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Abstract
Realizing a high solar light conversion magnitude in Cr,Nd: YAG transparent ceramic is crucial to its applications in solar pumped solid state lasers. In this study, high quality Cr,Nd:YAG transparent laser ceramics with homogeneous microstructure and theoretical transmittance were fabricated, and an efficient laser oscillation of watt-level was realized by pumping ceramic at 808 nm. There were no any characteristic absorptions corresponding to Cr2+ or Cr4+ ions detected, even when the Cr3+ ion doping concentration reached 0.6 at.%. Increasing Cr3+ and Nd3+ doping concentrations significantly enhanced the emission intensity of ceramics at 1.06 µm, and energy transfer efficiency of the 0.3 at.% Cr,Nd: YAG ceramics was increased from 14.9% to 36.9% when increasing Nd3+ ion concentration from 0.3 at.% to 1.0 at.%, with an increasing magnitude of 247.6%. The results indicated that Cr,Nd: YAG transparent ceramic is a promising gain medium for solar pumped solid state lasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the highly developed of science and technology, the exploration and utilization of aerospace resources have gradually become the key to the progress of human society [1,2]. As a crucial component of a national aerospace system, solar-pumped solid state lasers have considerable applications in the fields of information transmission and energy supply, which is of great strategic significance [3,4]. Currently, indirectly pumping mode has been widely applied in solar-pumped solid state lasers. It converts solar energy into electrical energy by means of solar cells, in order to provide power supply for the semiconductor pumping source. However, structure of indirectly pumping system is complicated, and its maximum light conversion efficiency is only 1.5% [5]. On the contrary, if applying a solar directly pumping mode to realize a light-light conversion, conversion efficiency of solar light in solar pumped solid state laser would be greatly improved. Additionally, solar directly pumping mode is conducive to the integration of the laser setup [6,7].
To meet the above requirements, preparation of the laser gain medium suitable for the solar directly pumping mode is necessary. Single crystals, glasses and transparent ceramics are the most frequently applied solid-state laser gain media, and transparent ceramics have the advantages of easy fabrication, low cost and flexibility in large size or composite structure preparation, etc., compared with those of single crystals or glasses [8–10]. It should be pointed out that yttrium aluminum garnet (Y3Al5O12, YAG) transparent ceramic processes excellent ion compatibility, easy preparation and high quantum efficiency [11,12]. Excellent physical and chemical properties of YAG enable it accommodate to the severe space environment such as extreme temperature (from -160 °C to 120 °C) and pressure (vacuum atmosphere). Therefore, YAG transparent ceramic has great developing potential for its real aerospace applications.
It is worth noting that among the ion doping system of YAG, absorption of Cr3+ ion has considerable matching degree with the solar spectrum. Because energy of solar radiation is basically concentrated in the visible range, and Cr3+ ion has wide ranged absorptions within the visible band with strong absorption intensities [13,14]. It has been proved by Ikesue et al. that the absorbed amount of solar radiation by Cr3+ ion was far stronger than that of Nd3+ ion [15]. Meanwhile, emission of Cr3+ ion overlaps the absorption of Nd3+ ion, and Cr3+ ion sensitized solar pumped 1 µm laser could be realized accordingly [16]. Through calculations, Zhang et al. proved that the absorbed solar energy by Cr,Nd: YAG transparent ceramics was as high as 38.44% of the solar constant, when the Cr3+ and Nd3+ ion doping concentrations were 0.1 at.% and 1.0 at.%, respectively [17]. In summary, Cr,Nd: YAG transparent ceramic is a splendid gain medium for the solar directly pumped solid state lasers.
