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Abstract
Developing a high quality ceramic laser gain medium for solar directly pumped solid state lasers is essential, and yet the light conversion efficiency of the gain media for solar pumping remains a challenge. In this study, Ce and Nd ions, co-doped YAG transparent ceramics with theoretical transmittance and stable Ce3+ valent state were developed, and revealed that the absorbed visible light and light conversion efficiency in Ce,Nd:YAG ceramics were 3.98 times and 1.34 times higher than those in widely reported Cr,Nd:YAG ceramics, respectively. A concentration matching principle between Ce3+ and Nd3+ ions in YAG was established, and a higher Nd3+ ion doping concentration with a relatively low Ce3+ concentration was favorable to improve both the light conversion efficiency and emission intensity at 1064 nm of Ce,Nd:YAG ceramics. Energy transfer efficiency from Ce3+ to Nd3+ of the 0.3 at.%Ce,1.5at.%Nd:YAG ceramic reached as high as 61.71% at room temperature. Surprisingly, it was further promoted to 64.31% at a higher temperature of 473 K. More excited electrons at the upper energy level of Ce3+ ion under the high temperature accounted for this novel phenomenon. This study proposes a new design strategy of gain materials for solar directly pumped solid state lasers.
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1. Introduction
Developing solar pumped solid state lasers for space stations and satellites facilitates the applications such as space energy transmission, space fine measurement and ultra-high speed information transmission, etc., which is crucial to the development of human society [1,2]. Traditional solar pumped solid state lasers use solar cells to convert solar energy into electricity, providing power to the LD pumping source to realize the solid state laser oscillation. The highest light conversion efficiency of this scheme is not exceeding 6%. Conversely, the solar directly pumping mode is characterized by converting solar light into laser directly using laser gain media, and thus promotes the solar light conversion efficiency to several tens of percents and the integration of the laser setup [3–5]. Therefore, it is essential to choose appropriate gain media for solar directly pumped solid state lasers.
Single crystals, glasses, and transparent ceramics are frequently used as the gain media for solid state lasers. Among them, transparent ceramic integrates the merits of both single crystal and glass, allowing it as a promising candidate for solar directly pumped solid state lasers [6–9]. It has been generally recognized by researchers that yttrium aluminum garnet (Y3Al5O12, YAG) transparent ceramic has distinct advantages of excellent ion compatibility, simple preparation and high quantum efficiency [10–12]. A desired development potential is expected for ceramic based solar pumped solid state lasers.
Nd:YAG transparent ceramics have been applied by researchers to realize solar directly pumped laser oscillations. For example, a 10.5 W continuous-wave laser oscillation was obtained from a φ4.0 × 35 mm Nd:YAG transparent ceramic laser rod by Vistas et al., by means of a solar laser head composed of a double-stage semispherical lens and a trapezoidal-shaped pumping cavity [13]. Xu et al. [14] obtained a 27 W solar pumped laser output power on the grooved Nd:YAG ceramic rod, with a slope efficiency of 9%. Nevertheless, the intrinsic narrow absorption peaks of Nd3+ ion dramatically limits its solar light absorption ability.
In order to overcome this problem, sensitize ions represented by trivalent Cr3+ are incorporated into Nd:YAG. According to the previous reports, Ohkubo et al. [15] combined a 4 m2 Fresnel lens and a pumping cavity as the secondary power concentrator, and realized a 80 W solar pumped solid state laser output using a 0.1at.%Cr, 1.0at.%Nd:YAG ceramic as the gain medium. It was demonstrated through simulation that the small-signal gain of Cr,Nd: YAG ceramic was 3-5 times higher than that of Nd: YAG ceramic. A solar convergence system with a 1.0 m2 effective collection area was applied by Liang et al., and a 32.5 W continuous-wave laser output with a slope efficiency of 6.7% was achieved at 1064 nm, by end pumping a φ4.5 × 35 mm Cr:Nd:YAG ceramic rod [16]. Saiki et al. [17] reported a white-light pumped Cr,Nd:YAG ceramic laser using a multi-amplifier system, and the obtained peak power was 1.6 kW.
