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Abstract
Intense multi-peak warm yellowish-white fluorescence emissions of Dy3+ are exhibited in alkaline-earth borate glasses under the excitation of a blue laser. The net emission power and net emission photon number are identified to be 498.23μW and 14.28 × 1014cps, respectively, and the quantum yield is as high as 23.10% in 2.0wt% Dy2O3 doped alkaline-earth borate glass under the excitation of a 453nm blue laser with 14.13mW power. Anticipation of fluorescence color reveals that white luminescence can be achieved when the intensity ratio between residual laser light and Dy3+ emission reaches the appropriate range. By the introduction of Ce3+, the excitation wavelength range and the emission intensity of Dy3+ in alkaline-earth borate glasses are expanded to UVB region and improved by a maximum sensitization factor of 38.2, respectively, demonstrating the utilizability of a UV pumping laser for Ce3+−Dy3+ codoped alkaline-earth borate glasses. Efficient fluorescence emission and realizable white lighting in Dy3+ doped alkaline-earth borate glasses under the laser excitation will promote the development of laser illumination devices.
© 2017 Optical Society of America
1. Introduction
Since the advent of high-power laser in the visible band, it has been attempted to apply to establish the illumination effect as pump source [1–4]. Light-emitting diodes (LEDs) as next generation of solid state lighting (SSL) have shown high potential for the replacement of conventional incandescent light bulbs and fluorescent lamps. Also based on the semiconductor light emitting, laser diodes (LDs) have characteristics of monochromaticity, polarization and parallelism on beam quality due to coherent phase photon emission [5–10], which is suitable for creating a more superior than the LED lighting device. High-power, high-brightness and diffraction-limited laser can be achieved by using fiber coupling technology, which provides support for promising and innovative laser illumination application, such as automobile headlights, instantaneous 3D imaging, fluorescence microscopy for optical measurement, and even micro-satellite lighting in aviation field [11–14]. Previous investigations have demonstrated that the combination of blue laser diode (GaN-blue chip) with yellow phosphor (YAG: Ce3+ phosphor) and RGB laser synthetic white laser are two main ways to realize visible white laser illumination, and researchers have been exploring more optimized and convenient laser lighting materials [15–22].
Trivalent rare-earth Dy3+ ions have been considered as promising luminescent centers for developing white light sources because its 4F9/2 excited state exhibits higher quantum efficiency and its characteristic emissions lead to generation of white light, and Dy3+ can be easily excited by commercial UV and blue laser [23–27]. Previous investigations on Dy3+ doped luminescent materials have demonstrated that in appropriate doping cases the quantum yield of Dy3+ can achieve single digit or even ten digit percentage [9, 28–30]. Particularly, the introduction of sensitizer Ce3+ enables Dy3+ to efficiently harvest UVB photon and enhance the visible emission [31–35]. Borate glasses doped with trivalent rare-earth ions are attractive to fabricate high quality laser illuminator due to excellent rare-earth ions solubility, high transparency, low melting temperature, and excellent thermal stability. Besides, alkaline-earth oxides can improve glass forming capability, and heavy metal oxides can result in good optical properties such as second harmonic generation [36–45]. There are previous reported that fluorescence emission depends on host material and rare earth ions concentration. However, little research deals with quantitative characterization for the dependence of white light luminescence on laser source. As a pump source, the path of the laser can be changed by reflection, and then the glass phosphor is excited and a white light is formed. A schematic diagram of glass phosphor for laser illumination is proposed and depicted in Fig. 1. This laser-lighting system is suitable for some special occasions owing to that photoluminescence avoids the danger of electricity generated by electric lighting.
In this work, Dy3+ doped alkaline earth borate glasses with warm yellowish-white fluorescence have been fabricated and characterized. Absolute characterization of glass samples under excitation of 453nm blue laser reveals the potential that combined white luminescence can be achieved when intensity ratio between residual laser and Dy3+ emissions reaches to the appropriate range, and the color coordinates and correlated color temperature (CCT) of white fluorescence by color anticipation are derived and calculated. High quantum yields of Dy3+ in alkaline-earth borate glasses are derived from absolute spectral parameters, and the effective excitation wavelength range and the emission intensity of Dy3+ in alkaline-earth borate glasses are remarkably expanded and improved by the introduction of Ce3+. Efficient fluorescence emission and realizable white lighting in Dy3+ doped alkaline-earth borate glasses under the laser excitation provides a promising direction to develop efficient optical devices for laser illumination.
2. Experiments
RE ions doped alkaline-earth borate glass samples were prepared from high-purity Li2CO3, NaNO3, Na2CO3, KNO3, K2CO3, ZnO, BaCO3, SrCO3, Sb2O3, H3BO3 powders according to the molar host composition of 6Li2O−4Na2O−2K2O−10ZnO−15BaO−7SrO−0.3Sb2O3− 55.7B2O3. In the formula, 0.1mol Na2O was introduced through NaNO3 and the other 3.9 mol Na2O was derived from Na2CO3, meanwhile, 0.05 mol K2O was introduced through KNO3 and the other 1.95 mol K2O was derived from K2CO3. Dopants were adopted as xCe(NO3)3 and yDy2O3 where x = 0, y = 0.1, 0.2, 0.5, 1.0, 2.0, 3.0 and x = 0.02, y = 0, 0.2, 0.5 in wt% based on the host glass weights. The well-mixed raw materials in alumina crucibles were heated at 1100 °C for 2 h using an electric furnace, and reducing atmosphere containing carbon monoxide was adopted in Ce3+ doping cases, then the molten glasses were poured into a graphite mold. The glasses were subsequently annealed at 500 °C for 2 h, and cooled down slowly to room temperature. For optical measurements, the annealed glass samples were sliced and polished into pieces with parallel sides.