According to the previous reports, a 80 W Cr,Nd: YAG ceramic based solar pumped solid state laser output was realized by Yabe et al. It proved that the small signal gain of Cr,Nd: YAG ceramic was 3-5 times higher than that of Nd: YAG ceramic through simulations [18]. Saiki et al. reported a white light excited laser output with a magnitude of kW level using Cr,Nd: YAG ceramic as gain medium, and the obtained light conversion efficiency was as high as 63% [19]. Through comparing the characteristics of Cr,Nd: YAG ceramic and single crystal based solar pumped lasers, Almeida et al. convinced that Cr,Nd: YAG ceramic has considerable advantages over its single crystal counterpart with respect to the laser performance. The obtained laser output power were 13.5 W for ceramic and 12.3 W for single crystal, respectively [20]. Shi et al., analyzed the transmittances and scattering coefficients of the fabricated Cr,Nd: YAG ceramics, and concluded that a 123 W solar pumped laser output could be realized, if side-pumping Cr,Nd: YAG ceramics with 6 mm diameter and 80 mm length using the optimized solar light collection system [21]. A 32.5W laser output with an optical conversion efficiency of 6.7% was realized by Liang et al, through pumping Cr,Nd: YAG ceramic rod (φ4.5 mm×35 mm) using a Fresnel lens of 1.0 m2 as the primary solar collection system [22]. Additionally, our group reported a successful regulation of the trivalent Cr3+ charge state in high quality 0.3 at.%Cr3+/1.0 at.%Nd: YAG transparent ceramics [23]. The above studies reported the preparation of high quality Cr,Nd: YAG transparent ceramics, as well as the corresponding ceramic based solid state laser outputs. However, all the obtained results were based on the constant Cr3+ and Nd3+ ion doping concentrations. It should be particularly pointed out that for the better application purpose of Cr,Nd: YAG transparent ceramic in solar pumped solid state lasers as gain medium, energy level matching between Cr3+ and Nd3+ ions should be further optimized.
Meanwhile, two fundamental characteristics are required for high quality Cr,Nd: YAG transparent ceramics: (1) an excellent light transmission performance and a stable trivalent state of Cr ion. Both divalent and tetravalent states of Cr ion would cause strong and wide ranged absorptions within the emission band of Nd3+ ion at ∼1 µm, which is fatal to the laser performance of ceramic [24]. According to our previous research, by using the Si-Mg additive system, divalent Cr2+ ion was detected in Cr,Nd: YAG ceramics when the Cr ion doping concentrations were equal or higher than 0.6at.%. Even annealing those ceramics in oxygen atmosphere under high temperature could not completely eliminate the residual Cr2+ ion [23]. (2) High conversion efficiency of solar light. Power density of solar light is far lower than that of the diode pumping source. Accordingly, ceramic with sufficient absorption and conversion amounts of solar light is required. Generally, it is of great significance to thoroughly perceive the directional correlation of the energy level matching between Cr3+ and Nd3+ ions under the promise of minimize the scattering loss of ceramics.
Based on the above analysis, in this work, aiming at obtaining Cr,Nd: YAG transparent ceramics with low scattering loss and stable Cr3+ charge state, effect of Cr3+ and Nd3+ ions doping on the microstructural, optical and luminescence performances of Cr,Nd: YAG ceramics, as well as the energy transfer performances between Cr3+ and Nd3+ ions were investigated systematically. High quality Cr,Nd: YAG transparent ceramics were fabricated through the vacuum sintering technique, and regulation laws of light conversion were obtained. This work provides theoretical guidance and experimental basis for the practical applications of Cr,Nd: YAG ceramic based solar pumped solid state lasers.
2. Materials and methods
2.1 Ceramic preparation
High purity Y2O3 (99.999%, Alfa Aesar, Ward Hill, America), Al2O3 (99.99%, Alfa Aesar, Ward Hill, America), Cr2O3 (99.999%, Alfa Aesar, Ward Hill, America) and Nd2O3 (99.999%, Alfa Aesar, Ward Hill, America) powders were selected as the raw materials. They were weighted precisely using an analytical balance. The detailed chemical formula, as well as the abbreviations of the corresponding Cr,Nd: YAG ceramics are shown in Table 1. 0.45 wt.% tetraethyl orthosilicate (TEOS, 99.6%, Alfa Aesar, Ward Hill, America) and 0.1 wt.% MgO (99.9%, Alfa Aesar, Ward Hill, America) were applied as the sintering additives. These raw powders and additives were mixed and ball milled for 20h under 200 rpm. After drying the slurries in an oven, they were crushed and sieved into powders, and then pressed into pellets with a diameter of 22 mm. The obtained pellets were cold isostatic pressed at 200 MPa for 8 min and then calcined at 1000 °C for 2h. The sintering process was carried out at 1760 °C for 20h in vacuum, and the obtained ceramics were then annealed at 1450 °C for 50h in air in a muffle furnace. Finally, all the ceramics were polished on both sides into 2 mm thickness for characterization.