However, an optimized light conversion efficiency from Cr to Nd in a 0.3at.%Cr, 1.0at.%Nd:YAG ceramic was 36.9%, according to our previous research [18]. Additionally, a light conversion efficiency of 52% was realized in a 2.0at.%Cr, 1.0at.%Nd:YAG ceramic by Lupei et al. [19]. These values should be further promoted, for the better application of transparent ceramics in solar directly pumped solid state lasers. In addition to Cr3+ ion, Ce3+ ion has broadband absorptions of strong intensities centered at ∼339 nm and ∼460 nm, respectively. Both of these absorptions exhibit an excellent matching with the solar spectrum [20–23]. The broad emission band ranged from 500-700 nm overlaps the absorption peaks of Nd3+ ion, and an effective energy transfer from Ce3+ to Nd3+ ions is expected. Payziyev et al. [24] demonstrated by simulation modeling that both the output power and slope efficiency of the Ce,Nd:YAG crystal were doubled compared to that of the Nd:YAG.
Investigations towards Ce,Nd:YAG crystals for solar directly pumped solid state lasers have been carried out by researchers recently [25–27]. By end-pumping a φ6mm × 95 mm Ce:Nd:YAG/YAG slot-bonded crystal rod, Cai et al. [28] realized a 26.93 W solar pumped continuous-wave laser output with a slope efficiency of 6.33%, over an effective solar energy collection area of 0.69 m2. A side-pumped dual-rod Ce:Nd:YAG solar laser was developed by Almeida et al. [29], and the maximum continuous-wave total solar laser power of 58 W was obtained. Garcia et al. [30] revealed that the resultant beam profile shape of a doughnut-shaped Ce:Nd:YAG solar laser was depended on the absorbed solar power, displaying a TEM00-mode profile at elevated input power. By comparing the solar-to-laser conversion efficiency and solar laser conversion efficiency of Ce,Nd:YAG rod under different weather conditions, Garcia et al. [31] demonstrated that a cloudy environment could be an asset for solar laser research.
In general, the relevant studies are basically focus on Ce,Nd:YAG crystals, rather than Ce:Nd:YAG ceramics [26,32]. Considering the intrinsic advantages of ceramic laser material over its single crystal counterpart, it is urgent to conduct a systematic investigation on Ce,Nd:YAG transparent ceramics.
It should be noted that an excellent optical quality is the precondition for Ce,Nd:YAG transparent ceramics applied in solar directly pumped solid state lasers. Li et al. [33] prepared Ce,Nd:YAG ceramics with different Ce3+ ion doping concentrations by the solid state reaction method, and the transmittances of all the ceramics were less than 80% at 1064 nm. Meanwhile, Samuel et al. [32] prepared a 0.05at.%Ce, 0.9at.%Nd:YAG ceramic with the transmittance of ∼80%, by means of the nanotechnology assisted ceramic preparation technique. These transmittances should be further promoted to suffice the need of solar pumped lasers.
Additionally, the broad absorption and emission bands of Ce3+ ion in Ce,Nd:YAG facilitate the conversion of solar light. In this regard, a desired light conversion in Ce,Nd:YAG transparent ceramic is highly expected, if an efficient energy level matching between Ce3+ and Nd3+ ions were realized. Particularly, we know almost all laser devices operation are accompanied by a high temperature (usually higher than hundreds °C) and it would deteriorate the laser output behaviors circularly. Therefore, it is interesting to figure out the light conversion behavior of Ce,Nd:YAG transparent ceramics at an elevated temperature.