The density of the 1.0wt% Dy2O3 doped alkaline-earth borate glasses was measured to be 3.07 g⋅cm−3, thus the number density of Dy3+ ions was calculated to be 9.186 × 1019 cm−3. Using a Metricon 2010 prism coupler, the refractive indices of the 1.0wt% Dy2O3 doped alkaline-earth borate glasses were measured to be 1.5914 and 1.5712 at 635.96 and 1546.9 nm, respectively. The refractive indices of the sample at all other wavelengths can be calculated by Cauchy’s equation with A = 1.5671 and B = 8932 nm2. Differential thermal analysis (DTA) scan was carried out by a WCR-2D differential thermal analyzer at a rate of 10 °C/min from the room temperature to 800 °C and the transition temperature (Tg) of the alkaline-earth borate glass is derived to be 476 °C. Absorption spectrum of the glass sample was recorded by a Perkin-Elmer UV/VIS/NIR Lambda 750 spectrophotometer. Visible fluorescence spectra were measured by a Hitachi F-7000 fluorescence spectrophotometer. Fluorescence decay curves were measured by a Jobin Yvon Fluorolog-3 spectrophotometer equipped with an R928 photomultiplier tube (PMT) detector and a flash Xe-lamp.
The absolute spectral parameters of Dy3+ doped glass samples were measured in an integrating sphere of 3.3 inch inner diameter (Labsphere) which was connected to an exciting 453 nm blue laser source and a QE65000 CCD detector (Ocean Optics) with 400μm-core and 600μm-core optical fibers, respectively. Also, the absolute spectral distributions in Ce3+−Dy3+ doping cases were measured by similar procedure in an integrating sphere of 25 cm inner diameter (Labsphere) with a 600 μm-core optical fiber, and the exciting 308 nm UVB light emitting diode (UVB-LED) was fixed at 20mA. A standard halogen lamp (Labsphere, SCL-050) was used to calibrate this measurement system, and spectral power distribution was obtained through fitting the factory data based on the black body radiation law. All the measurements were carried out at room temperature.
3. Results and discussion
3.1. Radiative transition properties of Dy3+ doped alkaline-earth borate glasses
The emission spectra of Dy3+ doped alkaline-earth borate glasses under 350nm excitation are shown in Fig. 2(a). The emission spectra have two visible emission bands which located at 482 and 575nm corresponding to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively [46–49]. The blue and yellow emission peaks result in a warm yellowish-white color to the human eyes, and in the inset of Fig. 2(a), the fluorescence photograph of 2.0wt% Dy2O3 doped alkaline-earth borate glasses under 350nm UVA radiation is exhibited. Figure 2(b) presents the excitation spectra of Dy3+ doped alkaline-earth borate glasses in the wavelength range of 200−520 nm monitoring at 574 nm emissions. The spectra consists of eight excitation bands peaking at 295, 325, 350, 364, 386, 425, 452 and 472nm, which are assigned to transitions from the ground state 6H15/2 to the various excited states of Dy3+ ions [50, 51]. The intensities of the emission and excitation peaks are rising with the appropriate increase of Dy3+ concentration. When the Dy3+ concentration further increases from 2.0wt% to 3.0wt%, concentration quenching is observed, indicating that the optimum Dy3+ doping concentration is about 2.0wt%. Intense excitation bands confirm that Dy3+ ions can be efficiently excited under the radiation of long-wavelength UV, violet and blue pumping laser.
Fig. 2 (a) Emission spectra of 0.2wt%, 0.5wt%, 1.0wt% 2.0wt% and 3.0wt% Dy2O3 doped alkaline-earth borate glasses under 350nm UVA excitation. Inset: fluorescence photograph of 2.0wt% Dy2O3 doped alkaline-earth borate glasses under 350nm UVA radiation. (b) Excitation spectra of 0.2wt%, 0.5wt%, 1.0wt% 2.0wt% and 3.0wt% Dy2O3 doped alkaline-earth borate glasses monitoring at 574 nm emission.
The absorption spectrum of Dy3+ doped alkaline-earth borate glasses is presented in Fig. 3. Absorption bands located at 385, 425, 451, 470, 749, 798, 895, 1086, 1264 and 1672 nm correspond to the transitions of Dy3+ from the ground state 6H15/2 to the specific excited states. The radiative transitions belonging to the 4f9 electronic configurations can be analyzed by the Judd-Ofelt theory based on the absorption of Dy3+ [52]. The Judd-Ofelt parameters for Dy3+ ions in alkaline-earth borate glasses are derived to be Ω2 = 4.52 × 10−20 cm2, Ω4 = 1.79 × 10−20 cm2, and Ω6 = 1.58 × 10−20 cm2 by a least-squares fitting approach, respectively. The root-mean-square deviation δrms is 1.7 × 10−7, indicating the calculation process is reliable. The larger Ω2 value for Dy3+ in alkaline-earth borate glasses is the direct consequence of strong inversion asymmetry and high covalency around Dy3+ ions, which is beneficial to achieving the intense fluorescence emission.
Fig. 3 Absorption spectrum of 1.0wt% Dy2O3 doped alkaline-earth borate glasses. Insets: (a) Fluorescence decay curves of the 4F9/2 level for 0.1wt%, 0.2wt%, 0.5wt%, 1.0wt% and 2.0wt% Dy2O3 doped alkaline-earth borate glasses. (b) Simulated emission cross-section profiles for the emission bands of 1.0wt% Dy2O3 in visible wavelength region.
Using these intensity parameters, some important radiative properties including spontaneous emission probabilities (Arad), luminescence branching ratios (β) and radiative lifetime (τrad) for the optical transitions of Dy3+ in alkaline-earth borate glasses were calculated and listed in Table 1. The predicted spontaneous emission probabilities Arad for the transitions 4F9/2→6H15/2, 6H13/2, 6H11/2, 6H9/2 and 6F11/2 are derived to be 143.5, 509.2, 58.4, 15.6 and 64.5s−1, and the corresponding branching ratios β account for 17.2%, 61.1%, 7.0%, 1.9% and 7.7%, respectively, showing that visible emissions of Dy3+ in alkaline-earth borate glasses are effective.
Table 1. Predicted spontaneous emission probabilities Arad, branching ratios β, and radiative lifetime τrad of Dy3+ in alkaline-earth borate glasses.
