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Abstract
High-brightness orangish red fluorescence emissions were captured in Eu3+ doped fluorophosphate (NBFP) glasses with outstanding rare earth (RE) ion solubility under laser excitation. Highly efficient emissions of Eu3+ doped NBFP glasses in the wavelength range of 580−720 nm make the phosphors potential candidates as a remarkable orangish red lighting source. The net emission power and the net emission photon number in 6.0wt% Eu2O3 doped NBFP glass were derived to be 6.48 mW and 2.08 × 1016 cps under the excitation of 465 nm laser with 53.46 mW optical power, respectively, and total measured quantum yield was as high as 54.03%. When the excitation power was increased to 561 mW, the luminous flux of 6.0wt% Eu2O3 doped NBFP glass was up to 31.21 lm, demonstrating that Eu3+ heavy-doped NBFP glasses are potential lighting source materials. Thus, the laser-driven high-brightness phosphors originating from the sufficient photon release of Eu3+ ions promote further development of orangish red lighting source.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-driven rare earth (RE) ions doped glass phosphors are potential illumination materials due to their long fluorescence lifetime, high brightness, and lower energy consumption [1,2]. With the commercialization of visible laser sources, the research of RE3+ ions (Dy3+, Sm3+, Tm3+, Eu3+, Pr3+) doped glasses are promoted [3–7]. Among the RE ions, Eu3+ as a desirable activator for numerous hosts has been considered as an excellent luminescent center because of its efficient 5D0→7F2 transition and heavy-doped characteristic. Moreover, Eu3+ heavy-doped glasses can release high-brightness orangish-red fluorescence to serve in various fields, such as medical diagnostics, solid-state laser lighting, emissive displays and other applications [8–14]. In particularly, Eu3+ ions exhibit admirable high-brightness fluorescence at 590–720 nm wavelength, which is situated in the maximum absorption regions of the photosensitizer currently employed in therapy and clinical trials. Thus, the exploration focusing on high-bright Eu3+ heavy-doped glasses driven by laser becomes urgent.
For Eu3+ doped glass phosphors, fluorophosphate glasses are promising candidates for photoluminescence materials due to their outstanding RE ions solubility, high transmittance, excellent thermal stability and high laser-damage threshold [15–25]. Furthermore, the fluoride in fluorophosphate glasses effectively reduces the presence of hydrogen and hydroxyl ions, which generates stronger fluorescent emission of RE ions [26–28]. In order to obtain high brightness in glass phosphors, laser-driven approach is employed due to its advantages of strong optical coherence and accurate emission wavelength [29]. In addition to this, the residual laser can be easily filtered out by the short-wave cut-off filter [30].
In this work, transparent Eu3+ heavy-doped fluorophosphate glasses (NBFP) were fabricated and characterized. High concentration Eu3+ ion and strong applicability host composition contribute to the generation of high-brightness orangish red fluorescence under the excitation of 465 nm laser excitation. A low non-radiative relaxation rate was identified and internal quantum yield of over 96% was derived from the lifetime analysis in the case when the Eu2O3 concentration is less than 6.0 wt%, providing an effective approach for synthesizing heavy-doped RE fluorescent material. In addition, the emission power and the measured quantum yield were calculated to be 6.48 mW and 54.03% under the excitation of 465 nm laser with 53.46 mW optical power, respectively. Furthermore, 31.21lm radiation luminous flux was obtained in 6.0wt% Eu2O3 doped NBFP glass when the excitation power was increased to 561mW, which was bright enough to become a practical material for light sources. These results indicate that high-bright orangish red NBFP glass phosphors are promising candidates to develop illumination devices.
2. Materials and experiments
Eu3+ heavy-doped NBFP glasses were prepared from high-purity NaPO3, BaF2 and Eu2O3 according to the molar composition of 37.5Na2O-25BaF2-37.5P2O5, In addition, 0.2wt%, 0.5wt%, 1.0wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 as dopants were adopted in NBFP glasses based on the host glass weights, respectively. Firstly, the well-mixed raw materials in alumina crucibles were put into electric furnace, then melted at 930 °C for 40 min. Secondly, the melts were poured into an aluminum mold for quenching and forming. Finally, the glasses were annealed at 390−400 °C for 8 h in order to remove residual stresses within the glasses, and after that cooled down slowly to room temperature. For further optical measurements, the glass samples were sliced and polished into the pieces with two parallel sides.
