White Upconversion Fluorescence In Tm³⁺/Ho³⁺/Yb³⁺ Doped Bismuth Tellurite Glass System

In recent years, great attention has been focused on multicolor source, which showsattract applications in the field of three-dimensional solid-state displays and backgroundillumination. And it provides an effective approach to generate multicolor visible emissionthrough infrared-to-visible frequency upconversion in rare earth doped materials. Rare earthions are suitable for upconversion emission center owing to their abundant electronic energylevels and narrow emission lines. The¹G₄â†'³H₆and³H₄â†'³H₆transitions of Tm³⁺locate atblue and near infrared upconversion spectral region, respectively, and the transitions of Ho³⁺are mainly situated in red and green spectral region. Based on primary colors principle, whitecolor simulation can be achieved by doping the different concentration of Tm³⁺and Ho³⁺andadjusting the primary colors upconversion fluorescences. Yb³⁺is an ideal sensitizer for Tm³⁺and Ho³⁺, which has simple energy levels and can avoid cross relaxation.In the oxide glasses, the phonon energy of tellurite glasses is lower than those in borate,phosphate, silicate and germanate glasses, in which rare earth ions can achieve efficientlyfluorescences emission. Based on traditional tellurite glasses, maximum phonon energies islower, refractive index are higher of the tellurite glasses after adding the oxide bismuth, and inthe meantime, high emission cross relaxation rate of rare earth ions will be realized and thetransition radiation will be obtained efficiently. Based on this consideration, Tm³⁺/Yb³⁺,Ho³⁺/Yb³⁺and Tm³⁺/Ho³⁺/Yb³⁺doped bismuth tellurite glasses have been designed andfabricated, and the upconversion fluorescence spectrum was investigated under974nm laserexcitation. The dependence of upconversion fluoresences intensity on the pumping power wasdescribed, the color coordinates of the Tm³⁺/Ho³⁺/Yb³⁺sample were calculated, and thedependence of color coordinates on the pumping power was studied. The results obtained areas follows:1. Tm³⁺/Yb³⁺co-doped7.5Li₂O–7.5K₂O–5BaO–5Bi₂O₃–75TeO2bismuth tellurite glasseshave been fabricated. Intensity blue and near infrared upconversion fluorescences wererecorded under974nm laser excitation, and the dependence of blue and near infrared upconversion fluoresences intensity on the pumping power was measured and described,indicating that the blue and near infrared emissions are due to a three-photon and to atwo-photon excitation process, respectively.2. Ho³⁺/Yb³⁺co-doped7.5Li2O–7.5K2O–5BaO–5Bi2O3–75TeO2bismuth tellurite glasseshave been prepared. Intensity red and green upconversion fluorescences were observed under974nm laser excitation, and the dependence of red and green upconversion fluoresencesintensity on the pumping power was measured and described, indicating that the red and greenemissions are both due to a two-photon excitation process.3. The fluoresences of Tm³⁺/Ho³⁺/Yb³⁺triply doped7.5Li2O–7.5K2O–5BaO–5Bi2O3–75TeO2bismuth tellurite glasses and the energy transfer among Tm³⁺, Ho³⁺, Yb³⁺have beeninvestigated under974nm laser excitation. While the doped concentration of Yb2O3, Tm2O3,and Ho2O3are1.6wt%,0.4wt%, and0.2wt%, respectively, the bismuth tellurite glasse showsbright white light. With the pumping power increasing, the intensity of white light increases.4. The color coordinates of Tm³⁺/Ho³⁺/Yb³⁺triply doped7.5Li2O–7.5K2O–5BaO–5Bi2O3–75TeO2bismuth tellurite glasses have been calculated, and the dependence of colorcoordinates on the pumping power was studied. With increasing pump power, the477nm blueemission band dramatically exceeds those of green and red emissions, due to the three-photonexcitation relation. Thus, the relative ratios of green, red and blue emission intensities can betuned by adjusting pumping power, and white lighting could be realized in Tm³⁺/Ho³⁺/Yb³⁺triply doped bismuth tellurite glasses.In the new bismuth tellurite glasses, Tm³⁺/Yb³⁺co-doped LKBBT glasses show brightblue and Ho³⁺/Yb³⁺co-doped LKBBT glasses present greenish-yellow color under974nmlaser excitation. Based on primary colors principle, white color simulation can be achieved bydoping the suitable concentration of Tm³⁺, Ho³⁺, and Yb³⁺in the bismuth tellurite glasses, andthe intensity and chroma of white light can be turned by adjusting the pumping power.These new phenomena and results of studies provide the theoretical basis and pledge ofnew materials for new types of fluorescent display devices and will be potentially employedin the application field of white light illumination.