Luminescence decay curves of the 4F9/2 level for 0.1wt%, 0.2wt%, 0.5wt%, 1.0wt% and 2.0wt% Dy2O3 doped alkaline-earth borate glasses are presented in Fig. 3(a). The fluorescent lifetimes (τexp) of the 4F9/2 level can be derived from the fluorescence decays by using the following formula [53]
where I(t) is the emission intensity at time t, and the related results are calculated and listed in Table 2. The decrease in fluorescence decay time as a function of Dy3+ concentration is due to the energy transfer via cross relaxation processes which have a negative influence on the luminescence quantum efficiency. Based on the experimental lifetimes, the quantum efficiency ηq for 4F9/2 level of Dy3+ in the visible region can be obtained by ηq = τexp/τrad. Meanwhile, the experimental lifetime (τexp) can be expressed as , where WMPR is the multiphonon relaxation (MPR) rate and WCR is the rate of cross-relaxation(CR), which should be dependent on the ion concentration [54]. The calculated WCR values corresponding to different doping concentrations are derived and summarized in Table 2, where the cross-relaxation process in the lowest-concentration doping is ignored. With the increase of Dy3+ concentration, the non-radiative relaxation rate WNR shows an upward trend.Table 2. Experimental average lifetimes τexp (ms), multi-phonon relaxation rates WMPR (s−1), cross-relaxation rates WCR (s−1) and non-radiation relaxation rates WNR (s−1) of Dy3+ in alkaline-earth borate glasses.
Figure 3(b) shows the stimulated emission cross-section σem of 1.0wt% Dy2O3 doped alkaline-earth borate glasses, which is evaluated from the experimental luminescence spectrum by the Füchtbauer-Ladenburg formula [55]
where n, Arad and N(λ) represent the refractive index, spontaneous transition probability and emission photon distribution, respectively. The obtained maximum values of σem-max for 4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2 and 4F9/2→(6H9/2, 6F11/2) emission transitions are 2.28 × 10−22, 17.98 × 10−22, 2.91 × 10−22 and 4.64 × 10−22 cm2, respectively. The σem-max value at 575 nm in this case is smaller than the reported values of Dy3+ doped aluminum-phosphate glasses (25.65 × 10−22 cm2) [56] and larger than Dy3+ doped BaF2–B2O3 (2.74 × 10−22 cm2) [57] and ZnO–PbO–P2O5 (8.80 × 10−22 cm2) glasses [58]. Long radiative lifetime and large emission cross section predict effective energy extraction of Dy3+ doped alkaline-earth borate glasses as illuminating luminous body.3.2 Absolute spectral parameters of Dy3+ doped alkaline-earth borate glasses
Here, an integrating sphere connected with a pumping laser was applied for the determination of absolute spectral parameter, which provided external quantum yield to assess luminescence materials. The net spectral power distributions of fluorescence were obtained under the excitation of a 453 nm blue laser with diverse pump powers as indicated in Fig. 4, and the near-white emission was observed in Dy3+ doped alkaline-earth borate glass samples under the excitation of blue laser in the photograph of Fig. 4. Each spectral power distribution curve consists of four emission bands located at 483, 575, 663 and 755 nm which are assigned to 4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2 and 4F9/2→(6H9/2, 6F11/2) transitions, respectively. Under the excitation of 453 nm laser with powers (Plaser) of 4.10, 7.31 and 14.13mW, net emission spectral powers for 0.5wt% Dy2O3 doped alkaline-earth borate glasses are obtained to 72.78, 128.65, and 236.64μW, and for 2.0wt% Dy2O3 doped alkaline-earth borate glasses are as high as 164.74, 261.52, and 498.23μW, respectively, confirming that high Dy3+ doping concentration contributes to more efficient utilization of laser.
Fig. 4 Net spectral power distribution in (a−c) 0.5wt% Dy2O3 and (d−f) 2.0wt% Dy2O3 doped alkaline-earth borate glasses under the 453 nm laser excitation. Insets: fluorescence photographs of Dy2O3 doped alkaline-earth borate glasses under the excitation of 453 nm laser in an integrating sphere. Net emission photon distribution in (g−i) 0.5wt% Dy2O3 and (j−l) 2.0wt% Dy2O3 doped alkaline-earth borate glasses under the 453nm laser excitation. Insets: details of related net absorption photon distribution.
Based on the net spectral power distributions, the photon distributions can be derived by
where λ is the wavelength, ν is the wavenumber, h is the Planck constant, c is the vacuum light velocity, and P(λ) is spectral power distribution. The abscissa of distribution spectrum is converted to wavenumber (cm−1) for accurate deconvolution. The net absorption and emission photon distribution curves of Dy3+ doped alkaline-earth borate glasses were derived from Eq. (3) with P(λ) as presented in Fig. 4. With the increase of the laser pump power, the emission photon numbers show an upward trend both for 0.5wt% and 2.0wt% Dy2O3 doped alkaline-earth borate glasses.Quantum yield (QY) is being used as a selection criterion of the luminescence materials for potential use in solid-state lighting and is defined as the number ratio of photons emitted to absorbed [59]. Namely,
The QYs of Dy3+ doped alkaline-earth borate glasses under the excitation of the 453 nm blue laser with diverse pump power were listed in Table 3. The total QYs of 0.5wt% and 2.0wt% Dy2O3 doped alkaline-earth borate glasses were calculated to be ~18% and ~23% in various excitation power densities, respectively. Under excitation of 453nm laser with 3.27W/cm2 power density, the quantum yield of 2.0wt% Dy2O3 doped alkaline-earth borate glasses reaches to be 23.74%, which are higher than 7% in Dy3+ doped YVO4 nanoparticles [60] and 8.90% in Dy3+ doped aluminium germanate (NMAG) glasses [61]. High photon capture efficiency further indicates the potential of Dy3+ doped alkaline-earth borate glasses as high quality phosphors for laser illumination.Table 3. Absorption and emission photon numbers and quantum yields in Dy2O3 doped alkaline-earth borate glasses under 453nm laser excitation.