Using the Metricon 2010 prism coupler, the refractive indices of 2.0wt% Eu2O3 doped NBFP glass were measured to be 1.5479 and 1.5342 at 635.96 and 1546.9 nm, respectively, the refractive indices of the glass samples at other wavelengths were obtained by the Cauchy’s equation n = A + B/λ2 with A = 1.5314 and B = 6668 nm2 [31]. The density of NBFP glass with 2.0wt% Eu2O3 was measured to be 3.31 g/cm3 by Archimedes method, and thus the number density of Eu3+ ions was calculated to be 2.22 × 1020 cm−3. Emission and excitation spectra were recorded by a Hitachi F-7000 fluorescence spectrophotometer, which were corrected with Rhodamine B method. The optical transmission spectrum was presented by a Lambda 950 spectrophotometer. The spectral resolutions for optical transmission and visible spectra are 0.05 nm and 1.0 nm, respectively. The X-ray diffraction (XRD) pattern spectrum was obtained by a Shimadzu XRD-7000 (Cu-Ka, 40 kV, 30 mA) diffractometer. Differential scanning calorimetry (DSC) was measured by using American TA company SDT-600 with the rate of 10 °C/min from room temperature to 800°C under N2 atmosphere (flow of 100 ml/min). Fluorescence decay curves were measured by a Jobin Yvon Fluorolog-3 spectrophotometer equipped with an R928 photomultiplier tube (PMT) detector, and a pulsed Xenon-lamp was adopted as the pump source. The absolute spectral parameters were obtained in a 25 cm inner diameter integrating sphere (Labsphere) which was connected to a QE65000 CCD detector (Ocean Optics) with a 600 μm-core optical fiber. A 465 nm diode laser were adopted as pumping source, and detailed optical powers in operating processes are presented in the following text. A standard SCL-050 halogen lamp (Labsphere) was employed for calibrating the measurement system. All the experiments were carried out at room temperature.
3. Results and discussion
3.1 Radiative transition properties of Eu3+ doped NBFP glasses
The normalized emission spectra of Eu2O3 doped NBFP glasses with different Eu3+ concentration are recorded under 395 nm excitation as shown in Fig. 1. The seven emission peaks are located at 537, 555, 579, 592, 615, 653 and 702 nm, respectively, which originate from the different excited levels of Eu3+ to the lower levels 7FJ (J = 0,1, 2, 4) transitions [32–34]. Among them, the 5D0→7F2 electric dipole transition at 615 nm is a more hypersensitive transition than others [35,36]. In the inset of Fig. 1, the brightness of the glass phosphors has an obvious upgrade with the increasing amount of Eu3+ ions in the clockwise direction, confirming the emission intensity is proportional to Eu2O3 doping concentration.
Fig. 1 Normalized emission spectra of 0.2wt%, 0.5wt%, 1.0wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 doped NBFP glasses under 395 nm excitation. Inset: fluorescent photographs with increasing dopant concentration in the clockwise direction under 395 nm excitation.
The normalized excitation spectra of Eu3+ doped NBFP glasses monitored at 615 nm are exhibited in Fig. 2. The spectra consist of ten excitation bands peaking at 252, 287, 299, 320, 362, 383, 395, 416, 466 and 527 nm. The broad charge transfer bands (CTS) in UV region located between 205 nm and 283 nm, which is due to the electron transition from O2− to 4f6 shell of Eu3+ ions [37], and other excitation bands are owing to the 4f–4f inner shell transitions of Eu3+ ions. The presence of widespread excitation bands indicates orangish red fluorescence can be achieved by UV/violet/blue/green laser pumping. Although the most efficient excitation wavelength is 395 nm in the glass system, 465 nm commercial laser is chosen as excitation source from the pratical perspective to pump orangish red light source materials in industrial development. The relationship between emission intensity and Eu2O3 doping concentration has been fitted, and intensity in high doping (>6wt%) cases are anticipated in the inset of Fig. 2. The concentration quenching in Eu3+-doped NBFP glasses is considered to occur at about 9.0wt% Eu2O3 doping.
Fig. 2 Normalized excitation spectra of 0.2wt%, 0.5wt%, 1.0wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 doped NBFP glasses monitoring at 615 nm emission. Inset: the relationship between emission intensity and Eu2O3 doping concentration.
3.2 Properties of thermodynamics and transmittance in Eu3+ doped NBFP glasses
The X-ray diffraction (XRD) pattern spectrum of 2.0wt% Eu2O3 doped NBFP glass exhibits two broad peaks and no sharp diffraction peak in Fig. 3, which indicates that the NBFP glasses system is an amorphous state. Moreover, the thermodynamic properties of 6.0wt% Eu2O3 doped NBFP glass are exhibited by DSC curve in Fig. 3(a). The transition temperature (Tg), the crystallization onset temperature (Tx) and the crystallization temperature (Tc) of Eu3+ heavy-doped NBFP glass were identified as 403 °C, 544 °C and 570 °C, respectively. The temperature difference values (ΔT = Tx−Tg) should be as large as possible to be considered as good optical glasses, and a ΔT value larger than 100 °C suggests excellent glass thermodynamic stability [38]. The ΔT of NBFP glass is calculated to be 141 °C, demonstrating the sample exhibits good stability when 6.0wt% Eu2O3 is heavily doped. Besides, in Fig. 3(b) Eu2O3 doped NBFP glass presents 80% transmittance in addition to the strong absorption bands of Eu3+ located at 300−403 nm and 1889−2246 nm in the transmission spectrum, indicating the energy of laser can be efficiently absorbed by RE ions, and the photon of Eu3+ can be adequately released. Thus, Eu3+ doped NBFP glasses with excellent transmittance and well thermodynamic stability are good candidates for illumination devices.