Based on the above analysis, in this work, Ce,Nd:YAG ceramics with high optical transmittance and stable Ce3+ valence state were prepared by vacuum sintering. Optical properties and energy transfer performances of Ce,Nd:YAG ceramics as the function of Ce3+ and Nd3+ ion doping concentrations were systematically evaluated. A desired energy transfer efficiency from Ce3+ to Nd3+ of 64.31% was realized in the prepared Ce,Nd:YAG transparent ceramic as elevating the measuring temperature to 473 K in this study, and convinced that Ce,Nd:YAG transparent ceramics had far superior solar light absorption conversion capacity than that of Cr,Nd:YAG ceramics. This study provides theoretical and experimental supports for the practical applications of Ce,Nd: YAG ceramics in solar pumped solid state lasers.
2. Materials and methods
2.1 Ceramic preparation
A series of Ce-Nd co-doped YAG transparent ceramics were prepared by the solid phase reaction method. High-purity Y2O3 (99.999%, Alfa Aesar, Ward Hill, America), Al2O3 (99.99%, Alfa Aesar, Ward Hill, America), CeO2 (99.99%, Alfa Aesar, Ward Hill, America), and Nd2O3 (99.999%, Alfa Aesar, Ward Hill, America) powders were selected as the raw materials. 0.5 wt.% tetraethyl orthosilicate (TEOS, 99.6%, Alfa Aesar, Ward Hill, America) and 0.1 wt.% MgO (99.9%, Alfa Aesar, Ward Hill, America) were used as the sintering aids. A 0.3at.%Cr, 1.0at.%Nd:YAG transparent ceramic was also prepared for the comparison purpose. The detailed chemical formulas and the corresponding abbreviations of the prepared ceramics are shown in Table 1. These raw powders and additives were placed into a ball milling jar and ball milled at 250 rpm for 15 h using high purity Al2O3 balls. The obtained slurry was dried at 50 °C for 24 h and sieved through a 200 mesh screen to obtain ceramic powders. These powders were calcined at 800 °C for 8 h in a muffle furnace to remove the volatile organic residuals. The calcined powders were pressed into disks with the diameter of 22 mm, and then cold isostatic pressed at 200 MPa and held for 5 min. The pressed green bodies were vacuum sintered at 1750 °C for 8 h using a tungsten mesh heated vacuum furnace. For the ceramic annealing treatment, Cr,Nd:YAG ceramics were annealed at 1450 °C for 25 h in air to realize the completely conversion of Cr2+→Cr3+. Ce,Nd:YAG ceramics were air annealed at 1000 °C for 10 h, in order to inhibit the adverse Ce3+→Ce4+ conversion [34]. Finally, all the ceramics were grinded into 2 mm thickness and then mirror polished on both surfaces using diamond slurry for characterization.
Table 1. Ingredients of Ce,Nd: YAG and Cr,Nd: YAG transparent ceramics
2.2 Characterization
An X-ray diffractometer (XRD, D2. Bruker, Karlsruhe, Germany) was used to analysis the phase component of the sintered ceramics. Polished surfaces of ceramics were observed by a scanning electron microscope (SEM; JSM- 6510, JEOL, Kariya, Japan), and the tested ceramics were thermal etched at 1450°C for 2 h for better observation. An UV-VIS-NIR spectrophotometer (Lambda 950, Perkin Elmer, Waltham, MA, America) with a standard, dual light beam arrangement with adjustable slit width was utilized to measure the in-line transmittances of the polished ceramics, and the scanning range was 200-1200 nm. A fluorescence spectrophotometer (FLS 980, Edinburgh Photonics, Edinburgh, England) was applied to analysis the photoluminescence (PL), photoluminescence excitation (PLE), fluorescence decay, and temperature-dependent fluorescence decay spectra of ceramics.
3. Results and discussion
3.1. Microstructural property
Figure 1 shows the photographs of the polished Ce,Nd:YAG and Cr,Nd:YAG transparent ceramics. All the ceramics exhibited a highly transparent appearance, and the words behind them could be clearly distinguished. Ce,Nd:YAG ceramics presented a faint yellow appearance, and this tint was slightly deepened when increasing Ce3+ ion doping concentration. There was no significant color difference between the unannealed and annealed Ce,Nd:YAG transparent ceramics. Color of the unannealed Cr,Nd:YAG was dark green, and a light green tint was presented after annealing Cr,Nd:YAG ceramics at 1450 for 25 h in air, which was the intrinsic color of Cr3+ ion.