3.3. Color anticipation of Dy3+ doped alkaline-earth borate glass phosphors with optimizating laser path
From the aspect of practicality, the intense yellowish-white light with adequate intensity and good directivity in 0.5wt% and 2.0wt% Dy2O3 doped alkaline-earth borate glasses provide favorable surroundings in developing laser illumination device. Under the excitation of a 453 nm blue laser, the fluorescence colors and intensities of Dy3+ doped alkaline-earth borate glasses vary as exhibited in Fig. 5, demonstrating the obvious photoluminescent improvement of Dy3+ emissions when the laser power increased from 4.10 to 14.13mW.
Fig. 5 Fluorescence photographs of (a−c) 0.5wt% Dy2O3 doped alkaline-earth borate glasses under the 453 nm laser excitation with power of 4.10, 7.31 and 14.13mW, respectively. Fluorescence photographs of (d−f) 2.0wt% Dy2O3 doped alkaline-earth borate glasses under the 453 nm excitation with laser power of 4.10, 7.31 and 14.13mW, respectively.
Owing to fixed branching ratio of Dy3+ intrinsic emission, white fluorescence for laser illumination can be achieved when intensity ratio between laser and Dy3+ emissions reaches the appropriate range. In this case, integral fluorescence emissions derived from the combination of the residual laser and Dy3+ spontaneous emission fluorescence can be realized to white. A series of relative spectral distributions whose intensity ratio between laser and Dy3+ emissions have been presupposed for Dy3+ doped alkaline-earth borate glasses under the 453 nm laser excitation are illustrated in Fig. 6 (a−c). An attractive fluorescence photograph of Dy3+ doped alkaline-earth borate glasses with extended laser path under the laser excitation is exhibited in the inset of Fig. 5, manifesting that multiple reflection of laser in glass samples will enable the Dy3+ absorption enhanced and the residual laser intensity reduced, that is, white fluorescence should be able to realize.
Fig. 6 Color coordinates in CIE 1931 chromaticity diagrams and (a−c) the presupposed relative spectral distributions for Dy3+ doped alkaline-earth borate glasses under 453nm laser excitation. Inset: fluorescence photographs of Dy2O3 doped alkaline-earth borate glasses with extended laser path under the laser excitation.
The CIE 1931 color coordinates for the white fluorescence of Dy3+ doped alkaline-earth borate glasses under these excitation conditions are calculated using the following formula
where X, Y, Z are the tree tristimulus values. The tristimulus values for a color with a spectral power distribution P(λ) are given bywhere λ is the wavelength of the equivalent monochromatic light. ,,are the three color-matching functions. In addition, using the color coordinates calculated above, the correlated color temperature (CCT) of the mixed white fluorescence corresponding different intensity ratios between the laser light and the Dy3+ emission bands is given by McCamy empirical formula [62]where is the inverse slope line, xe = 0.332 and ye = 0.186. The color coordinates and correlated color temperature (CCT) of white fluorescence in the cases of presupposed intensity radio between remaining laser and Dy3+ emissions are indicated in Fig. 6 and listed in Table 4. With varying the intensity ratios between the residual laser and the Dy3+ emissions, the fluorescence color coordinates move along the bottom left direction to the right boundary of the white region, passing through the pure white region and the nearest-white one is point 4 whose color coordinates and the corresponding CCT were derived to be (0.339, 0.329) and 5187K, respectively. The correlated CCT of the fluorescence located within the white region vary over a large range, which presents an excellent agreement with blackbody radiation curve to some extent. This performance provides the promise to achieve attractive warm, pure and cool white lights, which arouse the enthusiasm in developing laser illuminations.Table 4. CIE coordinates (x, y) and correlated color temperature (CCT) of the presupposed luminescence.
3.4. Expansion of excitation range on Ce3+−Dy3+ codoped alkaline-earth borate glasses
Although Dy3+ ions in alkaline-earth borate glasses can be effectively excited by UVA and blue light, the excitation in the UVB region remains to be limited. Ce3+ as a sensitizing donor can absorb energy in the UVB region and then efficiently transfer to other rare earth ions, such as Dy3+ [63, 64]. The emission spectrum of Ce3+ doped alkaline-earth borate glasses under 310nm UVB excitation is presented in Fig. 7(a), which consists of a broad emission band peaking at 360 nm attributed to 5d-4f transition of Ce3+. Bright blue fluorescence is achieved in Ce3+ doped alkaline-earth borate glasses, as indicated in the inserted photo of Fig. 7(a). The excitation spectrum of the glass samples monitoring at 360nm is comprised of an asymmetric broad band corresponding to the transition from 4f ground state to crystal field-splitting 5d state as shown in Fig. 7(b) and the intense excitation band at 313nm confirms that Ce3+ ions can be efficiently excited under the UVB radiation. Here, the possible energy transfer from Ce3+ to Dy3+ and potential emission channels are illustrated in Fig. 7(c).
Fig. 7 (a) Emission spectrum of 0.02wt% Ce(NO3)3 doped alkaline-earth borate glasses under 310nm UVB excitation. Insets: fluorescence photograph of 0.02wt% Ce(NO3)3 doped alkaline-earth borate glasses under 310nm UVB excitation. (b) Excitation spectrum of 0.02wt% Ce(NO3)3 doped alkaline-earth borate glasses monitoring at 360 nm emission. (c) Energy level diagram of Ce3+ and Dy3+ ion in alkaline-earth borate glasses. Possible energy transfer of excitation and emission mechanisms under UVB excitation are indicated.
The visible emission spectra of Ce3+–Dy3+ codoped and Dy3+ doped alkaline-earth borate glasses are presented in Fig. 8(a) and 8(c). A broad band in codoping cases centered at 357nm is observed due to the typical emission feature for the 5d-4f transition of Ce3+, and two emission bands peaking at 482 and 575 nm are attributed to the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+, respectively. Fluorescence intensity enhancement in the codoping cases is not only because of the emission of Ce3+, but also considered as the contribution of efficient energy transfer from Ce3+ to Dy3+ [65,66]. In addition, in the case of high Dy3+ concentration doping, fluorescence emission enhancement is more obvious because higher concentration Dy3+ increases the energy transfer probability from Ce3+ to Dy3+, which is further confirmed by the weakening of Ce3+ emission bands.