Fig. 3 XRD pattern spectrum of 2.0wt% Eu2O3 doped NBFP glass. Insets: DSC curve of 6.0wt% doped NBFP glass (a) and optical transmission spectrum of 2.0wt% Eu2O3 doped NBFP glass (b).
3.3 Quantitative characterization and brightness analysis of Eu3+ doped NBFP glasses
Absolute spectral parameters are obtained by the integrating sphere coupled with a CCD detector, providing measured quantum yield QYM to evaluate luminescence prospect of laser-driven light source materials [39]. As shown in Fig. 4, the net spectral power distributions are obtained for 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses under 465 nm laser excitation. Here, the spectral power distribution curves consist of six emission bands located at 579, 592, 614, 653, 701 and 804 nm, which are attributed to 5D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3, 5D0→7F4, and 5D0→7F6 transitions, respectively. The net emission powers of 2.0 wt% Eu2O3 doped NBFP glass are derived to be 1.25 and 2.72 mW under the excitation of 465 nm laser with of 25.19 and 53.46 mW optical powers, respectively. In addition, the corresponding net emission powers of 6.0 wt% Eu2O3 doped NBFP glass are as high as 2.99 and 6.48 mW under the 25.19 and 53.46 mW, respectively, revealing that heavy-doped Eu3+ ions and high laser power contribute to more intense orangish red emission. The color of the fluorescent changes from magenta to orangish red as exhibited in the inserted photographs of Fig. 4 with the increasing emission power of Eu3+ doped NBFP glasses.
Fig. 4 Net spectral power distribution curves of 2.0wt% (a-b) and 6.0wt% (c-d) Eu2O3 doped NBFP glasses under 465 nm laser excitation with different power. Insets: fluorescent photographs of 2.0wt% (a-b) and 6.0wt% (c-d) Eu2O3 doped NBFP glasses under 465 nm laser excitation with different power.
In this work, the net photon distribution is derived by the equation
in which is the wavenumber, c is the vacuum light velocity, h is the Planck constant, and is the net spectral power distribution [40]. As presented in Fig. 5, absorption and emission photon distributions of Eu3+ doped NBFP glasses are derived from Eq. (1) with corresponding . The absorption and emission photon numbers of these phosphors have been deduced and summarized in Table 1, and relevant results reveal that the Eu3+ ions doped NBFP glass system has outstanding photoluminescence behavior and excellent absorption capacity for the 465 nm laser beam.Fig. 5 Net emission photon distributions in 2.0wt% (a-b) and 6.0wt% (c-d) Eu2O3 doped NBFP glasses under the excitation 465 nm laser with different power. Inset: net absorption photon distributions under the 465 nm laser excitation, the area of red and blue rectangles stand for absorption and emission photon numbers, respectively.
Table 1. Absorption and emission photon numbers and measured quantum yields in Eu3+ doped NBFP glasses under 465 nm laser excitation.
The measured quantum yield is a selection criterion for assessing the effectiveness of photoluminescence materials for orangish red light sources, which is defined as the ratio of the number of emitted photons Nem to that of absorbed photons Nabs
All the QYM values of 5D0→7FJ (J = 0, 1, 2, 3, 4, 6) transition emissions for Eu3+ doped NBFP glasses have been obtained and listed in Table 1. The total QYM values vary from ∼40% to ∼54% when Eu3+ ions doping concentrations increase from 2.0wt% to 6.0wt%. The total QYM for 6.0wt% Eu2O3 heavy-doped NBFP glass is no less than 50% when optical power is adjusted from 25.19 to 53.46 mW. The total QYM for 6.0wt% Eu2O3 doped NBFP glass is larger than 9% in MoO3−ZnO−B2O3 crystallized glass [41], 13.4% in BaF2−SrF2−AlF3−YF3−Al(PO3)3 glass [42] and 3.2% in GaN layers [43]. Besides, the absorption coefficient 0.076 cm−1 for 7F0→5D2 in the 2.0wt% Eu2O3 doped NBFP glass is superior to ∼0.04 cm−1 in Eu2O3 doped Li2B4O7 glass [44], and in the perspective of photon release, the QYM of 2.0 wt% Eu2O3 doped NBFP glass is 40.74%, which is higher than 10.6% in Li2B4O7 glass. Thus, the Eu3+ heavy-doped NBFP glasses are preeminent candidates for 465nm laser driven materials in orangish red light source.Judd−Ofelt (J−O) intensity parameters Ωt (t = 2, 4, 6) are important indicators to evaluate the interaction between RE ion and host [45–47]. Intensity parameters are listed in Table 2, and Ω2 is obtained to be 7.461 × 10−20 cm2, which shows the higher asymmetry and the stronger covalency around Eu3+ ions. The presence of fluoride is beneficial for improving the Eu3+ ions solubility in the NBFP glasses. The Ω4 and Ω6 are calculated to be 6.687 × 10−20 and 1.507 × 10−20 cm2, respectively, which indicate the vibronic transitions of the Eu3+ ion-ligand bond and reflect the acid-basicity and the rigidity of NBFP glasses. Spontaneous transition probabilities Aij, branching ratios βij, and the radiative lifetimes τrad of 5D0 level of 2.0wt% Eu2O3 doped NBFP glass are derived and shown in Table 3. The Aij of the 5D0→7FJ (J = 1, 2, 4, 6) transitions are obtained to be 63.74, 248.30, 106.36 and 1.34 s–1, respectively. The fluorine ions in the glass induce structural changes in the vicinity of the Eu3+ ions, which in turn decrease the electronic transition probabilities leading to an increase in lifetimes of the 5D0 level. The βij are calculated to be 15.19%, 59.16%, 25.33% and 0.32% for 5D0→7FJ (J = 1, 2, 4, 6) transitions, respectively, predicting the efficient orangish red emissions of Eu3+ ions satisfy some special demands.