Figure 2(a) shows the XRD patterns of Ce,Nd:YAG ceramics. It could be seen that all the diffraction peaks were consistent well with YAG (PDF#33-0040). In Fig. 2(b)-(c), the main diffraction peak located at ∼33.5° was gradually shifted to lower angles with increasing Ce3+ and Nd3+ concentrations, due to the substitution of Ce3+ and Nd3+ ions into Y3+ site in YAG caused lattice expansion [35]. According to the formula design of Ce,Nd:YAG, both Ce3+ ion (r = 1.14 Å, CN = 8) and Nd3+ ion (r = 1.63 Å, CN = 8) replace the dodecahedral Y3+ (r = 1.02 Å, CN = 8) site in YAG lattice, and the schematic crystal structure sketch of Ce,Nd:YAG shown in Fig. 2(d) further illustrates the ion substitution process in Ce,Nd:YAG matrix.
Fig. 2. (a) XRD patterns of Ce,Nd:YAG ceramics and (b)-(c) enlarged diffraction peaks between 33° and 34°, (d) the corresponding schematic crystal structure sketch
Figure 3 displays the SEM images of the polished surfaces of the prepared ceramics. Obviously, all samples exhibited a uniform grain size distribution without any residual pores detected, and their grain sizes were several micrometers. Grain size of ceramics was initially increased and then decreased with increasing Ce3+ ion doping concentration. Because dual effects of sintering promotion and solute drag would be introduced by Ce3+ ion doping, and it could be deduced that the solute drag effect would be increasingly dominated with increasing Ce3+ doping concentration. Secondary phases were detected at the junctions of grain boundaries in the Ce07Nd10 ceramic, owing to the limited solid solubility of Ce3+ ion in YAG matrix [36]. Increasing Nd3+ ion doping concentration monotonously decreased the grain sizes of ceramics, and the trend was similar to that reported by Ikesue et al. [37].
Fig. 3. SEM micrographs of the thermal etched (a)Ce00Nd10, (b) Ce01Nd10, (c) Ce03Nd10, (d)Ce07Nd10, (e)Ce03Nd00, (f)Ce03Nd04, (g) Ce03Nd07, and (h) Ce03Nd15 ceramics.
3.2. Optical characteristics
In order to evaluate the optical qualities of the prepared Ce,Nd:YAG transparent ceramics, their in-line transmittances were measured. As is shown in Fig. 4(a), the Ce,Nd:YAG ceramics with relatively low Ce3+ ion doping concentrations (≤0.2 at.%) exhibited excellent light transmission properties, and their transmittances at 1064 nm almost reached the theoretical value of YAG. Further increasing Ce3+ concentration decreased the optical quality of ceramics, owing to the increased amount of Ce enriched precipitates located at grain boundaries, as shown in Fig. 3(d). From Fig. 4(b) it is obvious that at the fixed Ce3+ content, increasing Nd3+ ion doping concentration slightly affected the transmittances of ceramics. In fact, the sintering densification behavior of the multi-elements doped YAG ceramic is different with that the pure YAG. In this work, a low sintering temperature of 1750 °C was applied to compensate the aggravated sintering promotion effect introduced by Ce doping, and the low annealing temperature was beneficial to stabilize the Ce3+ valence state while maintaining the high transmittance of Ce,Nd:YAG ceramics.
