Study On High-order Multiphoton Upconversion Luminescence And Electronic Population Process In Rare Earth Doped Micro/Namomaterals

Frequency upconversion is one of the major routes for achieving short-wavelength light. The studies around upconversion luminescence materials and upconversion mechanism became research focus in the past decades. Blue, violet, and ultraviolet upconversion luminescence have been widely investigated throughout the scientific community owing to great fundamental interests and numerous applications in the fields of laser, communication, energy, medical treatment, photocatalysis, military, etc. Multiphoton upconversion is an anti-stokes process, which can convert low energy photon to high energy photon for satisfying requirements of real applications. Up to now, most studies about rare earth upconversion were limited in visible portion, and only a few papers have reported their ultraviolet upconversion luminescence. All these can attribute to the low transform efficiency during upconversion processes. Most upconversion luminescence of rare earth ions in the experiments came from two-photon or three-photon process, and some materials may present weak four-photon upconversion process under high power light excitation. However, higher-order upconversion processes are difficult to obtain. This bottleneck severely limited the use of upconversion technology in many fields. In view of the above questions, hexagonal NaYF4, with good upconversion performance, was selected as research subject in this paper. High-ordre multiphoton upconversion luminescence of some rare-earth ions were obtained firstly in our experiments. The light-matter interaction, electron transition, lattice relaxation, energy transfer, multiphoton upconversion luminescence and its interior mechanism in rare-earth ions doped materials were studied systematically through laser spectroscopic measurements and dynamic analysis and some of innovative results were obtained. Additionally, innovative research results were obtained in the field of rare-earth doped white upconversion luminescence. The major achievements obtained are as follow:1. NaYF4:Yb3+/Er3+microcrystals were prepared by using hydrothermal method. Under near-infrared light excitation, near-infrared to ultraviolet upconversion luminescence of Er3+were observed. Five-photon and six-photon upconversion processes of Er3+were obtained for the first time. Three-photon upconversion process of red emission of Er3+were confirmed experimentally for the first time. In Yb3+-Er3+ codoped system, energy transfer from Yb3+to Er3+played important roles in populating high-energy excited states of Er+. The power density-dependent populating processes of Er3+were discussed. Compared with low power density excitation, the ultraviolet emissions of Er3+came from higher-order processes under high power density excitation. The ultraviolet emissions came from 2I11/2 and 4D7/2 states of Er3+present power and temperature dependence, which demonstrated the existence of nonradiative relaxation process between these two states.2. NaYF4:Yb3+/Er3+/Gd3+and NaYF4:Yb3+/Tm3+/Gd3+microcrystals were prepared by using hydrothermal method. Under 980 nm excitation, ultraviolet upconversion luminescence of Gd3+were observed firstly in NaYF4 matrix. In the codoped system, Er3+and Tm3+act as "bridging" ions during energy transfer processes. Energy transfers from Er3+to Gd3+ions and from Tm3+to Gd3+ions played important roles in populating excited Gd3+. Experiments on concentration variation and dynamic analysis revealed the energy transfer processes between Er3+and Gd3+ and between Tm3+and Gd3+. Some of possible energy transfer routes were proposed based on experimental results. Power dependence of luminous intensity revealed the upconversion emissions of Gd3+came from five-photon and six-photon processes.3. Under 1560 nm infrared light excitation, ultraviolet (244 nm,256 nm,276 nm, 288 nm,306 nm,317 nm, and 335 nm) upconversion emissions of Er3+were observed firstly in NaYF4:Yb3+/Er3+microcrystals. Energy transfers between Er3+and Yb3+ were confirmed by spectral analysis. Excited state absorption of Er3+and energy transfers from Yb3+to Er3+are two major routes in populating high-energy excited states of Er3+. Ultraviolet emissions of Er3+came from high-order multiphoton upconversion processes. The highest-order upconversion process—eleven-photon upconversion process was observed in our experiments.4. Under 1560 nm excitation, ultraviolet (276.8 nm,279.6 nm,306 nm, and 311 nm) upconversion emissions of Gd3+were observed firstly inβ-NaYF4:Yb3+/Gd3+/Er3+microcrystals. In the codoped system, the "bridging" effect of Er3+played important roles in populating high-energy states of Gd3+ions. Energy transfers from Er3+to Gd3+were confirmed by spectral analysis and dynamic analysis. The thermal population between 6IJ multiplet is the main mode in producing their neighboring states. Power dependence of luminous intensity demonstrated the eight-photon and nine-photon upconversion processes of Gd3+.5. Rare-earth ions doped Gd2O3 nanotubes were synthesized by using a simple wet-chemical route at low temperature and ambient pressure followed by a subsequent heat treatment. SEM results depicted the hollow and tubular column feature of the nanotubes clearly. Under 980 nm excitation, room-temperature upconversion white luminescence was achieved in Gd2O3 matrix for the first time. The calculated CIE color coordinates fall well within the white region and shift only slightly when the power density changed in a wide region. In the Yb3+-Er3+-Tm3+codoped Gd2O3 nanotubes, blue and green upconversion emissions came from Tm3+and Er3+ respectively. Both Tm3+and Er3+ are effective to induce the red upconversion luminescence. Blue, green and red upconversion luminescence came from two-photon and three-photon processes, respectively. Dynamic analysis revealed the energy transfer from Tm3+ to Er3+ clearly.