Sensitivity Of Upconversion Mechanisms To Excitation Laser Wavelength In Er³⁺-doped YAG

In recent years, upconversion (UC) luminescence of rare-earth doped materials has attracted much interest due to their capability of converting radiation from longer wavelength to shorter one. However, UC luminescence intensity is relatively low, which makes it less practically valuable. So there has been a continuous interest in research of increasing UC efficiency.Besides the quantum efficiency of emitting level, the efficiency of excitation mechanism has an essential influence on UC emission intensity. By proper excitation mechanism, the fluorescence of favorable intensity could be produced. Various UC mechanisms could be realized as a function of the pumping characteristics such as wavelength, pulse or continuous excitation. So it is of great significance to have a good understanding of the UC mechanism dependence on excitation conditions in order to maximize UC efficiency in applications by suitable excitation condition. It is a common and efficient approach to obtain UV UC luminescence by cooperation of diverse UC mechanisms. But to our best knowledge very little attention is paid to the competition between different UC mechanisms on exciting wavelength in details when they cooperate for UC luminescence. In this paper, we reported on the sensitive dependence of the mechanism-competition on exciting wavelength in Er3+:YAG crystal. Moreover, the kinetics of fluorescence decay of 1S0 level in Pr3+:ZBLAN glass were studied by two-step excitation process.Firstly, we focused on the sensitive dependence of UC mechanisms on exciting wavelength in Er3+:YAG crystal.a) We reported on the sensitivity of UC mechanisms of 2P3/2 to excitation wavelength within a variation of only 0.5 nm in Er3+:YAG for the first time. The dominant UC mechanism for populating 2P3/2 will gradually change from ETU to ESA so long as the exciting wavelength is detuned by only 0.2 nm. To qualitatively understand the variation of UC mechanisms with exciting wavelength, a statistical analysis of the donor-acceptor distances could be utilized. In laser excitation region excited Er3+ ions randomly distribute, and if they are close enough to each other, energy transfer effective for 2P3/2 UC population will occur. Since the transfer probability depends on the interionic distances, when the excitation departs from resonance, the decrease of excited ions fades off the ETU process. In the case that ETU and ESA coexist, rapid decline of ETU processes makes ESA come to importance. Just like the UC mechanism dependence on dopant concentration, the sensitivity of fluorescence dynamics to exciting wavelength directly reflects the influence of excited ion density on energy transfer process. In order to understand in more detail the physical essence responsible for this sensitivity, we employed some rate equations to present a quantitative analysis for the UC emission from 2P3/2 as well as 2G9/2, and the theoretical results attested the sensitivity as revealed in the experiment. This sensitivity of UC mechanism deserves to be paid attention in the study of UC luminescence, especially, in the cases concerning the UC activity and the cases of those dopant ions with abundant energy level structure. It is known that the effect of concentration quenching is a main obstruction for producing high-efficient UC luminescence. Therefore, the study may illumine one to explore the sensitivity of UC mechanism on exciting wavelength in high-doped samples as well as the UC processes based solely on one mechanism.b) We focused on exploring an approximate quantitative method for analyses of different UC-mechanism contributions. By utilizing the UC excitation spectra of the related levels, an approximate analytical method for quantitative estimations of ESA and ETU contributions has been proposed. Our work is the first time to unambiguously give quantitative estimations of different UC-mechanism contributions. The sensitivities of the UC mechanism to exciting wavelength have been reproduced based on the analytical results deduced by this method, which proves the validity of this simple analytical method. This method is helpful for determining a suitable exciting wavelength to realize more diverse and effective UC excitation for an objective level. Furthermore, when UC mechanisms could not be distinguished by time-resolved fluorescence decay curves due to continuous wave excitation, the analytical method proposed here may illumine one to explore UC mechanisms and their respective contributions from excitation spectra. Moreover, the study of mechanism-sensitivity has been extended to a stage concerning multiple Stark levels. A more systematic study about mechanism-sensitivity has been reported, including the mechanism-sensitivity to the exciting wavelength under resonant excitations of different Stark levels and the variations of mechanism sensitivities when exciting different Stark levels from resonance to non-resonance. It is just the re-occurrence of the mechanism sensitivity in another UC scheme that lets us realize that the mechanism sensitivity is not a haphazard and it is worthy of being given quantitative analyses.c) Then, the study of mechanism-sensitivity has been extended to a stage concerning multiphoton UC mechanisms (cross relaxation (CR)) besides ESA and ETU processes. A novel sensitivity of laser-induced UC fluorescence dynamics to the exciting wavelength in Er3+:YAG crystal has been reported for 2Gg/2 and 2P3/2, populations of which originate from ETU/CR and ETU/ESA cooperation, respectively. The sensitivities to exciting wavelength variations are only 0.02 and 0.06 nm for 2G9/2 and 2P3/2. Since the competition between different UC mechanisms has a sensitive dependence on exciting wavelength, the work in this paper would provide a new idea for precisely determining a suitable exciting wavelength to make diverse the UC populating manners of an objective level for future studies on UV UC luminescence.d) The impacts of both the dopant concentration and the excitation wavelength on UC fluorescence have been taken into account together and it has been found that in samples with different concentrations though the UC fluorescence concerning the 2P3/2 and the 2G9/2 levels are obviously distinct, their sensitivities to exciting wavelength are very similar. Therefore, the work would provide a new idea for optimizing UC scheme design by proper determinations of both exciting wavelength and dopant concentration.Secondly, we experimentally reported on the energy position, branching ratios, lifetimes and emission cross sections of the 1S0 level in Pr3+:ZBLAN glasses by two-step excitation process:3H4→1D2→4f5d. The calculated branching ratios and lifetimes of 1S0 by the standard as well as the modified Judd-Ofelt theories have been compared with the measured ones. The results by the modified Judd-Ofelt theory, which more reasonably takes into account the effects of the configuration energies on the 5d perturbations to the 4f levels, have better agreements with the experimental results. The 1S0 luminescence by another two-step excitation process 3H4→3P0→4f5d has been also discussed. However, owing to adjacency of 1S0 to the 4f5d configuration, theoretical results by the modified theory are still unsatisfactory compared with the applications to other 4f lower-lying states of Pr3+ ion. It is obvious that large mixture of 4f5d configuration brings an important influence on luminescence property of the 1S0 state and hence improvement of applicability of the modified Judd-Ofelt theory for the 1S0 state is worthy in further study.