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High-quality (YbxY1-x)3(Sc1.5Ga0.5)Ga3O12 (Yb:YSGG) (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.5, and 1) single crystals have been successfully grown by the optical floating zone (OFZ) method. The phase purity and atomic structure were analyzed in detail by employing X-ray powder diffraction (XRPD) with Rietveld refinement. Chemical composition was measured and investigated by X-ray fluorescence (XRF). The results of thermal survey show that the thermal conductivity declines from 6.543 W·m−1·K−1 for YSGG to 3.708 W·m−1·K−1 for the 50 at.% Yb3+-doped crystal, but rises to 4.425 W·m−1·K−1 for the 100 at.% sample (YbSGG). The systematically spectroscopic investigations suggest that the crystal field of Yb3+ in Yb:YSGG crystals is almost independent of the Yb3+ dopant concentration and the fluorescence quenching only appears in the high Yb3+-doped crystal.
©2013 Optical Society of America
1. Introduction
The garnet family with general formula A3B2C3O12 (A = Ca, La, Tb, Y, Gd or Lu; B = Al, Ga, Fe, Sc; and C = Al, Fe, Ge, or Ga) has been frequently investigated as excellent host materials for rare-earth or transition-metal active ions, and has enjoyed wide application in the laser [1–3], scintillation [4], optical pressure sensor [5, 6], and magneto-optical fields [7, 8]. In the exploration of high-power lasers, a typical representative, yttrium aluminum garnet (YAG) doped with active ions, such as Nd3+, Ce3+, Yb3+, Er3+, etc, has had commercial success due to its isotropic lattice, stable physical properties and especially high thermal conductivity [9, 10]. In recent years, yttrium gallium garnet (YGG) and gadolinium gallium garnet (GGG) have attracted growing interest since they are of similar structure to YAG which augurs well for similar or even better properties in some aspects, based on the Neumann principle [8]. For instance, the spectral properties and laser operation of Nd:YGG and Yb:YGG crystals have been systematically investigated, and recently the Cr3+ crystal field in Cr:YGG nanocrystals was also measured [11–13]. The Nd:GGG crystal, because of its availability in a large size and its high optical homogeneity (without a “central core”), has exhibited excellent laser performance in the solid state heat capacity laser (SSHCL) field [14, 15].
In the garnet materials, Sc3+-doped crystals show some superiority over traditional garnets (like YAG), such as a higher active ion segregation coefficient and lower chemical stress in the axial direction during crystal growth [16]. Moreover, polyhedral distortion of the crystal structure by introducing Sc3+ ions can broaden the absorption and emission spectral bandwidths, properties that are more suitable for solid-state lasers [17]. As a representative material, it has been previously shown that an erbium doped yttrium scandium gallium garnet (YSGG) crystal operating at about 3 μm exhibited much better laser performance than Er:YAG, including a lower pump threshold, higher slope efficiency and larger energy conversion efficiency [18, 19]. In addition, the spectrum of Ho:YSGG and the laser operation of Tm:YSGG and Nd,Gd:YSGG have also shown potential for optical applications [20–22]. YSGG crystals are usually grown by the Czochralski (CZ) method [23]. Ga2O3 in the raw precursor materials easily volatilizes at such a high melting point (2150 K) [24], which leads to a chemical valence change in Ga in the as-grown crystal. Moreover, since the growth uses an iridium crucible in an oxygen-free (as well as N2 or Ar free) atmosphere, crucible pollution and the presence of oxygen vacancies present two other tough problems. Thus, the optical floating zone (OFZ) method, where crystal growth can be carried out in a high oxygen atmosphere without using a crucible, is quite suitable for the growth of gallium garnet crystals [25]. Using this technology, the above-mentioned problems associated with the CZ method can be avoided.