Table 1. Ingredients of Cr,Nd: YAG transparent ceramics
2.2 Characterization
Phase compositions of ceramics were measured by an X-ray diffraction machine (XRD, D2, Bruker, Karlsruhe, Germany). Morphologies of the polished and fracture surfaces of ceramics were identified by a scanning electron microscope (SEM; JSM- 6510, JEOL, Kariya, Japan). Ceramics were thermal etched at 1450 °C for 2h in air for the better observation of their polished surfaces. An energy dispersive X-ray spectrometer (EDS, Inca X-Max, Oxford Instruments, Oxford, England) connected with the SEM was utilized to recognize the element distribution of ceramics. An optical transmission microscopy (Axio Scope. A1; Carl Zeiss, Oberkochen, Germany) was applied to investigate the microstructural property of ceramics. For the characterization of in-line transmittance of ceramics, 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, and the scanning range was 200-1200 nm. Photoluminescence (PL), photoluminescence excitation (PLE) and fluorescence decay spectra were identified by a fluorescence spectrophotometer (FLS 980, Edinburgh Photonics, Edinburgh, England). All the above measurements were carried out at ∼25 °C.
3. Results and discussion
3.1 Microstructural property
Figure 1(a) shows the photographs of Cr,Nd: YAG transparent ceramics with different Cr and Nd3+ ion doping concentrations sintered at 1760 °C for 20h before annealing. All the fabricated Cr,Nd: YAG ceramics exhibited transparent appearance, and the words behind them could be clearly distinguished. Color of the ceramic without Cr ion doping was lavender, i.e., the intrinsic color of Nd3+ ion. The observed color of ceramics with Cr ion doping was dark green, and this tint was darkened when increasing Cr ion doping concentration. Increasing Nd3+ ion doping concentration barely changed the color of ceramics. After annealing ceramics at 1450 °C for 50h in air (Fig. 1(b)), color of all the Cr ion doped ceramics were light green, indicating that the valence state of Cr ion in ceramics was regulated effectively.
Fig. 1. Appearances of the vacuum sintered Cr,Nd: YAG transparent ceramics before and after annealing.
Figure 2(a) displays the XRD patterns of Cr,Nd: YAG ceramics. It could be clearly seen from the XRD diffraction characteristics that all the peaks were consistent with the cubic YAG phase (PDF: 97-001-6825), indicating that doping Cr2O3 and Nd2O3 did not change the structure of YAG matrix. Additionally, there was no obvious peak shift observed from the main diffraction peaks of ceramics. In this study, the maximum doping concentrations of Cr and Nd3+ ions were 0.6 at.% and 1.0 at.%, respectively. The applied ion doping concentrations were relatively low that hardly altered the XRD peak positions of ceramics [25,26]. According to the experimental design, Nd3+ ion (r = 1.63 Å, CN = 8) substitutes the dodecahedral Y3+ (r = 1.02 Å, CN = 8) site, while Cr3+ ion (r = 0.69 Å, CN = 6) occupies the octahedral Al3+ (r= 0.54 Å, CN = 6) site in YAG lattice. To further explain the ion substitution procedure of Cr3+ and Nd3+ ions into YAG matrix, schematic crystal structure sketch of Cr,Nd: YAG transparent ceramic is exhibited in Fig. 2(b).
Fig. 2. (a) XRD patterns and (b) schematic crystal structure sketch of Cr,Nd: YAG transparent ceramic.
Figure 3(a)-(h) show the SEM micrographs of the thermal etched surfaces of Cr,Nd: YAG ceramics. It was obvious that a homogeneous microstructure was exhibited in all ceramics, and there was no any residual pore or secondary phase detected, ensuring their excellent optical qualities. Cr00Nd10 ceramic without Cr ion doping exhibited the smallest grain size compared with those of the Cr ion doped ceramics. Increasing Cr ion doping concentration increased the grain size of ceramics (Fig. 3(a)-(d)). Therefore, it could be inferred that similar to the effect of sintering additive, Cr2O3 raw material could promote grain growth of YAG ceramics during sintering. This phenomenon was in accordance with that of Zhou et al.'s observations [23]. From Fig. 3(e)-(h) it could be seen that grain size of ceramics was slightly influenced by Nd3+ ion doping. Ikesue et al. indicated that Nd2O3 could effectively restrain grain growth of YAG transparent ceramic during high temperature sintering [27]. According to the fracture surfaces of ceramics shown in Fig. 3(a’)-(h’), it was obvious that a fully dense microstructure was provided in all the sintered Cr,Nd: YAG ceramics, and their fracture modes were mainly characterized by transgranular.