Absorption performances of Ce,Nd:YAG and Cr,Nd:YAG transparent ceramics were evaluated and shown in Fig. 5. In Fig. 5(a), there was no any absorption peak derived from Ce4+ ion centered at ∼300 nm observed from all the Ce3+ ion doped ceramics, indicating that the applied annealing scheme hardly influence the valence stability of Ce3+ ion in Ce,Nd:YAG ceramics [38]. The absorption bands centered at 340 nm and 460 nm were correspond to the 2F5/2 →5d2 and 2F7/2→5d1 transitions of Ce3+ ion, respectively. It is noteworthy that the absorption coefficient of the Ce3+ ion in the Ce01Nd10 ceramic at 460 nm reached 39.35 cm-1, which was far stronger than that of Nd3+ ion. Absorptions of the Ce,Nd:YAG ceramics with Ce3+ ion doping concentrations higher than or equal to 0.2 at.% at 460 nm reached the detection limit of the measuring instrument. The sharp absorption peaks in Fig. 5(b) were ascribed to the absorptions of Nd3+ ion, and the characteristic absorption bands located at 430 nm and 590 nm were caused by Cr3+ ion, corresponding to the 4A2→4T1 and 4A2→4T2 transitions, respectively [39].
It is universally acknowledged that ceramics with a desired solar light absorption ability within the visible region is beneficial to improve its light conversion efficiency and laser performance. Therefore, a solar irradiance spectrum was plotted in Fig. 6, in order to further evaluate the solar light absorption ability of the prepared transparent ceramics [40]. Obviously, the broad absorption bands of both Ce3+ and Cr3+ ions located in the visible region were highly coincident with the solar spectrum, so that solar light could be effectively absorbed by the corresponding ceramics [41]. Integral areas of all the absorption bands of the prepared ceramics within the visible region (370 nm-780 nm) were calculated. It is noteworthy to find out that the calculated integral absorption areas of Ce01Nd10, Cr03Nd10 and Ce00Nd10 ceramics were 1872, 471 and 81, respectively, with the ratio of 23.1: 5.8: 1. It indicates that the visible light absorption of Ce01Nd10 ceramic was 3.98 times higher than that of Cr03Nd10 ceramics. Evidently, the Ce,Nd:YAG transparent ceramic has a desired application potential for solar directly pumped solid-state lasers.
Fig. 6. Absorption spectra of Ce00Nd10, Ce01Nd10 and Cr03Nd10 of AM1.5 direct solar spectrum at 300-800 nm wavelength range
3.3. Ce3+ ion sensitized luminescence and fluorescence decay performances
Luminescence and fluorescence decay performances of the prepared Ce,Nd:YAG transparent ceramics were monitored to further assess their practicability as gain media for solar directly pumped solid state lasers. Figure 7 illustrates the emission characteristics of the vacuum sintered Ce,Nd:YAG transparent ceramics with and without the post annealing treatment, and the exciting wavelength was 450 nm. The emission band ranged from 500 nm to 700 nm centered at 550 nm was the characteristic emission band of Ce3+ ion, corresponding to its 5d1→2F5/2 transition. It is interesting to discover that luminescence intensities of all the annealed ceramics were higher than that of the corresponding unannealed ceramics. Because defects represented by oxygen vacancies are readily to generated as electron traps when sintering ceramics in vacuum. A portion of the electrons that participate in luminescence would be captured by those electron traps to deteriorate the luminescence intensities of ceramics [42]. In this regard, despite vacuum sintering is beneficial to maintain the valence stability of the trivalent Ce3+ ion in Ce,Nd:YAG ceramics, annealing ceramics in oxygen enriched atmosphere is still necessary. However, Li et al. [38] indicated that weak absorptions derived from Ce4+ ions were detected, after annealing the 1.0at.% Ce:YAG ceramic at 1000 °C for 10 h in air, with 0.5 wt.% TEOS as the sintering additive. In this study, owing to the relatively low Ce ion content, the generation of Ce4+ ion was completely inhibited under the same annealing condition.
