Efficient upconversion (UC) microcrystals are important for the development of novel photonic markers, solid-state lasers, displays, etc. Herein, we report on greatly enhanced UC emission in β-NaYF4:20Yb,2Er microcrystals via a simple ion-exchange reaction. The ion exchange accompanying with an ionic redistribution process is demonstrated. By simply controlling the amount of NaF (0–8.0 mmol), reaction temperature (140–220 °C), and duration (0–144 h), enormous enhancement of the UC emission up to ~4200 times is obtained compared with the pre-synthesized β-NaYF4:20Yb,2Er microcrystals. The decrease of body defects and change of local structure around lanthanide may account for the UC emission enhancement. This method may apply to other UC materials to increase the UC emission efficiency.

Monodisperse β-NaYF4:Yb3+,Tm3+ (NYF) hexagonal microplates with efficient near-infrared (NIR)-to-NIR up-conversion (UC) developed via a new ion-exchange modified hydrothermal method are reported. Ion exchange modification (IEM) not only significantly increases the UC intensity by up to 1000 times and prolongs the emission lifetimes of Tm3+ and Yb3+, but also enables the monodisperse morphology and size of hexagonal microplates to be maintained. A high UC quantum efficiency (QE) of 3.1% is obtained for IEM β-NaYF4:20%Yb3+,1%Tm3+ when excited with 980 nm light at a power density of 10 W cm−2. The UC emission properties can be finely tailored by changing the NaF/NYF molar ratio (NYF represents lanthanide doped β-NaYF4) and the doping concentration of Tm3+. The two photon NIR UC emission centered at ∼803 nm (arising from Tm3+: 3H4 → 3H6) dominates the UC emission spectrum and high concentrations of Tm3+ favor NIR UC further. A proof-of-concept optical image of printed patterns is demonstrated to verify their applications in security. These results suggest the promising applications of the newly developed monodisperse β-NaYF4:Yb3+,Tm3+ hexagonal microplates in security, luminescent labels, solid-state lasers, amplifiers, and biomedicine.

•The spatial distribution homogeneity of Tm3+ improves as the La/Al ratio increases.•The structure of glass was studied systematically by 27Al NMR, FTIR and Raman.•The effect of La/Al ratio on the spectroscopic properties of Tm3+ ion was studied.Lanthanum aluminosilicate (LAS) glasses with compositions (Tm2O3)0.8-(Al2O3)12-x (La2O3)x-(SiO2)87.2 (x = 0, 1.2, 4.5 mol%; La/Al = 0, 1/9, 3/5) were prepared via a sol–gel method combined with high temperature sintering. The total amount of Al2O3 and La2O3 was 12 mol%. The effect of the La/Al ratio on the structure and spectroscopic properties of Tm3+ has been studied in detail. The structure of Tm3+ doped LAS glasses was studied via 27Al nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The spinning sideband patterns of the 27Al NMR spectra showed that the population of Tm3+ ions in the vicinity of Al3+ decreased with an increase in the La2O3 concentration. FTIR and Raman spectra suggest that glasses with high La/Al ratios have less interconnected glass networks. The spectroscopic properties of Tm3+ ions were investigated via the Judd–Ofelt method using the measurement of absorption and emission spectra. Both the 1.8 μm fluorescence intensity and the relative intensity of up-conversion emission of Tm3+ ions decrease with an increase in the La/Al ratio. This indicates reduced cross relaxation between Tm3+ ions in glass with a high La/Al ratio. We conclude that the spatial distribution homogeneity of Tm3+ ions in LAS glass improves when the La/Al ratio increases.Download high-res image (272KB)Download full-size image