Fig. 8 Emission spectra of (a) 0.5wt% Dy2O3 (curve 1) and 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 (curve 2) and (b) 2.0wt% Dy2O3 (curve 3) and 0.02wt% Ce(NO3)3-2.0wt% Dy2O3 (curve 4) doped alkaline-earth borate glasses under 310nm UVB radiation. Insets of (a) and (b): details of emission spectra in the spectral region of 440–600nm. Excitation spectra of (c) 0.5wt% Dy2O3 (curve 5) and 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 (curve 6) and (d) 2.0wt% Dy2O3 (curve 7) and 0.02wt% Ce(NO3)3-2.0wt% Dy2O3 (curve 8) doped alkaline-earth borate glasses monitoring at 574 nm emission. Insets of (c) and (d): sensitization coefficient of 574 nm fluorescence for comparison of glasses containing Ce(NO3)3 and not containing.
Figure 8(b) and 8(d) show the excitation spectra of Ce3+–Dy3+ codoped and Dy3+ doped alkaline-earth borate glasses monitoring at 574nm emission, respectively. The efficient excited wavelength range of Dy3+ in alkaline-earth borate glasses covers the whole UVB, UVA, violet and blue spectral ranges well, indicating that Dy3+ doped glass samples can be effectively excited by near-ultraviolet, violet and blue pumping sources, respectively. Based on the excitation spectra, the intensity ratio of 574nm emissions between Ce3+–Dy3+ codoped and Dy3+ doped alkaline-earth borate glasses under different excitation wavelengths was obtained as shown in the insets of Fig. 8(b) and 8(d). The intensity ratio possesses large values in excitation wavelength range of 260–340nm, which is in good accordance with the UVB region, and the maximum sensitization factors reach to be 31.2 and 38.2 under 309nm excitation for 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 codoped versus 0.5wt% Dy2O3 doped, and 0.02wt% Ce(NO3)3-2.0wt% Dy2O3 codoped versus 2.0wt% Dy2O3 doped glasses, respectively, indicating that Ce3+ can also be an efficient sensitizer for harvesting UVB photon and greatly enhancing the visible emissions of Dy3+ in Ce3+–Dy3+ codoped alkaline-earth borate glasses.
Effective photon capture makes it a better candidate to achieve remarkable efficiency enhancement for laser illumination. Under the excitation of a 308nm UVB LED, net emission spectral power distribution and derived net photon distribution in Ce3+–Dy3+ codoped and Dy3+ doped alkaline-earth borate glasses are shown in Fig. 9, and corresponding photos with enhanced fluorescence are shown in the insets of Fig. 9, further demonstrating the obvious quality improvement of Dy3+ emissions by the assistance of Ce3+ in the Ce3+–Dy3+ codoped alkaline-earth borate glasses.
Fig. 9 Net spectral power distribution in (a) 0.2wt% Dy2O3 (curve 1) and 0.02wt% Ce(NO3)3-0.2wt% Dy2O3 (curve 2) and (b) 0.5wt% Dy2O3 (curve 3) and 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 (curve 4) doped alkaline-earth borate glasses under 308nm UVB LED excitation. Net emission photon distribution in (c) 0.2wt% Dy2O3 (curve 5) and 0.02wt% Ce(NO3)3-0.2wt% Dy2O3 (curve 6) and (d) 0.5wt% Dy2O3 (curve 7) and 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 (curve 8) doped alkaline-earth borate glasses under 308nm UVB LED excitation. Insets: the luminescence photographs of (Ι) 0.2wt% Dy2O3, (ΙΙ) 0.02wt% Ce(NO3)3-0.2wt% Dy2O3, (ΙΙΙ) 0.5wt% Dy2O3 and (ΙV) 0.02wt% Ce(NO3)3-0.5wt% Dy2O3 doped alkaline-earth borate glasses under 308 nm UVB LED excitation in an integrating sphere. Sample size: 24.80 × 14.14 × 3.62 mm3 (0.2wt% Dy2O3); 25.26 × 15.06 × 3.40 mm3 (0.02wt% Ce(NO3)3-0.2wt% Dy2O3); 22.34 × 15.44 × 3.00 mm3 (0.5wt% Dy2O3); 27.32 × 17.66 × 3.16 mm3 (0.02wt% Ce(NO3)3-0.5wt% Dy2O3).
4. Conclusion
Dy3+ doped alkaline earth borate glasses with intense warm yellowish-white fluorescence were fabricated and characterized. Quantitative characterization under the excitation of 453nm blue laser with 14.13mW power indicates that net emission power and net emission photon number are identified to be 498.23μW and 14.28 × 1014cps, and the quantum yield is as high as 23.10% in 2.0wt% Dy2O3 doped alkaline-earth borate glasses. Anticipation of fluorescence color reveals that white luminescence can be achieved when intensity ratio between residual laser and Dy3+ emission reaches to appropriate range, and color coordinates and correlated color temperature (CCT) of optimal white fluorescence are derived to (0.339,0.329) and 5187K, respectively. By the introduction of Ce3+, the effective excitation wavelength range and the emission intensity in Dy3+ doped alkaline-earth borate glasses are remarkably expanded and improved by a maximum sensitization factor of 38.2 in the UVB region, indicating the utilizability of UV laser pumping for Ce3+−Dy3+ codoped alkaline-earth borate glasses. Realizable white light combination of Dy3+ emissions with the laser manifests that the alkaline-earth borate glasses have great potential in developing laser illumination devices.
Funding
Natural Science Foundation of Liaoning Province, China (2015020179); the CityU Strategic Grant (7004788); and the National Natural Science Foundation of China (61275057).