Table 2. Photon number ratios and intensity parameters (Ω2, Ω4 and Ω6) in Eu3+ doped NBFP glasses under 465 nm laser excitation.
Table 3. Spontaneous transition probabilities Aij, branching ratios βij and radiative fluorescent lifetime τrad of 5D0 level in Eu3+ doped NBFP glasses.
Fluorescence decay curves of 5D0 level in Eu3+ doped NBFP glasses monitoring at 615 nm under 465 nm laser excitation are presented in Fig. 6. The experimental average lifetime τexp-avg of the 5D0 level for Eu3+ doped NBFP glass can be derived from the fluorescence decay curves using the following equation
where I(t) is the emission intensity [48,49]. The related results are calculated and listed in Table 4. The τexp-avg are obtained to be 2.36, 2.34, 2.32 and 2.29 ms for 0.2wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 doped NBFP glass samples, respectively, indicating the effect of Eu3+ concentration on the τexp-avg in NBFP glass system is not significant. Based on the τexp-avg and τrad, the lifetime-based quantum yields QYL of the 5D0 level for the NBFP glasses in the visible region can be obtained by QYL = τexp-avg/τrad, the QYL of 0.2wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 doped NBFP glasses are up to be 99.2%, 98.3%, 97.5% and 96.2%, respectively, and the variation of QYL displays an extremely slow decrease tendency with the increasing of Eu3+ ions.Fig. 6 Fluorescence decay curves of 0.2wt%, 2.0wt%, 4.0wt% and 6.0wt% Eu2O3 doped NBFP glasses monitoring at 615 nm under 465 nm laser excitation.
Non-radiative relaxation rate WNR includes multi-phonon relaxation WMPR rate and cross relaxation rate WCR. When the lowest concentration 0.2wt% Eu2O3 is adopted, the cross relaxation behavior is negligible, thus, WMPR can be calculated by the following equation
[50]. Furthermore, the cross relaxation rates for other Eu2O3 doped NBFP glasses are derived by substituting the WMPR into Eq. (4). The corresponding parameters τexp−avg, QYL, WCR, and WMPR are listed in Table 4 for various Eu3+ doped glasses. The ∼17 s−1 WNR value of 6.0wt% Eu2O3 doped glass is lower than 61 s−1 in P2O5−K2O−Al2O3−PbF2−Na2O glass [51], 272 s−1 in PbF2−TeO2−H3BO3 glass [52], 289 s−1 in TeO2−La2O3−10TiO2 glass [53]. The low WNR rate indicates there is ultrahigh concentration quenching even if Eu3+ ions are adequately heavily doped in NBFP glasses.Table 4. Experimental average fluorescent lifetimes τexp-avg, lifetime-based quantum yield QYL values, multi-phonon relaxation rates WMPR and cross relaxation rates WCR of Eu3+ doped NBFP glasses.
For the potential illumination material, the characterization of absolute spectral parameter under laser pumping is essential to assess practical level of photoluminescence materials [54–56]. As shown in Fig. 7, the net spectral power distributions for 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses were obtained when 465 nm laser optical power was incresed 561 mW. Meanwhile, the corresponding net emission spectral powers for Eu2O3 doped NBFP glasses were up to 52.67 and 116.52 mW under 465 nm laser excitation, respectively. The peak position and shape of the net spectral power distributions are as same as the spectral characteristic of Fig. 4. However, the net emission power for 6.0wt% Eu2O3 doped glass is above 18 times than that of Fig. 4(d).
Fig. 7 Net spectral power distributions for 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses under 465 nm laser excitation with 561 mW power. Insets: fluorescent photographs of 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses under 465 nm laser excitation with 561 mW.