Fig. 3. SEM micrographs of thermal etched surfaces and fracture surfaces of (a)-(a’) Cr00Nd10, (b)-(b’) Cr02Nd10, (c)-(c’) Cr04Nd10, (d)-(d’) Cr06Nd10, (e)-(e’) Cr03Nd00, (f)-(f’) Cr03Nd03, (g)-(g’) Cr03Nd06 and (h)-(h’) Cr03Nd10 ceramics.
In order to further explore the elemental distribution of Cr,Nd: YAG ceramics, an EDS analysis was carried out. The selected sample for the EDS analysis was Cr06Nd10 with the highest Cr and Nd3+ ion doping concentrations. According to the EDS mapping results shown in Fig. 4(a)-(g), it was obvious that all the Cr, Al, Y, O and Nd elements were homogeneously distributed in the ceramic block, indicating that the solid phase reaction process was completely finished, and the applied doping ions (Cr and Nd3+ ions) were solid soluted into YAG matrix at the designed ion doping concentrations.
3.2 Optical and laser characteristic
In-line transmission spectra of Cr,Nd: YAG ceramics are exhibited in Fig. 5. Remarkably, broad absorption bands centered at ∼1.1 µm were observed from all the vacuum sintered Cr ion doped ceramics without annealing (Fig. 5(a)-(b)). It has been proved by researchers that this band should be attributed to the absorption of Cr2+ ion [24]. The existent of Cr2+ ion is harmful to the laser performance of ceramics, owing to the highly overlapped Cr2+ absorption band and Nd3+ emission band. In this study, vacuum sintering was applied to promote densification of ceramics, and oxygen vacancies along with free electrons would be generated inevitably, owing to the oxygen partial difference between ceramic bulk and furnace, see Eq. (1) [28]:
In this case, if a free electron was captured by a trivalent Cr3+ ion, a divalent Cr2+ ion would be generated according to Eq. (2), resulting in the observed dark green tint of ceramic (Fig. 1(a)), along with the broad absorption band at ∼1 µm. Optical quality of Cr,Nd: YAG transparent ceramics was deteriorated, accordingly.
Fig. 5. In-line transmission spectra of Cr,Nd: YAG transparent ceramics (a) before annealing and (b) after annealing.
Accordingly, high temperature annealing in oxygen enriched atmosphere is necessary for Cr,Nd: YAG ceramics, in order to eliminate the generated Cr2+ ions. From Fig. 5(c)-(d) it was evident that after annealing ceramics at 1450 °C for 50h in air, the absorptions around 1 µm were disappeared, even when the Cr ion doping concentration was up to 0.6 at.%. It indicates that Cr2+ ions were completely eliminated during the high temperature annealing process. Additionally, it should be pointed out that in addition to the elimination of Cr2+ ion, there was no characteristic absorption of Cr4+ ion (800-1200 nm) observed from all the ceramics. Because 0.45 wt.% TEOS (decomposed into SiO2 at high temperature) was adopted as the sintering additive in this study, and the oxidation of trivalent Cr3+ ion was thus inhibited, owing to the charge compensation effect of Si4+ ion [29]. In general, an effective regulation of trivalent Cr3+ ion was realized, by utilizing TEOS and MgO as sintering additives, as well as the high temperature annealing technique.
Notably, transmittances of all the annealed Cr,Nd:YAG samples at 1064 nm almost reached the theoretical limit of YAG (∼84.6%), and the observed transmission curves were flat, indicating the good optical transmittance and small scattering loss of the as-fabricated ceramics. Consequently, the obtained ceramics in this study were qualified for solid state lasers as laser gain media.