Fig. 7. PL spectra of Ce,Nd: YAG ceramics with and without the post annealing treatment between 500 nm and 700 nm
As are shown in Fig. 7(a)-(b), luminescence intensity of Ce,Nd:YAG ceramics at 550 nm was initially increased, and reached the maximum when the Ce3+ ion doping concentration was 0.2 at.%. Further increasing the Ce3+ ion concentration decreased the luminescence intensity, owing to the concentration quenching [43]. Figure 7(c)-(d) indicate that increasing Nd3+ ion doping concentration at the fixed Ce3+ content monotonously decreased the emission intensity of Ce3+ ion at 550 nm, with a sharp decreasing magnitude, since the broad emission band of Ce3+ ion overlapped the main absorption peaks (primarily located at 530 nm (4I9/2 → 2K13/2 + 4G7/2 + 2G9/2) and 588 nm (4I9/2 → 2G7/2 + 2 G5/2)) of Nd3+ ion. It indicates an effective energy transfer from Ce3+ to Nd3+ ions occurred in the prepared Ce,Nd:YAG ceramics [44].
Figure 8 shows the emission characteristic of the prepared ceramics between 900 nm and 1200 nm at the exciting wavelength of 450 nm. Incorporating the Ce3+ or Cr3+ ion in Nd:YAG ceramic significantly enhanced its emission intensity at 1064 nm, i.e., the 4F3/2→4I11/2 transition of Nd3+ ion. Interestingly, emission intensities of ceramics within the infrared region were not significantly influenced by Ce3+ ion doping in Fig. 8(a), and the Ce01Nd10 ceramic owned the highest emission intensity at 1064 nm. In Fig. 7(b), Ce02Nd10 ceramic obtained the dominant emission intensity at 550 nm, which was different from that observed in Fig. 8(a). In fact, a competitive relationship is existed between the emitting and sensitizing effect of Ce3+ ion. It can be deduced that increasing Ce3+ ion doping concentration had a more profound influence on promoting its luminescence at 550 nm, rather than accelerating the energy transfer from Ce3+ to Nd3+ ions. It should be pointed out in Fig. 8(b) that emission intensities of all the Ce3+ and Nd3+ co-doped ceramics at 1064 nm were far stronger than that the Cr03Nd10 ceramic. Almedia et al. [26,45] found that the highest solar laser power obtained in the Ce:Nd:YAG ceramic was 1.19 times higher than that in the Cr:Nd:YAG ceramic of the identical ion doping concentration. It further illustrated the anticipated application potential of Ce,Nd:YAG transparent ceramics for solar directly pumped solid state lasers. Meanwhile, emission intensity of Ce,Nd:YAG ceramics at 1064 nm was increased sharply with increasing Nd3+ ion doping concentration. It can be concluded that a higher Nd3+ ion concentration combining a relatively low Ce3+ content is conducive to enhance the Ce3+ sensitized emission intensity of Nd3+ ion.
Obviously, both the solar light absorption and sensitized emission intensities of the Ce,Nd:YAG ceramic were far superior than that of Cr,Nd:YAG ceramics. Despite increasing Cr3+ concentration could further enhance the corresponding absorption and emission intensities in Cr,Nd:YAG ceramics, maintaining the +3 valence state of Cr3+ ion at a high Cr doping concentration is still a challenging issue, since both the Cr2+ and Cr4+ ions in Cr,Nd:YAG cause severe absorptions that overlap the lasing wavelength of Nd3+ ion [46–48]. According to our previous research, divalent Cr2+ ions were detected in Cr,Nd: YAG ceramics when the Cr ion doping concentration reached 0.6 at.%. Residual absorptions originated from Cr2+ ions were still remained, even annealing these ceramics in a high temperature and oxygen enriched atmosphere [46].
