of events leads to excitation that follows a geometric progression of excited ions, abruptly increasing photon absorption at ~1064 nm. With large numbers of ions at the 4F3/2 level, transitions characterizing upconversion (UC) throughout the visible and infrared photoluminescent spectrum can be observed. The influence of neodymium concentrations on different samples was also investigated, revealing a strong dependence on cross-relaxation for photon avalanche mechanisms, due to the proximity of ions. Thus, an energy-looping mechanism was proposed for concentrations from 5% to 20% Nd3+, and for concentrations greater than 20%, the photon avalanche is reestablished. The results of this dissertation are published in the journals Applied Physics Letters with the title “Photon-avalanche-like upconversion in NdAl3(BO3)4 nanoparticles” and Optical Materials with the title “Energy-looping and photon-avalanche-like phenomena in NdxY1.00-xAl3(BO3)4 powders excited at 1064 nm”.