Nowadays, the ytterbium laser has attracted a great deal of attention in ultra-short pulsed, kilowatt level high-power continuous-wave (cw) laser systems and tunable laser applications. Many host materials for the Yb3+ ions have been studied, such as YAG, Y2SiO5, KGd(WO4)2, and Sr3Y(BO3)3 [3, 17, 26]. Doping with this ion may exhibit some advantages over traditional Nd3+-doped lasers, such as weak concentration quenching and broad absorption and emission bandwidths. Therefore, it is assumed that the Yb:YSGG crystal should be an excellent laser gain material. Unfortunately, the Yb3+ ion in YSGG has been mainly treated as a sensitizer [27, 28], and the effect of Yb3+-doped concentration on its own structure, thermal properties and spectrum has not yet been systematically studied, to the best of our knowledge.
Previous studies have shown that the congruent melting composition YSGG is Y3Sc1.43Ga3.57O12 [24]. Since the segregation coefficient of Sc is less than 1 in Sc3+-doped garnets [29], we used the chemical formula (YbxY1-x)3(Sc1.5Ga0.5)Ga3O12 (Yb:YSGG) to prepare the precursor materials, where the proportional value of Sc is 1.5. On the basis of X-ray powder diffraction (XRPD), X-ray fluorescence (XRF) and Rietveld refinement, the crystal structure and chemical composition were investigated in detail. In addition, the effect of Yb3+-doped concentration for the thermal properties, spectrum, and fluorescence lifetime were also studied and discussed. All the results indicate that Yb:YSGG is a promising candidate for high-power and ultrafast laser systems.
2. Experimental Section
2.1 Synthesis of polycrystalline materials and single-crystal growth
The raw polycrystalline precursor materials were prepared with commercially available Yb2O3, Y2O3, Sc2O3, and Ga2O3 powders (99.99%), and combined according to their respective proportions. In order to avoid the change in the Ga chemical valence caused by the volatilization of Ga2O3, an extra of 2 wt.% of Ga2O3 was added. The materials were mixed, ground, remixed in sequence, and then heated in a platinum crucible at 1273 K for 10 h to eliminate absorbed water and fractional organic compounds that had been introduced in previous processing. The solid-state reaction was completed based on the following chemical reaction:
The initial stoichiometric mixture was pressed by a feed rod with dimensions of Ø7 × 60 mm3 into a corresponding mold, and then sintered at 1773 K for 6 h in air. Yb:YSGG crystals were all grown by the OFZ method with a four-ellipsoidal-mirror furnace (Crystal Systems Inc., FZ-T-12000-X-I-S-SU). A [111] oriented YAG single crystal was used as the seed and the crystals were grown in a 300 mL/min oxygen atmosphere (purity > 99.9%). In order to release the thermal stress in the as-grown crystals, they were all annealed in air at 1273 K for 30 h.
2.2 Determination of crystal structure and chemical composition
XRPD was used to study the structure and phase purity of the as-grown crystals with an X-ray powder diffractometer (Bruker AXS, D8 Advance) at RT. The measured data from 15° to 91° were recorded with a step size of 0.015° and a counting time of 0.9 s, and the refinement data on YSGG, 5° < 2θ < 125°, was surveyed with 0.01°/step and 6 s/step. Rietveld refinement was performed with a Pawley fitting to the whole powder pattern decomposition (WPPD) procedure with TOPAS v.3 software [30]. A Chebyshev Polynomial was used to fit the background and a modified Thompson-Cox-Hasting pseudo-Voigt (TCHZ_PV) function was employed to fit the peak shape. The elemental composition of Yb, Y, Sc, and Ga in the as-grown crystals was measured using a Rigaku ZSX Primus II XRF spectrometer.
2.3 Thermal survey
The thermal expansion with the sample dimensions of 3 × 3 × 5 mm3 was measured by a Perkin–Elmer thermal-mechanical analyzer over the temperature range from 293 K to 673 K. The specific heat measurement was performed on a (Mettler Toledo DSC822e) differential scanning calorimeter (DSC) with the temperature range from 293 to 573 K. The thermal diffusion coefficient was measured by the laser flash method using a laser flash apparatus (NETZSCH LFA457) on square wafers (4 × 4 × 1 mm3) over the temperature range of 293 to 573 K.
