Colorful Upconversion Fluorescence In Tm³⁺/Ho³⁺/Yb³⁺ Doped Sodium-aluminum-tellurite Ceramics

In recent years, there are many reports about upconversion luminescence phenomena inthe materials of rare earth doped crystals, glasses, ceramics and so on. One of the extensiveattentions has been focused on turning infrared-to-visible to multicolor visible emissionthrough frequency upconversion. By virtue of this feature, upconversion materials can beworked as infrared light display materials that used in night vision, infrared quantum counters,LED and other laser materials, which can improve the sensitivity of infrared detector withlong wave and it has the huge potential in the areas of national economy and national defencebuilding. The transitions of Tm³⁺are mainly located at blue and near infrared upconversionspectral region and the transitions of Ho³⁺are mainly situated in red and green spectral region.By adding different concentration of Tm³⁺and Ho³⁺and adopting the Tricolor fluorescencegenerating by upconversion, finally white light can be get by more multicolor upconversionfluorescence changes. Yb³⁺is an ideal sensitizer for Tm³⁺and Ho³⁺, which has simple energylevels and can avoid cross relaxation.Recently, the study of RE visible upconversion fluorescence are mainly focused ontransparent glass materials, especially fluoride glass and oxide glass. Because the traditionaloxide glass and fluoride glass can not meet the requirement to improve the luminousefficiency and stability. The new sodium aluminum tellurite ceramics is an opaque glass withlow phonon energy and high light absorption ability. It not only has the high mechanicalstrength feature of tellurite glass and the high stability feature of alumina glass, but also hasthe feature of only about700cm-1low phonon energy, which can solve the problem of lowefficiency of the up-conversion luminescence caused by high phonon energy. In addition, itshigh light absorption ability has an important application meaning to improve luminousefficiency.Based on this consideration, Tm³⁺/Yb³⁺, Ho³⁺/Yb³⁺and Tm³⁺/Ho³⁺/Yb³⁺dopedsodium-aluminum-tellurite ceramics have been designed and fabricated, and the upconversionfluorescence spectrum was investigated under974nm laser excitation. The dependence of upconversion fluoresences intensity on the pumping power was described, the colorcoordinates of the Tm³⁺/Ho³⁺/Yb³⁺sample were calculated, and the dependence of colorcoordinate on the pumping power was studied. The results obtained are as follows:1. The Tm³⁺/Yb³⁺doped sodium-aluminum-tellurite ceramics（Na2O－Al2O3－TeO2）were prepared by the solid-state melting method. The blue, red, and near infraredupconversion fluorescences were recorded under974nm laser excitation, and the blueupconversion emission is about10times stronger than the red emission, and the luminescencecolor is nearly pure blue. Based on the dependence of blue and near infrared upconversionfluoresences intensity on the pumping power was measured and described, indicating that theblue and red emission signals are due to a three-photon excitation process, and the nearinfrared emission signal is due to a two-photon excitation process.2. Ho³⁺/Yb³⁺co-doped sodium-aluminum-tellurite ceramics have been fabricated. Theblue, green, and near infrared upconversion fluorescences were recorded under974nm laserexcitation, the red fluorescence intensity is more than twice than that of the green light. Thehuman eyes are very sensitive to green light, thus, greenish-yellow is observed. Based on thedependence of blue and near infrared upconversion fluoresences intensity on the pumpingpower, the red, green and near infrared emissions are both due to a two-photon excitationprocess.3. Three different concentration ratio of Tm³⁺/Ho³⁺/Yb³⁺co-dopedsodium-aluminum-tellurite ceramics have been fabricated, The fluoresences and the energytransfer among Tm³⁺, Ho³⁺, Yb³⁺have been investigated under974nm laser excitation. Thered, green, and blue upconversion fluorescences were recorded under974nm laser excitation（LD）. The red emission band correspond to the1G4�'3F4transitions of Tm³⁺and5F5�'5I8transitions of Ho³⁺, the green emission band correspond to the5F4,5S2�'5I8transitions of Ho³⁺,the blue emission band correspond to the1G4�'3H6transitions of Tm³⁺. With theconcentration of Yb³⁺and the increasing of pumping power, the intensity of upconversionfluorescences increases.4. With increasing pump power, the477nm blue emission band dramatically exceedsthose of green and red emissions, due to the three-photon excitation relation in theTm³⁺/Ho³⁺/Yb³⁺triply doped sodium-aluminum-tellurite ceramics. The color coordinates ofthe sample are calculated，and the dependence of color coordinate on the pumping power is described. With the pumping power increasing, the color coordinate marks move along thedown-left direction in1931-CIE chromaticity diagram.The results indicate that the relative ratios of green, red and blue emission intensities canbe tuned by adjusting pumping power, and thus the fluorescence color tunability was achieved.It confirms that Tm³⁺/Ho³⁺/Yb³⁺doped sodium-aluminum-tellurite ceramics are effectiveupconversion materials and a high spot in potential application in potential application ofupconversion fluorescence are provided.