A schematic diagram was plotted to further illustrate the energy transfer process from Ce3+ to Nd3+ ions. As can be seen from Fig. 9, when the exciting light is absorbed by Ce3+ ions, the electrons from the 2F5/2 ground state of Ce3+ ions will be transferred to the 5d1 (2A1 g) and 5d2 (2B1 g) excited states, and the excited electrons from the 5d2 (2B1 g) state will be non-radiatively relaxed to the lower 5d1 state. Then, the electrons located at the 5d1 state will be further radiatively returned to the 2F5/2 ground state, accompanied by the broad band emission located within 500-700 nm. Note that this emission overlaps the absorptions of Nd3+ ions (mainly located at 530 nm (4I9/2→4G7/2) and 588 nm (4I9/2→2G5/2)). Therefore, a cross-relaxation process (path (a)) is inevitable [32], when ceramics being excited by a 450 nm source. The absorption cross-section of Nd3+ ion in YAG matrix is relatively small, so path (a) should not be considered as a mainstream energy transfer path contributing to the 1064 nm emission. Path (b) is a cooperative down-conversion process [26,49], since the band gap between the 5d1 and 2F5/2 levels is approximately twice compared to that between the 4F3/2 and 4I11/2 levels of Nd3+ ion. In this regard, one photon transferred from the 5d1 (2A1 g) level of Ce3+ ion will be divided into two infrared photons emitting at 1064 nm. Considering the lifetime of the Nd3+ ion emission at 1064 nm is relatively short, the photon would leap rapidly to the 4I11/2 level, after being transferred to the 4F3/2 level. It is beneficial to the continuous energy transfer from 5d1 (2A1 g) to 4F3/2 levels. Therefore, path (b) is the dominate energy transfer path in Ce,Nd:YAG system.
To further evaluate the energy transfer performance from Ce3+ to Nd3+ ions in Ce,Nd:YAG transparent ceramics, their fluorescence decay behaviors (λex = 450 nm) were measured, and the fitted results are shown in Fig. 10(a)-(b). From Fig. 10(a), it can be seen that variation of the fitted lifetimes from Ce01Nd10 to Ce07Nd10 ceramics was declined modestly. This decline should be attributed to the enhanced inter-activation among Ce3+ ions. Meanwhile, secondary phase within ceramics might cause energy consumption and generate traps in ceramic matrix [50]. Hence the presented secondary phase in the Ce07Nd10 ceramic further accelerated its fluoresce quenching behavior. The fitted lifetimes from Ce03Nd00 to Ce03Nd15 ceramics were decreased sharply from 89.403 ns to 34.236 ns, illustrating that the energy transfer from Ce3+ to Nd3+ ions was significantly enhanced with increasing Nd3+ ion doping concentration.
Fig. 10. (a)-(b) Fluorescence decay curves of Ce,Nd:YAG transparent ceramics and (c) energy transfer efficiencies of Ce,Nd:YAG ceramics at different Nd3+ ion doping concentrations; temperature dependence fluorescence lifetimes of (d) Ce:YAG and (e) Ce,Nd:YAG ceramics, (f) PL spectra of the Ce: YAG ceramics annealed at different temperatures
Considering the fluorescence lifetimes from Ce01Nd10 to Ce03Nd10 ceramics were similar (Fig. 10(a)). In this regard, a series of Ce,Nd:YAG ceramics with a Ce3+ ion doping concentration of 0.3 at.% was chosen to evaluate their light conversion behaviors, for the better comparison with that of Cr03Nd10 ceramic. Light conversion efficiency from Ce3+ to Nd3+ ions in Ce,Nd:YAG ceramics can be obtained according to the following equation:
Energy transfer behavior of Ce,Nd:YAG transparent ceramic at high temperatures should be emphasized, and the corresponding temperature dependence fluorescence decay curves are shown in Fig. 10(d)-(e). It could be seen that fluorescence lifetimes of Ce03Nd00 ceramics were increased, whereas that of the Ce03Nd15 ceramic were slightly decreased, with increasing the operating temperature. The detailed temperature dependence light conversion efficiencies of Ce03Nd15 ceramic is shown in Table 2, and it was amazing that energy transfer efficiency from Ce3+ to Nd3+ of Ce03Nd15 was further promoted to 64.31% at a higher temperature of 473 K.