References and links
1. N. W. Rosemann, J. P. Eußner, A. Beyer, S. W. Koch, K. Volz, S. Dehnen, and S. Chatterjee, “A highly efficient directional molecular white-light emitter driven by a continuous-wave laser diode,” Science 352(6291), 1301–1304 (2016). [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. M. Matthews, F. Pomel, C. Wender, A. Kiselev, D. Duft, J. Kasparian, J. P. Wolf, and T. Leisner, “Laser vaporization of cirrus-like ice particles with secondary ice multiplication,” Sci. Adv. 2(5), e1501912 (2016). [CrossRef] [PubMed]
4. A. Steude, E. C. Witts, G. B. Miles, and M. C. Gather, “Arrays of microscopic organic LEDs for high-resolution optogenetics,” Sci. Adv. 2(5), e1600061 (2016). [CrossRef] [PubMed]
5. Y. Kaganovskii, A. M. Korsunsky, and M. Rosenbluh, “Ripples in amorphous chalcogenide films under homogeneous laser illumination,” Mater. Lett. 183, 156–160 (2016). [CrossRef]
6. R. Porras-Aguilar, K. Falaggis, and R. Ramos-Garcia, “Error correcting coding-theory for structured light illumination systems,” Opt. Lasers Eng. 93, 146–155 (2017). [CrossRef]
7. C. Basavapoornima, C. K. Jayasankar, and P. P. Chandrachoodan, “Luminescence and laser transition studies of Dy3+: K–Mg–Al fluorophosphate glasses,” Physica B 404(2), 235–242 (2009). [CrossRef]
8. N. Vijaya, K. Upendra Kumar, and C. K. Jayasankar, “Dy3+-doped zinc fluorophosphate glasses for white luminescence applications,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 113, 145–153 (2013). [CrossRef] [PubMed]
9. A. N. Meza-Rocha, R. Lozada-Morales, A. Speghini, M. Bettinelli, and U. Caldiño, “White light generation in Tb3+/Eu3+/Dy3+ triply-doped Zn(PO3)2 glass,” Opt. Mater. 51, 128–132 (2016). [CrossRef]
10. U. Caldiño, A. Lira, A. N. Meza-Rocha, E. Pasquini, S. Pelli, A. Speghini, M. Bettinelli, and G. C. Righini, “White light generation in Dy3+-and Ce3+/Dy3+-doped zinc–sodium–aluminosilicate glasses,” J. Lumin. 167, 327–332 (2015). [CrossRef]
11. R. Porras-Aguilar, K. Falaggis, and R. Ramos-Garcia, “Optimum projection pattern generation for grey-level coded structured light illumination systems,” Opt. Lasers Eng. 91, 242–256 (2017). [CrossRef]
12. M. A. Neil, A. Squire, R. Juskaitis, P. I. Bastiaens, and T. Wilson, “Wide-field optically sectioning fluorescence microscopy with laser illumination,” J. Microsc. 197(1), 1–4 (2000). [CrossRef] [PubMed]
13. E. Kristensson, Z. Li, E. Berrocal, M. Richter, and M. Aldén, “Instantaneous 3D imaging of flame species using coded laser illumination,” Proc. Combust. Inst. 36(3), 4585–4591 (2017). [CrossRef]
14. C. G. Lyu, S. Gao, and J. C. Yang, “An optimisation design of adaptive illumination for a multi-reflective 3D scene,” Opt. Lasers Eng. 93, 128–138 (2017). [CrossRef]
15. I. I. Kindrat, B. V. Padlyak, and A. Drzewiecki, “Luminescence properties of the Sm-doped borate glasses,” J. Lumin. 166, 264–275 (2015). [CrossRef]
16. I. I. Kindrat, B. V. Padlyak, S. Mahlik, B. Kukliński, and Y. O. Kulyk, “Spectroscopic properties of the Ce-doped borate glasses,” Opt. Mater. 59, 20–27 (2016). [CrossRef]
17. L. Shamshad, G. Rooh, K. Kirdsiri, N. Srisittipokakun, B. Damdee, H. J. Kim, and J. Kaewkhao, “Effect of alkaline earth oxides on the physical and spectroscopic properties of Dy3+-doped Li2O-B2O3 glasses for white emitting material application,” Opt. Mater. 64, 268–275 (2017). [CrossRef]
18. F. Zaman, G. Rooh, N. Srisittipokakun, H. J. Kim, E. Kaewnuam, P. Meejitpaisan, and J. Kaewkhao, “Scintillation and luminescence characteristics of Ce3+ doped in Li2O–Gd2O3–BaO–B2O3 scintillating glasses,” Radiat. Phys. Chem. 130, 158–163 (2017). [CrossRef]
19. A. C. Rocha, L. H. Andrade, S. M. Lima, A. M. Farias, A. C. Bento, M. L. Baesso, Y. Guyot, and G. Boulon, “Tunable color temperature of Ce3+/Eu2+, 3+ co-doped low silica aluminosilicate glasses for white lighting,” Opt. Express 20(9), 10034–10041 (2012). [CrossRef] [PubMed]
20. J. Zhong, H. Liang, B. Han, Z. Tian, Q. Su, and Y. Tao, “Intensive emission of Dy3+ in NaGd(PO3)4 for Hg-free lamps application,” Opt. Express 16(10), 7508–7515 (2008). [CrossRef] [PubMed]
21. Z. Guosheng, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26(10), 5366–5381 (1982). [CrossRef]
22. B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6(6), 355–359 (2012). [CrossRef] [PubMed]
23. P. P. Pawar, S. R. Munishwar, and R. S. Gedam, “Intense white light luminescent Dy3+ doped lithium borate glasses for W-LED: a correlation between physical, thermal, structural and optical properties,” Solid State Sci. 64, 41–50 (2017). [CrossRef]
24. K. Das, A. Marathe, X. Zhang, Z. Zhao, and J. Chaudhuri, “Superior white light emission and color tunability of tri-doped YBO3:Tb3+, Eu3+ and Dy3+ for white light emitting diodes,” Rsc Adv. 6(97), 95055–95061 (2016). [CrossRef]
25. G. R. Dillip, G. B. Kumar, V. R. Bandi, M. Hareesh, B. D. P. Raju, S. W. Joo, L. K. Bharat, and J. S. Yu, “Versatile host-sensitized white light emission in a single-component K3ZnB5O10:Dy3+ phosphor for ultraviolet converted light-emitting diodes,” J. Alloys Compd. 699, 1108–1117 (2017). [CrossRef]
26. H. L. Sang, S. R. Bae, G. C. Yong, and W. J. Chung, “Visible spectroscopic properties of SiO2–Na2O–Al2O3–LaF3 glass ceramics doped with Dy3+ and Ho3+ under blue LED excitation,” J. Non-Cryst. Solids 431, 126192 (2015).