Figure 6 shows the absorption spectra of the vacuum sintered Cr,Nd: YAG transparent ceramics after annealing at 1450 °C for 50h. Two broad absorption bands centered at ∼430 nm and ∼590 nm were observed from all the Cr3+ ion doped ceramics, corresponding to the 4A2→4T1 and 4A2→4T2 transitions of Cr3+ ion, respectively. Increasing Cr3+ ion doping concentration enhanced the intensities of these bands. The observed broad absorption bands of Cr3+ ion highly overlapped the solar spectrum within the visible range, and solar light could be effectively absorbed by the prepared ceramics. Notably, absorption coefficient of the Cr06Nd10 ceramic at 430 nm was as high as 8.3 cm-1. Ceramic with a strong absorption characteristic within the visible range is conducive to the light conversion efficiency and output power of ceramic based solar pumped solid state lasers. The observed sharp absorption peaks were ascribed to the absorptions of Nd3+ ion, and the strongest peak position was located at 808 nm, corresponding to the 4I9/2→2H9/2+4F5/2 transition.
Optical transmission micrographs of the annealed Cr,Nd: YAG transparent ceramics are displayed in Fig. 7. It was obvious that no any scattering center observed from the depth direction of all ceramics, providing an optically homogeneous microstructure. The observed phenomenon was in accordance with the excellent transmission performance of ceramics shown in Fig. 5(c)-(d). The presented light green color of ceramics was the intrinsic color of Cr3+ ion in YAG matrix, and this tint was darkened when increasing Cr3+ ion doping concentration.
Fig. 7. Optical transmission micrographs of (a) Cr03Nd00, (b) Cr03Nd03, (c) Cr03Nd06, (d) Cr03Nd10, (e) Cr00Nd10, (f) Cr02Nd10, (g) Cr04Nd10 and (h) Cr06Nd10.
A laser setup was established to further evaluate the optical quality of the prepared Cr,Nd:YAG transparent ceramics. Figure 8(a) indicates the schematic sketch of the laser setup. The applied resonant cavity was plane-concave, and a semiconductor laser with a central wavelength of 808 nm was used as the pumping source. A coupling focusing lens (1: 0.8) was utilized to focus the pump light onto the plane mirror. An antireflective film (AR) at 808 nm was coated on the one end, whereas a high reflective (HR) film at 1064 nm was coated on the other end of the mirror to reduce the cavity loss. As a representative, the prepared Cr03Nd10 ceramic (3 mm×3 mm×5 mm) without coating was selected as the laser gain medium. Transmittances of the employed output couplers at 1064 nm were 2%, 3% and 8%, respectively, and the radius of curvature of all the output coupler was 200 mm.
Fig. 8. (a) Laser setup and (b) the corresponding laser oscillation characteristics of Cr03Nd10 ceramic.
The corresponding laser oscillation characteristics of the Cr03Nd10 ceramic was plotted in Fig. 8 (b). Monitoring at 1064 nm, laser oscillations could be observed by using the designed output couplers, and the laser threshold was ∼100 mW. Under the pump power of 8.6 W, the highest output power and slope efficiency could be acquired by employing the output coupler with 3% transmission, corresponding to 1.9 W and 22.5%, respectively. It should be pointed out that the obtained laser oscillation characteristics could be further optimized, if coating Cr,Nd:YAG ceramic on both sides before lasing. In general, the fabricated Cr,Nd:YAG transparent laser ceramic in this study meets the laser application standard, and is qualified for solar pumped solid state laser as gain medium.
3.3 Cr3+ ion sensitized luminescence performance
Luminescence performance of Cr,Nd: YAG transparent ceramics was monitored, in order to further evaluate their feasibility as laser gain media for solar pumped solid state lasers. Figure 9(a)-(b) exhibit the PLE spectra of the annealed ceramics. Monitoring at 1064 nm, the characteristic PLE peaks of Nd3+ ion centered at 808 nm could be observed. Simultaneously, broad PLE peaks located within the visible range could be observed from all the Cr3+ and Nd3+ ion doped ceramics, indicating that these ceramics could be excited effectively by solar light to realize the 1064 nm emission. Increasing Cr3+ and Nd3+ doping concentrations enhanced the PLE intensities of ceramics, and the variation trend of the PLE spectra was similar to that of the absorption spectra shown in Fig. 6.