Table 2. Light conversion efficiencies of Ce03Nd15 ceramic at different temperatures
In general, lattice vibration would be accelerated at an elevated temperature to aggravate the non-radiative transition of active ions, leading to a decreased fluorescence lifetime [54,9]. Nevertheless, phenomenon of the increased fluorescence lifetime with elevating temperature in the prepared Ce03Nd00 (Ce:YAG) ceramic was observed, which was in contradict to the above theory. In this study, all the Ce doped ceramics were annealed at 1000 °C for 10 h in air to inhibit the adverse Ce3+→Ce4+ conversion, and the applied annealing temperature was lower than the conventional annealing temperature of YAG transparent ceramics. In this regard, a small fraction of the residual defects such as oxygen vacancies were still remained within ceramic as electron traps [42,55]. At an elevated temperature, the captured electrons would be released moderately from those trap centers, and thus prolonged the fluorescence lifetime of ceramic [56].
In order to further convince this viewpoint, the vacuum sintered Ce03Nd00 were further annealed at 1450 °C for 10 h in air to analysis its luminescence performance. As can be seen from the PL spectra shown in Fig. 10(f), luminescence intensity of the Ce03Nd00 ceramic annealed at 1450 °C was stronger than that annealed at 1000 °C. It confirms that a portion of defects were still exhibited in the Ce03Nd00 when the annealing temperature was 1000 °C. Despite electrons would be released from these defects at an elevated temperature, it hardly contributed to the enhancement of luminescence intensity of Ce03Nd00 ceramic.
An energy band model is plotted in Fig. 11 to describe the novel thermal behavior. At room temperature, electrons located at the 5d1(2A1 g) and 5d2 levels are inclined to relaxed radiatively to the ground state of Ce3+ ions (Fig. 11(a)). As can be seen in Fig. 11(b), under a high operating temperature, portion of the electrons stored in the 5d1 level, as well as the electrons captured by the electron traps are thermally excited to the upper 5d1(2A1 g) and 5d2 levels, and the radiative transition process of Ce3+ ion is thus retarded, i.e., temperature accelerated retention effect. The accumulated electrons within the 5d1(2A1 g) and 5d2 levels increases the probability of the electron migration from Ce3+ to Nd3+ ion, resulting in the enhanced light conversion efficiency from Ce3+ to Nd3+ of Ce,Nd:YAG transparent ceramics.
Fig. 11. Schematic diagrams illustrating light conversion behaviors of Ce,Nd:YAG ceramics at (a) room temperature and (b) high temperature
4. Conclusion
In summary, an excellent solar light absorption and conversion behavior were realized in pure phase Ce,Nd:YAG transparent ceramics with the transmittance of 84.4% at 1064 nm, and convinced that Ce,Nd:YAG transparent ceramics had far superior solar light absorption conversion capacity than that of Cr,Nd:YAG ceramics. The integral absorption areas of Ce01Nd10, Cr03Nd10 and Ce00Nd10 ceramics within the visible region were 1872, 471 and 81, respectively, with the ratio of 23.1: 5.8: 1. It was noteworthy to find out that the energy transfer efficiency from Ce3+ to Nd3+ of 0.3at.% Ce,Nd:YAG ceramics was increased from 29.36% to 61.71% with an increment of 210.2%, when increasing Nd3+ ion doping concentration from 0.3 at.% to 1.5 at.%, and it was further promoted to 64.31% as elevating the measuring temperature to 473 K. A reasonable energy band model was proposed illustrating the excited electrons are more inclined to maintain at the upper energy level of Ce3+ ion at a high temperature. Finally, this study confirmed that Ce,Nd:YAG transparent ceramic has highly potential application in solar directly pumped solid state lasers, and provides a new insight of the design strategy of gain materials for high power lasers.
Funding
National Key Research and Development Program of China (2023YFB3506600, 2023YFB3501700, 2023YFB3507900); National Natural Science Foundation of China (52202135, 61975070, 52302141); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); International S&T Cooperation Program of Jiangsu Province (BZ2023007); Key Research and Development Project of Jiangsu Province (BE2023050, BE2021040); Natural Science Foundation of Jiangsu Province (BK20221226); Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX22_1255); Special Project for Technology Innovation of Xuzhou City (KC23380, KC21379, KC22461, KC22497); Open Project of State Key Laboratory of Crystal Materials (KF2205).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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