Jurgis Grube defend PhD thesis "Luminescence processes in Er3+ doped NaLaF4"

This Thesis is dedicated to the spectroscopic studies of Er³⁺ doped NaLaF₄, which is a promising material for obtaining up-conversion luminescence. During the process of up-conversion, the photons with lower energy (usually – infrared radiation) are converted to the photons with higher energy (visible and ultraviolet radiation). In the NaLaF₄:Er³⁺ luminescence spectrum, Er³⁺ characteristic luminescence bands with a dominant green luminescence band (⁴S_3/2→⁴I_15/2, 540 nm) can be observed. As Er³⁺ concentration increases (0.1 – 10 mol%), concentration quenching is observed for the green luminescence band, which is associated with interaction processes between Er³⁺. The studies of luminescence spectra and kinetics show that up‑conversion luminescence is excited by two processes: excited state absorption and energy transfer. The contribution of these processes to the excitation of green up-conversion luminescence depends on the excitation wavelength. Analysis of the measured luminescence spectra and luminescence decay kinetics at different excitation wavelengths (482 – 490 nm) at low temperatures (15 K) confirm that Er³⁺ can incorporate into several non-equivalent sites in crystal structure of NaLaF₄. It is shown that energy transfer occurs between Er³⁺ located in non-equivalent sites. Up-conversion luminescence spectrum is a superposition of Er³⁺ luminescence spectra appearing as Er³⁺ occupies different positions at the crystalline structure of NaLaF₄. Spectroscopic data obtained at different temperatures (15 – 300 K) allow to conclude that apart from ⁴S_3/2 state, an adjacent ²H_11/2 state is also involved in Er³⁺ interaction processes leading to the concentration quenching for the green luminescence. Key words: luminescence, up-conversion luminescence, NaLaF₄, time resolved spectroscopy, site-selective spectroscopy.