Fig. 9. (a)-(b) PLE spectra of the annealed Cr,Nd: YAG ceramics monitored at 1064 nm, (c)-(f) PL spectra of the annealed Cr,Nd: YAG ceramics excited at 430 nm.
Emission characteristic of Cr3+ ion between 600 nm and 800 nm was monitored, as is shown in Fig. 9(c)-(d). The selected exciting wavelength was 430 nm. PL intensity of ceramics was increased with increasing Cr3+ ion doping concentration (Fig. 9(c)). Apparently, emission of Cr3+ ion overlapped the main absorption peaks of Nd3+ ion, and the absorbed solar energy by Cr3+ ion could be transferred to Nd3+ ion, accordingly. Additionally, there was no any emission detected from the Nd3+ ion doped ceramic (Cr00Nd10) between 600 nm and 800 nm. From Fig. 9(d) it was found that increasing Nd3+ ion doping concentration sharply decreased the emission intensity of Cr3+ ion. Therefore, it could be speculated an effective energy transfer from Cr3+ ion to Nd3+ ion was occurred.
Emission characteristic at 1064 nm determines the laser performance of Cr,Nd:YAG ceramics as laser gain media. Accordingly, emissions between 900 nm and 1100 nm were monitored by exciting ceramics at 430 nm (Fig. 9(e)-(f)). PL intensity of Cr00Nd10 ceramic without Cr3+ ion doping at 1064 nm was extremely weak. Introducing Cr3+ ion significantly enhanced the PL intensities of ceramics at 1064 nm. Accordingly, it was convinced that the absorbed energy by Cr3+ ion was effectively transferred to Nd3+ ion. Obviously, emission intensity of ceramics at 1064 nm was increased sharply with increasing Cr3+ and Nd3+ ion doping concentrations, indicating that a high dose ion doping was conducive to the emission performance of Cr,Nd: YAG ceramic at 1 µm.
To further provide a clear insight concerning the energy transfer from Cr3+ to Nd3+ ions in Cr,Nd:YAG transparent ceramics, their fluorescence decay curves were measured (λex = 430 nm, λem = 710 nm), and the fitted results are shown in Fig. 10. All the decay curves were fitted well by the double exnential decay function. As could be seen from Fig. 10(a), at the fixed Cr3+ ion doping concentration (0.3 at.%), with increasing Nd3+ ion doping concentration from 0 at.% to 1.0 at.%, the fitted lifetimes of ceramics were decreased from 1808.42 µs to 1140.26 µs. The decreased lifetimes of ceramics should be attributed to the enhanced energy transfer from Cr3+ to Nd3+ ions. Besides, from Fig. 10(b) it was obvious that at the fixed Nd3+ ion doping concentration (1.0 at.%), the obtained lifetimes of ceramics were fluctuated slightly, when increasing Cr3+ ion doping concentration.
Energy transfer efficiency from Cr3+ to Nd3+ ions could be evaluated according to the measured luminescence decay behaviors of ceramics, according to Eq. (3) [30]:
where ηT, τ and τ0 are the energy transfer efficiency, the fitted lifetime of the sensitizer Cr3+ ion in the presence and without Nd3+ ion, respectively. The calculated energy transfer efficiencies of Cr03Nd03, Cr03Nd06 and Cr03Nd10 ceramics were 14.9%, 26.0% and 36.9%, respectively, with an increasing magnitude of 247.6%. Also, Lupei et al. proved that energy transfer efficiency of Cr,Nd:YAG ceramics was promoted from 52% to 77%, when increasing Cr3+ ion doping concentration from 1.0 at.% to 2.0 at.% at the fixed Nd3+ ion concentration (1.0 at.%) [31]. Therefore, it could be speculated that high doping concentrations of active ions is beneficial to promote energy transfer efficiency of Cr,Nd:YAG transparent ceramics.Schematic diagram of the energy transfer process in Cr,Nd: YAG ceramic is displayed in Fig. 11. When the solar energy was absorbed by Cr3+ ions, electrons from the 4A2 ground state were pumped to the excited states (4A2→4T1 and 4A2→4T2 transitions), and then moved to the transient 2E state of Cr3+ ions. After that, electron transfer could be conducted through two paths, i.e., radiative and non-radiative transitions [32]. For the radiative transition, a portion of the electrons located at the 2E state transmitted directly to the 4A2 ground state of Cr3+ ions, accompanied by the emission within 600-800 nm (Fig. 9(c)-(d)). Simultaneously, the remaining electrons were transferred to the upper energy levels of Nd3+ ions (2E→4F3/2 transition, i.e., non-radiative transition), and a Cr3+ ion sensitized emission at 1064 nm (4F3/2→4I11/2 transition) could be realized, accordingly. Furthermore, energy transfer time (τT) from the 2E state of Cr3+ ion to the 4F3/2 state of Nd3+ ion was derived from Eq. (4) [32,33]:
according to Eq. (4), the obtained energy transfer times of Cr03Nd03, Cr03Nd06 and Cr03Nd10 ceramics were 10679.2 µs, 5141.1 µs and 3086.2 µs, respectively.As has mentioned, the enhanced conversion efficiency of solar energy in Cr,Nd: YAG ceramics should be based on the fabrication of ceramics with low scattering loss and a stable trivalent Cr3+ ion charge state. The critical point to reach this goal is to optimize the applied sintering additives. According to our previous study, absorption of Cr2+ ion was still detected after annealing 0.6at.%Cr/1.0at.%Nd:YAG ceramics at 1450 °C for 25h when applying 0.5 wt.% TEOS and 0.1 wt.% MgO as sintering additives [23]. Decreasing the addition amount of TEOS is beneficial to eliminate the residual Cr2+. However, the reduced addition amount of TEOS is not conducive to the densification of YAG ceramics. Despite divalent additives (e.g., MgO or CaO) is capable of compensating the adverse charge compensation effect caused by TEOS, tetravalent Cr4+ ion would be appeared if adding an improper amount of divalent additives at a low TEOS addition amount [34]. In this regard, a competitive relationship is existed among the addition of additives, optical quality of ceramics and charge state of Cr ion. For the real application of Cr,Nd: YAG transparent ceramic for solar pumped solid state lasers, utilizing the synergistic effect of TEOS and divalent additives, combining the advanced sintering technique (e.g. HIP sintering, etc.) is crucial to further promote their qualities.
4. Conclusion
In summary, pure phase Cr,Nd: YAG transparent laser ceramics with theoretical transmittance at 1064 nm were fabricated to verify their energy transfer performances. Notably, there was no any absorption corresponding to Cr2+ or Cr4+ ions detected, even when the Cr3+ ion doping concentration reached 0.6 at.%. An efficient laser oscillation of watt-level at 1064 nm was realized in uncoated Cr,Nd:YAG ceramic, and the obtained slope efficiency was 22.5%. An effective energy transfer from Cr3+ to Nd3+ ions was verified, and sharp emission peaks located at 1064 nm were detected when exciting Cr,Nd: YAG ceramics at 430 nm. It was found that increasing Cr3+ and Nd3+ ion doping concentrations was beneficial to promote the light conversion efficiency and emission intensity at 1064 nm of Cr,Nd: YAG transparent ceramics, and energy transfer efficiency of the 0.3 at.% Cr,Nd: YAG ceramics was increased from 14.9% to 36.9% with an increasing magnitude of 247.6%, when increasing Nd3+ ion doping concentration from 0.3 at.% to 1.0 at.%. Finally, this study proposed the feasibility of Cr,Nd: YAG transparent ceramics for solar pumped lasers.
Funding
Open Project of State Key Laboratory of Advanced Materials and Electronic Components (FHR-JS-202011017); Special Project for Technology Innovation of Xuzhou City (KC19250, KC20201, KC20244); Natural Science Foundation of the Jiangsu Higher Education Institutes of China (19KJB430018, 20KJA430003); International Science and Technology Cooperation Program of Jiangsu Province (BZ2019063, BZ2020030, BZ2020045); Undergraduate Research & Practice Innovation Program of Jiangsu Province (202010320082); Natural Science Foundation of Jiangsu Province (BK20191467); Key Research and Development Project of Jiangsu Province (BE2019033); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); National Natural Science Foundation of China (51902143, 61775088, 61971207, 61975070); National Key Research and Development Program of China (2021YFB3501700).
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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