Study On Relationship Between Enhanced Ultraviolet Upconversion Emission Efficiency And Particle Size

In recent years, short-wavelength solid-state lasers in the ultraviolet to green spectral range have attracted much attention due to a wide range of applications including high-density optical data storage, all color displays, undersea communications, biomedicine and infrared sensors. The phenomenon of frequency upconversion by using the intrinsic energy level matching of certain rare-earth ions, as one of the available approaches exploring short-wavelength solid-state lasers, has been investigated widely. Ultraviolet and violet upconversion solid-state lasers are the next objective of the researchers. For the realization of ultraviolet and violet upconversion solid-state lasers, it urgently needs to develop new high efficient ultraviolet upconversion materials. So, the study of Rare earth doped optical materials has been investigated widely.Because upconvertion property depends on the material structure, we believe that the particle size also affect the upconvertion property. Compare to the properties of bulk materials, they are obviously different in nano-sized materials. It was reported that the upconvertion emission efficiency became high as the particle size decreased. In our group, we have observed enhanced ultraviolet upconvertion emissions. But the contrary conclusion was also reported. In this paper, we synthesized different ultraviolet upconversion emission materials by different methods, characterized their properties, and made a research on the relationship between the ultraviolet upconvertion emission efficiency and the particle size. The main contents in this thesis are as follows.1. CaF₂:Yb³⁺/Tm³⁺ nanometer thin films were synthesized by a Sol-gel method, and their UC optical properties were studied at room temperature. Ultraviolet upconversion fluorescence was observed. The experimental results indicated that the ¹I₆â†'³F₄(～347 nm) and ¹D₂â†'³H₆(～362 nm) emissions were much stronger than the ¹D₂â†'³F₄(～452 nm) and ¹G₄â†'³H₆(～476 nm) emissions. To study the relationship between the emission efficiency and the particle size, a bulk material was obtained after melting the powder synthesized by the sol-gel method. It emitted weak ultraviolet and intense blue light under 980 nm excitation. However, the powder obtained by lapping the bulk material emitted weak blue and intense ultraviolet light. It was obvious that the upconvertion emission efficiency became high as the particle size decreased from millimeter-scale to nano-sized.2. CaF₂:Yb³⁺/Tm³⁺ nanocrystals were synthesized through a hydrothermal method. About 50～1200 nm samples were obtained after annealing. Under 980 nm excitation, enhanced ultraviolet light was observed. They were mainly the ¹I₆â†'³F₄(～347 nm) and ¹D₂â†'³H₆(～362 nm) emissions. We studied the proportion of every emission. It was found that the ultraviolet upconversion emission efficiency increased at first, then decreased with the increase of particle size from 50 nm to 1200 nm . The emission efficiency was the highest when the particle size was around 600nm. This indicated that, in nano-sized scale, the ultraviolet upconversion efficiency had a maximum when the particle was at a certain grain size.