Luminous flux is defined as a derived quantity from radiant flux by evaluating the radiation power that based on its perception upon the standard photometric observer. As presented in Fig. 8, the total luminous fluxes ΦV of the material are deduced under 465 nm laser excitation with 561 mW power by the equation
where is the luminous flux, is relative eye sensitivity, and Km is the maximum luminous efficacy at 555 nm (683 lm/W) [57–59]. The luminous flux distributions of the 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses are presented in Fig. 8 under the excitation of 465 nm laser with 561 mW optical power, and relevant results are summarized in Table 5. The pie charts display the increase of the emission luminous fluxes and the decline of residual laser luminous flux by increasing Eu2O3 doping concentration, providing a high-efficient approach to improve usage of the 465 nm laser by adopting Eu3+ heavy-doped NBFP glasses. The total emission luminous fluxes of 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses are 14.27 and 31.21 lm. The Eu2O3 heavy-doped NBFP glass with 31.21 lm luminous flux is bright enough to be a practical light source, further demonstrating its potential as illumination material.Fig. 8 Luminous flux distributions of 2.0wt% and 6.0wt% Eu2O3 doped NBFP glasses under 465 nm laser excitation with 561 mW power. Insets: pie charts display the percentage of the luminous fluxes between the orangish red emission and those of residual laser.
Table 5. Emission luminous fluxes, residual laser luminous fluxes, and total luminous fluxes from 2.0wt% and 6.0wt% Eu2O3 doped NBFP glass under the excitation of 465 nm laser with 561 mW power.
4. Conclusion
Eu3+ heavy-doped fluorophosphate glass (NBFP) phosphors were prepared, and the high-brightness orangish red fluorescence exhibits their potential as light source. Low non-radiative relaxation rate contributes to the lifetime-based quantum yield of over 96% and ultrahigh concentration quenching in the Eu3+ heavy-doped fluorescent material. The net emission power and the net emission photon number in 6.0wt% Eu2O3 doped NBFP glass were derived to be 6.48 mW and 2.08 × 1016 cps under 465 nm laser with 53.46 mW optical power pumping, respectively, and measured quantum yield is calculated to be 54.03%. Furthermore, the luminous flux is up to 31.21 lm when the excitation power is increased to 561mW, which can satisfy the essential demands of practical brightness illumination. The high-brightness orangish red fluorescence demonstrates the superiority of laser-driven high-brightness Eu3+ heavy-doped NBFP glasses as lighting source, which promotes the further development of orangish red illumination.
Funding
Natural Science Foundation of Liaoning Province, China (Grant No. 20170540068); Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. CityU 11218018).
References
1. J. Rocha, L. D. Carlos, F. A. A. Paz, and D. Ananias, “Luminescent multifunctional lanthanides-based metal-organic frameworks,” Chem. Soc. Rev. 40(2), 926–940 (2011). [CrossRef] [PubMed]
2. M. Ozaki, J. Kato, and S. Kawata, “Surface-plasmon holography with white-light illumination,” Science 332(6026), 218–220 (2011). [CrossRef] [PubMed]
3. S. Pleasants, “Animal vision: Colour perception,” Nat. Photonics 8(4), 267 (2014).
4. B. Zhou, B. Shi, D. Jin, and X. Liu, “Controlling upconversion nanocrystals for emerging applications,” Nat. Nanotechnol. 10(11), 924–936 (2015). [CrossRef] [PubMed]
5. D. C. Zhou, R. F. Wang, X. J. He, J. Yi, Z. G. Song, Z. W. Yang, X. H. Xu, X. Yu, and J. Qiu, “Color-tunable luminescence of Eu3+ in PbF2 embedded in oxyfluoroborate glass and its nanocrystalline glass,” J. Alloys Compd. 621, 62–65 (2015). [CrossRef]
6. S. L. Dong, S. Ye, L. L. Wang, X. Y. Chen, S. B. Yang, Y. J. Zhao, J. G. Wang, X. P. Jing, and Q. Y. Zhang, “Gd3B(W,Mo)O9:Eu3+ red phosphor: from structure design to photoluminescence behavior and near-UV white-LEDs performance,” J. Alloys Compd. 610, 402–408 (2014). [CrossRef]