27. C.Y. Yu, Z.W. Yang, A.J. Huang, Z.Z. Chai, J.B. Qiu, Z.G. Song, and D.C. Zhou. “Photoluminescence properties of tellurite glasses doped Dy3+ and Eu3+ for the UV and blue converted WLEDs,” Journal of Non-Crystalline Solids 457, 1−8 (2017). [CrossRef]
28. Ł. Sójka, Z. Tang, H. Zhu, E. Bereśpawlik, D. Furniss, A. B. Seddon, T. M. Benson, and S. Sujecki, “Study of mid-infrared laser action in chalcogenide rare earth doped glass with Dy3+, Pr3+ and Tb3+,” Opt. Mater. Express 2(11), 1632–1640 (2012). [CrossRef]
29. G. Lakshminarayana, S. O. Baki, A. Lira, I. V. Kityk, U. Caldiño, K. M. Kaky, and M. A. Mahdi, “Structural, thermal and optical investigations of Dy3+-doped B2O3–WO3–ZnO–Li2O–Na2O glasses for warm white light emitting applications,” J. Lumin. 186, 283–300 (2017). [CrossRef]
30. C. B. A. Devi, S. Mahamuda, M. Venkateswarlu, K. Swapna, A. S. Rao, and G. V. Prakash, “Dy3+ ions doped single and mixed alkali fluoro tungsten tellurite glasses for laser and white LED applications,” Opt. Mater. 62, 569–577 (2016).
31. A. Strzęp, W. Ryba-Romanowski, and M. Berkowski, “Spectral characteristics of visible luminescence in Gd2SiO5–Lu2SiO5(LGSO) solid solution crystals co-doped with Ce3+ and Dy3+,” Opt. Mater. 37, 862–865 (2014). [CrossRef]
32. J. Qiao, J. Zhang, X. Zhang, Z. Hao, W. Deng, Y. Liu, L. Zhang, L. Zhang, H. Zhao, and J. Lin, “Formation condition of red Ce3+ in Ca3Sc2Si3O12:Ce3+, N3- as a full-color-emitting light-emitting diode phosphor,” Opt. Lett. 38(6), 884–886 (2013). [CrossRef] [PubMed]
33. T. Wang, F. F. Huang, W. Q. Cao, Y. Y. Guo, R. S. Lei, R. G. Ye, J. J. Zhang, and S. Q. Xu, “Positive influence of Ce3+ on effective transfer Yb3+:2F5/2 → Ho3+:5I6 in silica-germanate glass for mid-infrared applications,” Opt. Mater. Express 7, 1048–1095 (2017). [CrossRef]
34. Q. Wang, W. H. Zhang, S. Y. Ouyang, Y. P. Zhang, and H. P. Xia, “Luminescent properties of Ce3+-doped transparent oxyfluoride glass ceramics containing BaGdF5 nanocrystals,” J. Rare Earths 411(1), 13–19 (2015). [CrossRef]
35. H. Masai and T. Yanagida, “Emission property of Ce3+-doped Li2O-B2O3-SiO2 glasses,” Opt. Mater. Express 5(8), 1851 (2015). [CrossRef]
36. V. P. Tuyen, B. Sengthong, V. X. Quang, P. V. Do, H. V. Tuyen, L. X. Hunga, N. T. Thanhd, M. Nogamia, T. Hayakawag, and B. T. Huy, “Dy3+ ions as optical probes for studying structure of boro-tellurite glasses,” J. Lumin. 178, 27–33 (2016). [CrossRef]
37. R. E. Hayek, F. Ferey, P. Florian, A. Pisch, and D. R. Neuville, “Structure and properties of lime alumino-borate glasses,” Chem. Geol. (2016).
38. G. Samdani, G. Ramadevudu, M. N. Chary, and M. Shareefuddin, “Physical and spectroscopic studies of Cr3+ doped mixed alkaline earth oxide borate glasses,” Mater. Chem. Phys. 186, 382–389 (2017). [CrossRef]
39. N. Soga, K. Hirao, M. Yoshimoto, and H. Yamamoto, “Effects of densification on fluorescence spectra and glass structure of Eu3+-doped borate glasses,” J. Appl. Phys. 63(9), 4451–4454 (1988). [CrossRef]
40. W. H. A. Kamaruddin, M. S. Rohani, M. R. Sahar, H. Liu, and Y. Sang, “Synthesis and characterization of lithium niobium borate glasses containing neodymium,” J. Rare Earths 34(12), 1199–1205 (2016). [CrossRef]
41. B. Padlyak, W. Ryba-Romanowski, R. Lisiecki, O. Smyrnov, A. Drzewiecki, Y. Burak, V. Adamiv, and I. Teslyuk, “Synthesis and spectroscopy of tetraborate glasses doped with copper,” J. Non-Cryst. Solids 356(37-40), 2033–2037 (2010). [CrossRef]
42. S. Chen, Q. Yang, R. K. Brow, K. Liu, K. A. Brow, Y. Ma, and H. Shi, “In vitro stimulation of vascular endothelial growth factor by borate-based glass fibers under dynamic flow conditions,” Mater. Sci. Eng. C 73, 447–455 (2017). [CrossRef] [PubMed]
43. Y. B. Saddeek, K. Aly, G. Abbady, N. Afify, K. S. Shaaban, and A. Dahshan, “Optical and structural evaluation of bismuth alumina-borate glasses doped with different amounts of (Y2O3),” J. Non-Cryst. Solids 454, 13–18 (2016). [CrossRef]
44. S. Loosa, M. Mungraa, B. Ahrensa, R. L. Leonardc, A. Evansc, J. A. Johnsonc, F. Steudel, and S. Schweizera, “Concentration-dependent luminescence and energy transfer in Tb3+/Eu3+ doped borate and fluorozirconate glasses,” J. Lumin. 187, 298–303 (2017). [CrossRef]
45. H. L. Wen, P. A. Tanner, and B. M. Cheng, “Optical properties of 3dN transition metal ion-doped lead borate glasses,” Mater. Res. Bull. 83, 400–407 (2016). [CrossRef]