•Nd3 +-doped CaO-Al2O3-BaO glasses were prepared.•The amount of NBOs increases with the increase in CaO/Al2O3 ratio.•Their broadband emission properties were discussed.•Structure-property relationships of these glasses were investigated.Nd3 +-doped CaO-Al2O3-BaO glasses with varying CaO/Al2O3 ratios were synthesized, and their thermal, structural, absorption, and emission properties were characterized as functions of the CaO/Al2O3 ratio. The thermal stability of the glasses was analyzed by differential scanning calorimetry. The Raman spectra indicated that the amount of AlOAl bridging oxygen bonds decreased and the amount of the Alnon-bridging oxygen (NBO) bonds increased with an increase of the CaO/Al2O3 ratio. A broadband emission was observed in the fluorescence spectra, and the full width at half maximum bandwidth was found to increase from 38.6 nm to 41.2 nm with the increase of CaO/Al2O3 ratio. Meanwhile, the fluorescence peak shifted from 1064 nm to 1068 nm. Based on the Judd–Ofelt parameters, the transition probabilities, emission cross-sections and gain parameters of the glasses were calculated to evaluate the optical amplification performances; the stimulated emission cross-section decreased from 2.11 × 10− 20 to 1.35 × 10− 20 cm2 with the increase in the CaO/Al2O3 ratio. These results indicate that Nd3 +-doped CaO-Al2O3-BaO aluminate glasses with a broadband fluorescence bandwidth and suitable gain property have potential for the application in high-energy ultrashort-pulse lasers.Download high-res image (211KB)Download full-size image

•Nd3+-doped CaO-Al2O3-MgO-BaO-B2O3 glasses were prepared.•The redistribution of NBOs was found in the glass network.•Their spectroscopic properties were measured at room temperature.•Structure-property relationships of these glasses were investigated.Nd2O3-doped calcium aluminate glasses was synthesized with the following compositions: (100−x)(33Al2O3-62CaO-2MgO-3BaO)-xB2O3-0.5Nd2O3 (x = 0, 2.5, 5, 7.5, 10). The Raman, absorption, and emission spectra were measured to characterize the structure and spectroscopic properties of these glasses. The glass thermal stability was studied using differential scanning calorimetry (DSC) tests. Both the Raman spectra and DSC results indicated a decrease in the non-bridging oxygens (NBOs) in the [AlO4]− network with an increase in the B2O3 content. The J-O intensity parameter Ω2, covalency degree of the Nd-O bond, and emission bandwidth of the Nd3+ ions decreased with the B2O3 content. The stimulated emission cross-section and optical gain property increased with an increase in the B2O3 content. The tunable gain property and broadband emission feature of the Nd3+-doped CaO-Al2O3-B2O3-MgO-BaO calcium aluminate glass suggested a potential application to a high-energy ultra-short-pulse laser.

In this paper, the thermal stability and reliability properties of (Sr, Ca) AlSiN3:0.05Eu2+ phosphors prepared by solid-state reaction in 1.1 MPa N2 atmosphere were studied. The internal quantum efficiency of (Sr, Ca) AlSiN3:0.05Eu2+ could reach as high as about 92% after Sr ions replacing Ca upon excitation in blue light range (450–470 nm). As the temperature increased from 300 to 500 K, PL intensity of SrxCa1−xAlSiN3:0.05Eu2+ phosphor decreases by 66.24, 27.87 and 16.66% of the initial PL intensity for x = 0.2, 0.5 and 0.8, respectively. The relative PL intensity of SrxCa1−xAlSiN3:0.05Eu2+ phosphor decrease to 98.67% from 96.87% as x increased to 0.8 from 0.2 after storing into an ambient condition of 120 °C for 1 h and −40 °C for 1 h by turns for five times. Reliability test results show that, with x value is from 0.2 to 0.8, the relative PL intensity of SrxCa1−xAlSiN3:0.05Eu2+ phosphor is from 95.87 to 86.51% after the ambient condition of 85 °C/RH 85% for the exposure time of 168 h, from 99.86 to 92.45% after soaking the phosphors into boiled water for 3 h. It can be summarized that the addition of Sr increased the thermal stability largely and deteriorates the reliability of the phosphors slightly. And the reactivity of phosphor with water is the main reason for reducing the reliability of the phosphors.