7. H. Rahimian, Y. Hatefi, A. D. Hamedan, and S. P. Shirmardi, “Structural and optical investigations on Eu 3+ doped fluorophosphate glass and nano glass-ceramics,” J. Non-Cryst. Solids 487, 46–52 (2018). [CrossRef]
8. E. G. Rowse, S. Harris, and G. Jones, “The switch from low-pressure sodium to light emitting diodes does not affect bat activity at street lights,” PLoS One 11(3), e0150884 (2016). [CrossRef] [PubMed]
9. O. Carrión, A. R. J. Curson, D. Kumaresan, Y. Fu, A. S. Lang, E. Mercadé, and J. D. Todd, “A novel pathway producing dimethylsulphide in bacteria is widespread in soil environments,” Nat. Commun. 6(1), 6579 (2015). [CrossRef] [PubMed]
10. F. A. La Sorte, D. Fink, J. J. Buler, A. Farnsworth, and S. A. Cabrera-Cruz, “Seasonal associations with urban light pollution for nocturnally migrating bird populations,” Glob. Change Biol. 23(11), 4609–4619 (2017). [CrossRef]
11. T. Cowan and G. Gries, “Ultraviolet and violet light: attractive orientation cues for the indian meal moth, plodia interpunctella,” Entomol. Exp. Appl. 131(2), 148–158 (2009). [CrossRef]
12. J. Rajagukguk, J. Kaewkhao, M. Djamal, R. Hidayat, and Y. Ruangtaweep, “Structural and optical characteristics of Eu3+ ions in sodium-lead-zinc-lithium-borate glass system,” J. Mol. Struct. 1121, 180–187 (2016). [CrossRef]
13. H. Segawa, N. Hirosaki, S. Ohki, K. Deguchi, and T. Shimizu, “Exploration of metaphosphate glasses dispersed with Eu-doped SiAlON for white LED applications,” Opt. Mater. 42, 399–405 (2015). [CrossRef]
14. M. Kemere, J. Sperga, U. Rogulis, G. Krieke, and J. Grube, “Luminescence properties of Eu, RE3+ (RE= Dy, Sm, Tb) co-doped oxyfluoride glasses and glass-ceramics,” J. Lumin. 181, 25–30 (2017). [CrossRef]
15. M. Zhu, X. Z. Song, S. Y. Song, S. N. Zhao, X. Meng, L. L. Wu, C. Wang, and H. J. Zhang, “A temperature-responsive smart europium metal-organic framework switch for reversible capture and release of intrinsic Eu3+ ions,” Adv. Sci. (Weinh.) 2(4), 1500012 (2015). [CrossRef] [PubMed]
16. G. Galleani, Y. Ledemi, E. S. de Lima Filho, S. Morency, G. Delaizir, S. Chenu, J. R. Duclere, and Y. Messaddeq, “UV-transmitting step-index fluorophosphate glass fiber fabricated by the crucible technique,” Opt. Mater. 64, 524–532 (2017). [CrossRef]
17. R. Balda, J. Fernández, J. L. Adam, and M. A. Arriandiaga, “Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu3+-doped fluorophosphate glass,” Phys. Rev. B Condens. Matter 54(17), 12076–12086 (1996). [CrossRef] [PubMed]
18. R. J. Amjad, M. R. Dousti, M. R. Sahar, S. F. Shaukat, S. K. Ghoshal, E. S. Sazali, and N. Fakhra, “Silver nanoparticles enhanced luminescence of Eu3+-doped tellurite glass,” J. Lumin. 154(1), 316–321 (2014). [CrossRef]
19. Q. Zhang, X. F. Liu, Y. B. Qiao, B. Qian, P. D. Guo, J. Ruan, Q. L. Zhou, J. R. Qiu, and D. P. Chen, “Reduction of Eu3+ to Eu2+ in Eu-doped high silica glass prepared in air atmosphere,” Opt. Mater. 32(3), 427–431 (2010). [CrossRef]
20. G. H. Liu, J. T. Li, and L. Wu, “Preparation and optical properties of Eu-doped Y2O3−Al2O3−SiO2 glass,” Mater. Res. Bull. 48(10), 3934–3938 (2013). [CrossRef]
21. G. Chu, X. S. Wang, T. R. Chen, W. Xu, Y. Wang, H. W. Song, and Y. Xu, “Chiral electronic transitions of YVO4: Eu3+ nanoparticles in cellulose based photonic materials with circularly polarized excitation,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(14), 3384–3390 (2015). [CrossRef]
22. X. Li, H. Zhong, B. Chen, G. Sui, J. Sun, S. Xu, L. Cheng, and J. Zhang, “Highly stable and tunable white luminescence from Ag-Eu3+ co-doped fluoroborate glass phosphors combined with violet LED,” Opt. Express 26(2), 1870–1881 (2018). [CrossRef] [PubMed]
23. B. J. R. S. Swamy, B. Sanyal, R. Vijay, P. R. Babu, D. K. Rao, and N. Veeraiah, “Influence of copper ions on thermoluminescence characteristics of CaF2–B2O3–P2O5, glass system,” Ceram. Int. 40(2), 3707–3713 (2014). [CrossRef]