46. P. P. Pawar, S. R. Munishwar, S. Gautam, and R. S. Gedam, “Physical, thermal, structural and optical properties of Dy3+ doped lithium alumino-borate glasses for bright W-LED,” J. Lumin. 183, 79–88 (2017). [CrossRef]
47. S. Liu, Y. Liang, M. Tong, D. Yu, Y. Zhu, X. Wu, and C. J. Yan, “Photoluminescence properties of novel white phosphor of Dy3+-doped LaBSiO5 glass,” Mater. Sci. Semicond. Process. 38, 266–270 (2015). [CrossRef]
48. M. V. Rao, B. Shanmugavelu, and V. V. R. K. Kumar, “Optical absorption and photoluminescence studies of Dy3+ doped alkaline earth bismuth borate glasses,” J. Lumin. 181, 291–298 (2017). [CrossRef]
49. J. Li, R. Wei, X. Liu, and H. Guo, “Enhanced luminescence via energy transfer from Ag+ to RE ions (Dy3+, Sm3+, Tb3+) in glasses,” Opt. Express 20(9), 10122–10127 (2012). [CrossRef] [PubMed]
50. K. V. Rao, S. Babu, G. Venkataiah, and Y. C. Ratnakaram, “Optical spectroscopy of Dy3+ doped borate glasses for luminescence applications,” J. Mol. Struct. 1094, 274–280 (2015). [CrossRef]
51. C. M. Dodson, J. A. Kurvits, D. Li, M. Jiang, and R. Zia, “Magnetic dipole emission of Dy3+:Y2O3 and Tm3+:Y2O3 at near-infrared wavelengths,” Opt. Mater. Express 4(11), 2441–2450 (2014). [CrossRef]
52. D. D. Chen, Q. Qian, M. Y. Peng, S. H. Xu, Z. M. Yang, Q. Y. Zhang, and Z. H. Jiang, “Effect of Ce3+, Dy3+, and Tb3+ additions on the spectroscopic properties of Er3+/Yb3+ codoped tellurite glasses,” Physica B 405(21), 4453–4456 (2010). [CrossRef]
53. M. R. Rahimi, G. J. Yun, G. L. Doll, and J. S. Choi, “Effects of persistent luminescence decay on mechanoluminescence phenomena of SrAl2O4:Eu2+, Dy3+ materials,” Opt. Lett. 38(20), 4134–4137 (2013). [CrossRef] [PubMed]
54. D. Rajesh, K. Brahmachary, Y. C. Ratnakaram, N. Kiran, A. P. Baker, and G. G. Wang, “Energy transfer based emission analysis of Dy3+/Eu3+ co-doped ZANP glasses for white LED applications,” J. Alloys Compd. 646, 1096–1103 (2015). [CrossRef]
55. F. Rivera-López, P. Babu, L. Jyothi, U. R. Rodríguez-Mendoza, I. R. Martín, C. K. Jayasankar, and V. Lavín, “Er3+–Yb3+ codoped phosphate glasses used for an efficient 1.5 μm broadband gain medium,” Opt. Mater. 34(8), 1235–1240 (2012). [CrossRef]
56. F. Wang, B. Chen, Y. B. Pun, and H. Lin, “Dy3+ doped sodium–magnesium–aluminum–phosphate glasses for greenish–yellow waveguide light sources,” J. Non-Cryst. Solids 391, 17–22 (2014). [CrossRef]
57. Y. Dwivedi and S. B. Rai, “Spectroscopic study of Dy3+ and Dy3+/Yb3+ ions co-doped in barium fluoroborate glass,” Opt. Mater. 31(10), 1472–1477 (2009). [CrossRef]
58. R. J. Amjad, M. R. Sahar, S. K. Ghoshal, M. R. Dousti, and R. Arifin, “Synthesis and characterization of Dy3+ doped zinc–lead–phosphate glass,” Opt. Mater. 35(5), 1103–1108 (2013). [CrossRef]
59. R. Chen, Y. Tian, B. Li, C. Wang, X. Jing, J. Zhang, and S. Xu, “Infrared fluorescence, energy transfer process and quantitative analysis of thulium-doped niobium silicate-germanate glass,” Infrared Phys. Technol. 79, 191–197 (2016). [CrossRef]
60. N. S. Singh, R. S. Ningthoujam, M. N. Luwang, S. D. Singh, and R. K. Vatsa, “Luminescence, lifetime and quantum yield studies of YVO4: Ln3+, (Ln3+ = Dy3+, Eu3+) nanoparticles: Concentration and annealing effects,” Chem. Phys. Lett. 480(4-6), 237–242 (2009). [CrossRef]
61. H. Y. Li, L. F. Shen, E. Y. B. Pun, and H. Lin, “Dy3+ -doped germanate glasses for waveguide-typed irradiation light sources,” J. Alloys Compd. 646, 586–591 (2015). [CrossRef]
62. C.S. McCamy, “Correlated Color Temperature as an Explicit Function of Chromaticity Coordinates,” Color. Res. Appl. 17, 142−144 (1992) 142. [CrossRef]
63. H. Masai and T. Yanagida, “Emission property of Ce3+-doped Li2O-B2O3-SiO2 glasses,” Opt. Mater. Express 5(8), 1851 (2015). [CrossRef]
64. S. Cui, D. L. Zhao, J. Huang, H. Y. Fu, J. Y. Qian, Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+/Dy3+-codoped fluorosilicate glass ceramics,” J. Rare Earths 30(4), 304–309 (2012). [CrossRef]
65. S. Kaur, P. Kaur, G. P. Singh, D. Arora, S. Kumar, and D. P. Singh, “White light emission of Ce3+ sensitized Sm3+ doped lead alumino borate glasses,” J. Lumin. 180, 190–197 (2016). [CrossRef]
66. H. S. Jang, H. Y. Kim, Y. S. Kim, H. M. Lee, and D. Y. Jeon, “Yellow-emitting γ-Ca2SiO4:Ce3+, Li+ phosphor for solid-state lighting: luminescent properties, electronic structure, and white light-emitting diode application,” Opt. Express 20(3), 2761–2771 (2012). [CrossRef] [PubMed]