•The optimum Al2O3/Ho2O3 mole ratio of Ho-doped silica glass for 2 μm was discussed.•The structure of sample prepared by sol–gel method can be investigated.•The spectroscopic properties of sample prepared by sol–gel method was studied.•Ho3+-doped silica glass with a broad composition region are prepared.A series of Ho3+-doped silica glasses containing different Al2O3/Ho2O3 mole ratios were prepared successfully using a sol–gel method combined with high temperature sintering. The X-ray diffraction (XRD), nuclear magnetic resonance (NMR) spectra of 27Al, and X-ray photoelectron spectra (XPS) of O1s, as well as the absorption and fluorescence spectra of the Ho3+ ions in these silica glasses, were recorded and analyzed systematically. The 27Al-NMR spectral studies indicated that Al3+ ions entered the glass structure mainly in sixfold and fivefold co-ordination for the series of sol–gel HAS glasses with constant 0.2 mol% Ho2O3 content. The NMR and XPS results obtained indicated a relationship between the amounts of Al2O3 and Ho2O3 and the Al coordination. Based on the structure and spectroscopic properties of the Ho3+-doped silica glasses, it was concluded that the optimum Al2O3/Ho2O3 mole ratio for the Ho3+/Al3+ co-doped silica glass for a 2-μm laser was 15.

•Silica glasses doped with P5 +, Nd3 + and Al3 + were successfully prepared.•Their optical and spectroscopic properties were measured at room temperature.•Structure–property relationships of these glasses were investigated.Glasses with compositions of (95.8-x) SiO2–4Al2O3–xP2O5–0.2Nd2O3 (in mol%, x = 0–10) were prepared. Structural variation caused by P5 + was investigated by Judd–Ofelt parameters and Raman spectra. Results reveal that Nd3 + preferentially coordinates to PO sites at molar ratio of P5 +/Al3 + > 1 and thus it leads to a sudden change in fluorescence effective linewidth, stimulated emission cross section, fluorescence lifetime and non-radiative transition rate of Nd3 + ions. The fluorescence lifetime changes in the range of 315–379 μs. The emission cross section changes in the range of 1.39–1.79 × 10− 20 cm2. With the increase of P5 +/Al3 + ratio, the fluorescence effective linewidth varies from 45.77 to 37.03 nm. The shortest fluorescence lifetime was observed at molar ratio of P5 +/Al3 + = 1. The maximum non-radiative transition rate of 1694/s occurred at molar ratio of P5 +/Al3 + = 1.25. The highest emission cross section of 1.79 × 10− 20 cm2 was obtained at the highest P5 +/Al3 + molar ratio. Furthermore, structure–property relationship in these glasses was discussed through Judd–Ofelt parameters and luminescent behaviors.

•Silica glasses doped with P5+, Yb3+, Al3+ were successfully prepared.•Their optical and spectroscopic properties were measured at room temperature.•Structure–property relationships of these glasses were investigated.Two series of silica glasses [(95.9−x)SiO2–4Al2O3–xP2O5–0.1Yb2O3 (in mol%, x=0–10) and (92−y) SiO2–4Al2O3–4P2O5–yYb2O3 (in mol%, y=0–0.2)] were prepared by sol–gel method combined with high-temperature sintering. The relationship between the glass structure and spectroscopic properties is investigated. Significant alterations in density, refractive index, absorption and emission cross sections, fluorescence lifetime, and scalar crystal field NJ are observed at a molar ratio of P5+/Al3+=1 due to the transformation of P5+ structure type from AlPO4-like to PO4 units. This structural change is clearly observed in Raman spectra. In addition, NMR experimental results suggest that Yb3+ is mainly located in AlPO4-like but not SiO4 units at a molar ratio of P5+/Al3+=1, and also Raman spectra reveals the presence of P=O linkage in samples with molar ratio of P5+/Al3+>1. This further demonstrates that it is due to the formation of P=O site but not the simple addition of P2O5 in Yb3+/Al3+/P5+ co-doped silica glass to significantly decrease the absorption and emission cross sections of Yb3+. It reflects in the change of Yb3+ Stark splitting, and is revealed by the decline of Yb3+ asymmetry degree.