24. K. Binnemans, D. R. Van, C. Gorller-Walrand, and J. L. Adam, “Spectroscopic properties of trivalent lanthanide ions in fluorophosphate glasses,” J. Non-Cryst. Solids 238(1–2), 11–29 (1998). [CrossRef]
25. T. Wei, F. Z. Chen, Y. Tian, and Q. S. Xu, “Efficient 2.7 μm emission and energy transfer mechanism in Er3+ doped Y2O3 and Nb2O5 modified germanate glasses,” J. Quant. Spectrosc. Ra. 133, 663–669 (2014). [CrossRef]
26. X. Wen, G. Tang, J. Wang, X. Chen, Q. Qian, and Z. Yang, “Tm3+ doped barium gallo-germanate glass single-mode fibers for 2.0 μm laser,” Opt. Express 23(6), 7722–7731 (2015). [CrossRef] [PubMed]
27. M. Secu, C. E. Secu, and C. Ghica, “Eu-doped CaF2 nanocrystals in sol-gel derived glass-ceramics,” Opt. Mater. 33(4), 613–617 (2011). [CrossRef]
28. V. Vijaya, V. Venkatramu, P. Babu, C. K. Jayasankar, U. R. Rodríguez-Mendoza, and V. Lavín, “Spectroscopic properties of Sm3+ ions in phosphate and fluorophosphate glasses,” J. Non-Cryst. Solids 365, 85–92 (2013). [CrossRef]
29. T. S. Gonçalves, R. J. M. Silva, M. de Oliveira Jr., C. R. Ferrari, G. Y. Poirier, H. Eckert, and A. S. S. de Camargo, “Structure-property relations in new fluorophosphate glasses singly-and co-doped with Er3+ and Yb3+,” Mater. Chem. Phys. 157, 45–55 (2015). [CrossRef]
30. B. D. Wilts, T. M. Trzeciak, P. Vukusic, and D. G. Stavenga, “Papiliochrome II pigment reduces the angle dependency of structural wing colouration in nireus group papilionids,” J. Exp. Biol. 215(Pt 5), 796–805 (2012). [CrossRef] [PubMed]
31. R. Zhang, H. Lin, Y. L. Yu, D. Q. Chen, J. Xu, and Y. S. Wang, “A new-generation color converter for high-power white led: transparent Ce3+:YAG phosphor-in-glass,” Laser Photonics Rev. 8(1), 158–164 (2014). [CrossRef]
32. H. Gebavi, D. Milanese, R. Balda, M. Ivanda, F. Auzel, J. Lousteau, J. Fernandez, and M. Ferraris, “Novel Tm3+-doped fluorotellurite glasses with enhanced quantum efficiency,” Opt. Mater. 33(3), 428–437 (2011). [CrossRef]
33. R. Bagga, V. G. Achanta, A. Goel, J. M. F. Ferreira, N. P. Singh, D. P. Singh, V. Contini, M. Falconieri, and G. Sharma, “Luminescence study of mixed valence Eu-doped nanocrystalline glass–ceramics,” Opt. Mater. 36(2), 198–206 (2013). [CrossRef]
34. D. K. Singha, P. Majee, S. K. Mondal, and P. Mahata, “A Eu-doped Y-based luminescent metal-organic framework as a highly efficient sensor for nitroaromatic explosives,” Eur. J. Inorg. Chem. 2015(8), 1390–1397 (2015). [CrossRef]
35. H. Guo, X. Wang, J. Chen, and F. Li, “Ultraviolet light induced white light emission in Ag and Eu3+ co-doped oxyfluoride glasses,” Opt. Express 18(18), 18900–18905 (2010). [CrossRef] [PubMed]
36. P. Adhikary, S. Garain, S. Ram, and D. Mandal, “Flexible hybrid Eu3+ doped P(VDF-HFP) nanocomposite film possess hypersensitive electronic transitions and piezoelectric throughput,” J. Polym. Sci. Pol. Phys. 54(22), 2335–2345 (2016). [CrossRef]
37. Q. L. Ma, W. S. Yu, X. T. Dong, J. X. Wang, G. X. Liu, and J. Xu, “Electrospinning fabrication and properties of Fe3O4/Eu(BA)3phen/PMMA magnetic–photoluminescent bifunctional composite nanoribbons,” Opt. Mater. 35(3), 526–530 (2013). [CrossRef]
38. W. C. Wang, J. Yuan, L. X. Li, D. D. Chen, Q. Qian, and Q. Y. Zhang, “Broadband 2.7 μm amplified spontaneous emission of Er3+ doped tellurite fibers for mid-infrared laser applications,” Opt. Mater. Express 5(12), 2964–2977 (2015). [CrossRef]
39. S. Fischer, B. Fröhlicha, H. Steinkempera, K. W. Krämerb, and J. C. Goldschmidta, “Absolute upconversion quantum yield of β-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections,” Sol. Energy Mater. Sol. Cells 122(10), 197–207 (2014). [CrossRef]
40. C. Würth, M. Grabolle, J. Pauli, M. Spieles, and U. Resch-Genger, “Relative and absolute determination of fluorescence quantum yields of transparent samples,” Nat. Protoc. 8(8), 1535–1550 (2013). [CrossRef] [PubMed]
41. L. Aleksandrov, T. Komatsu, R. Iordanova, and Y. Dimitriev, “Structure study of MoO3−ZnO−B2O3 glasses by raman spectroscopy and formation of α-ZnMoO4 nanocrystals,” Opt. Mater. 33(6), 839–845 (2011). [CrossRef]
