Study On Dependence Of Competitions Between Different Upconversion Mechanisms To Exciting Wavelength In Er³⁺: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.Firstly, we focused on the sensitive dependence of UC mechanisms on exciting wavelength in Er3+(8.4at%):YAG crystal. a) We reported on the sensitivity of UC mechanisms of 2P3/2 to excitation wavelength within a variation of only 0.02 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 0.02 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. 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) Then, the study of mechanism-sensitivity has been extended to the energy level 2G9/2,which can be obtain by cooperation of ETU and CR. The sensitivities to exciting wavelength variations are only 0.02 and 0.06 nm. c) 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. 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.Therefore, the work would provide a new idea for optimizing UC scheme design by proper determinations of both exciting wavelength and dopant concentration.