•The high holmium doped silica glass was realized.•The spectroscopic properties of sample prepared by sol–gel can be investigated.•The optimum Al2O3/Ho2O3 mole ratio of sample for 2μm was disscussed.•Samples have a larger measured lifetime and quantum efficiency.•Detailed concentration quenching have been discussed.Two series of Ho3+ and Al3+ co-doped silica glasses containing different Al2O3/Ho2O3 mole ratios and concentrations of Ho3+ ions were successfully prepared by the sol–gel method combined with high temperature sintering. Using the fluorescence spectra and fluorescence lifetimes, it was found that the optimum Al2O3/Ho2O3 mole ratio of Ho3+ and Al3+ co-doped silica glass for realizing a 2-μm laser is 15. Ho3+ and Al3+ co-doped silica glasses with compositions of xHo2O3–15xAl2O3–(100−16x) SiO2 (in mol%, x=0.14, 0.2, 0.3, 0.4, and 0.5) were designed to investigate the concentration dependence of the 2.0-μm emission and concentration quenching of the 5I7 emission. The Judd–Ofelt intensity parameters and the stimulated emission and absorption cross-sections of 0.3Ho2O3–4.5Al2O3–95.2SiO2 were systematically studied. The analysis on the relationship between the 5I7 lifetime and the Ho3+doping concentration reveals that self-quenching for a rather large Ho3+ doping range is well described by a limited diffusion process, and that the critical concentration for self-quenching is 4.84×1020 ions/cm3. The results show that Ho3+ and Al3+co-doped silica glass is an ideal candidate for realizing ~2-μm laser.

•Silica glasses doped with F−, Yb3+ Al3+ were successfully prepared.•Their optical and spectroscopic properties were measured at room temperature.•The effects of F− on Yb3+ absorption and emission were investigated.Yb3+/Al3+-co-doped silica glasses with different F− content were prepared in this work by sol–gel method combined with high temperature sintering. XRF, FTIR and XPS methods were used to confirm the presence of F−. The effects of F− on the optical and spectroscopic properties of these glasses have been investigated. It is worth to notice that the F−/Si4+ mass ratio equal to 9% is a significant value showing a real change in the variation trends of numerous following parameters: refractive index, UV absorption edge, absorption and emission cross sections, scalar crystal-field NJ and fluorescent lifetimes. Furthermore, introduction of F− can adjust the refractive index of Yb3+/Al3+-co-doped silica glass and it is useful for large mode area (LMA) fibers.

•The high thulium doped silica glass was realized.•The thulium doped silica glass has a large stimulated emission cross section.•Samples have a long fluorescence lifetime and a low hydroxyl content.•The crossrelaxation was used to explain the optimum thulium doping concentration.Tm3+/Al3+ co-doped silica glass was prepared by sol–gel method combined with high temperature sintering. Glasses with compositions of xTm2O3–15xAl2O3–(100 − 16x) SiO2 (in mol%, x = 0.1, 0.3, 0.5, 0.8 and 1.0) were prepared. The high thulium doped silica glass was realized. Their spectroscopic parameters were calculated and analyzed by Judd–Ofelt theory. Large absorption cross section (4.65 × 10−21 cm2 at 1668 nm) and stimulated emission cross section (6.00 × 10−21 cm2 at 1812 nm), as well as low hydroxyl content (0.180 cm−1), long fluorescence lifetime (834 μs at 1800 nm), large σem × τrad (30.05 × 10−21 cm2 ms) and large relative intensity ratio of the 1.8 μm (3F4 → 3H6) to 1.46 (3H4 → 3F4) emissions (90.33) are achieved in this Tm3+/Al3+ co-doped silica glasses. According to emission characteristics, the optimum thulium doping concentration is around 0.8 mol%. The cross relaxation (CR) between ground and excited states of Tm3+ ions was used to explain the optimum thulium doping concentration. These results suggest that the sol–gel method is an effective way to prepare Tm3+ doped silica glass with high Tm3+ doping and prospective spectroscopic properties.