42. T. B. De. Queiroz, M. B. S. Botelho, T. S. Gonçalves, R. Dousti, and A. S. S. D. Camargo, “New fluorophosphate glasses co-doped with Eu3+ and Tb3+ as candidates for generating tunable visible light,” J. Alloys Compd. 647, 315–321 (2015). [CrossRef]
43. W. D. A. M. de Boer, C. McGonigle, T. Gregorkiewicz, Y. Fujiwara, S. Tanabe, and P. Stallinga, “Optical excitation and external photoluminescence quantum efficiency of Eu3+ in GaN,” Sci. Rep. 4(1), 5235 (2015). [CrossRef] [PubMed]
44. I. I. Kindrat and B. V. Padlyak, “Luminescence properties and quantum efficiency of the Eu-doped borate glasses,” Opt. Mater. 77, 93–103 (2018). [CrossRef]
45. S. Babu and Y. C. Ratnakaram, “Emission characteristics of holmium ions in fluoro-phosphate glasses for photonic applications,” AIP Conf. Proc. 1731(1), 070001 (2016). [CrossRef]
46. F. Huang, Y. Zhang, L. L. Hu, and P. D. Chen, “Judd–Ofelt analysis and energy transfer processes of Er3+ and Nd3+ doped fluoroaluminate glasses with low phosphate content,” Opt. Mater. 38, 167–173 (2014). [CrossRef]
47. S. K. Gupta, N. Pathak, and R. M. Kadam, “An efficient gel-combustion synthesis of visible light emitting barium zirconate perovskite nanoceramics: probing the photoluminescence of Sm3+ and Eu3+ doped BaZrO3,” J. Lumin. 169, 106–114 (2016). [CrossRef]
48. Y. Jin, J. H. Zhang, and W. P. Qin, “Photoluminescence properties of red phosphor Gd3Po7:Eu3+ for UV-pumped light-emitting diodes,” J. Alloys Compd. 579, 263–266 (2013). [CrossRef]
49. G. Lakshminarayana, J. Qiu, M. G. Brik, G. A. Kumar, and I. V. Kityk, “Spectral analysis of Er3+-, Er3+/Yb3+-and Er3+/Tm3+/Yb3+-doped TeO2-ZnO-WO3-TiO2-Na2O glasses,” J. Phys. Condens. Matter 20(37), 375101 (2008). [CrossRef] [PubMed]
50. X. Q. Xiang, B. Wang, H. Lin, J. Xu, J. M. Wang, T. Hu, and Y. S. Wang, “Towards long-lifetime high-performance warm w-LEDs: Fabricating chromaticity-tunable glass ceramic using an ultra-low melting Sn-PFO glass,” J. Eur. Ceram. Soc. 38(4), 1990–1997 (2018). [CrossRef]
51. U. Caldiño, I. Camarillo, A. Speghini, M. Bettinelli, and T. B. De. Queiroz, “Down-shifting by energy transfer in Tb3+/Dy3+ co-doped zinc phosphate glasses”,” J. Lumin. 161, 142–146 (2015). [CrossRef]
52. C. R. Kesavulu, K. K. Kumar, N. Vijaya, K. S. Lim, and C. K. Jayasankar, “Thermal, vibrational and optical properties of Eu3+-doped lead fluorophosphate glasses for red laser applications,” Mater. Chem. Phys. 141(2−3), 903–911 (2013). [CrossRef]
53. M. V. V. Kumar, B. C. Jamalaiah, K. R. Gopal, and R. R. Reddy, “Novel Eu3+-doped lead telluroborate glasses for red laser source applications,” J. Solid State Chem. 184(8), 2145–2149 (2011). [CrossRef]
54. W. Stambouli, H. Elhouichet, B. Gelloz, and M. Férid, “Optical and spectroscopic properties of Eu-doped tellurite glasses and glass ceramics,” J. Lumin. 138(6), 201–208 (2013). [CrossRef]
55. L. G. Dai, L. Wang, H. Q. Jia, W. X. Wang, J. M. Zhou, W. M. Liu, and H. Chen, “Realization of high-luminous-efficiency InGaN light-emitting diodes in the “green gap” range,” Sci. Rep-UK. 5, 10883 (2015).
56. D. G. Li, W. P. Qin, S. H. Liu, W. B. Pei, Z. Wang, P. Zhang, L. L. Wang, and L. Huang, “Synthesis and luminescence properties of RE3+ (RE= Yb, Er, Tm, Eu, Tb)-doped Sc2O3 microcrystals,” J. Alloys Compd. 653, 304–309 (2015). [CrossRef]
57. J. Rajagukguk, J. Kaewkhao, M. Djamal, R. Hidayat, and Y. Ruangtaweep, “Structural and optical characteristics of Eu3+ ions in sodium-lead-zinc-lithium-borate glass system,” J. Mol. Struct. 1121, 180–187 (2016). [CrossRef]
58. L. Li, H. T. Lin, S. T. Qiao, Y. Zou, S. Danto, K. Richardson, J. D. Musgraves, N. S. Lu, and J. J. Hu, “Integrated flexible chalcogenide glass photonic devices,” Nat. Photonics 8(8), 643–649 (2014). [CrossRef]
59. X. M. Zang, D. S. Li, E. Y. B. Pun, and H. Lin, “Dy3+ doped borate glasses for laser illumination,” Opt. Mater. Express 7(6), 2040–2054 (2017). [CrossRef]