2.4 Optical and spectroscopic characterization
The RT absorption and transmission spectra were measured using a spectrophotometer (JASCO model V-570) on double-sided polished samples (3 × 3 × 0.5 mm3), with the spectral resolution of 0.2 nm and 2 nm, respectively. The RT fluorescence lifetime was measured by the time-correlated single-photon counting (TCSPC) method with an Edinburgh Instruments FLS920 fluorescence spectrometer equipped with an ANDO Shamrock SR-303i high-resolution optical spectrum analyzer and a tunable Opolette (HE) 355 II (5 ns, 20 Hz) pump source. The excitation and detection wavelengths are 930 nm and 1026 nm, respectively. In order to weaken the effect of radiation trapping, all the polished samples were prepared with a thickness of 0.5 mm and a focusing prism was placed in front of the sample. The focusing spot size is about Ø 0.5 mm.
3. Results and discussions
3.1 Crystal growth
As Fig. 1 shows, the Yb:YSGG single crystals were grown by the OFZ method. Although some crystal surfaces are rough due to the slight volatilization of Ga2O3, the as-grown crystals were largely transparent thanks to growing under an O2 atmosphere without using a crucible, thereby avoiding the appearance of Yb2+ ions, oxygen vacancies, and crucible pollution [25, 31], which always occur in the traditional CZ method. To grow complex multiple-component compounds, many variables need to be taken into account in the OFZ method, such as diameter, growth speed and rotation rate [25, 32]. Considering that the volatilization of Ga2O3 will damage the transmission properties of the quartz tube when the seed rod diameter becomes bigger and the growth speed becomes slower, the pump power must be continuously increased to maintain a constant melting zone length. However, a change of pump power will cause a fluctuation of the solid-liquid interface, and thus is very unfavorable for obtaining stable crystal growth. In addition, the use of optical heating produces a steeper temperature gradient along the growth direction than the CZ method does, and so a crystal rod of large diameter is more likely to lead to cracking due to thermal stress. After repeated experiments and literature review, the principal growth parameter values were optimized and are summarized as follows: the diameter of the seed rod should be less than 8 mm, with 7 mm the optimum value, the diameter of the crystal rod should not exceed 6 mm in the constant-diameter control portion, the crystal growth speed is fixed at 6 mm/h, and the rotation rates of the seed and feed rods are fixed at 15 rpm in opposite directions.
3.2 Analysis of structure and composition
The XRD diagram is presented in Fig. 2 (a). It can be seen that the Yb:YSGG single crystals retain almost the same structure as YGG (ICSD No. 43-512), and like Y3Sc2Ga3O12 (ICSD No. 25-1246), some new diffraction planes appear, for instance, the (220) set. As shown in Fig. 2 (b), with an increase in Yb3+ dopant concentration, the diffraction peaks of the (400), (420) and (422) faces are shifted to a higher scattering angle. Figure 3 displays the lattice constants of the Yb:YSGG crystals, and it demonstrates an approximately linear decrease as the Yb3+ concentration increases. These tendencies are quite reasonable based on the Bragg diffraction equation and Vegard’s law, with the effective ionic radius of Yb3+ being slightly smaller than that of Y3+ (rYb = 0.985 and rY = 1.019 Å for CN = 8) [33].
The Rietveld refinement of YSGG is shown in Fig. 4 and clearly indicates that the crystal structure belongs to space group . The atomic positions obtained from refinement are shown in Table 1. As the inset of Fig. 4 depicts, the ions occupy positions that correspond to the other Sc3+-doped garnets [7, 16, 33]. The larger Y3+ and Yb3+ ions are located at the dodecahedral sites, while the Sc3+ ions, as well as a small number of Ga3+ ions (rSc = 0.745 and rGa = 0.620 Å for CN = 6) are located at the octahedral sites; the other Ga3+ ions (rGa = 0.47 Å for CN = 4) are located at the tetrahedral sites. The lattice constant of YSGG is 12.433 Å, a value which is slightly larger than that of YGG (12.280 Å) due to the relative magnitudes of the ionic radii rSc>rGa at the octahedral sites [34]. From the bond lengths of YSGG and YGG displayed in Table 2, we see that when doping with Sc3+ions, the Sc–O or Ga–O bond lengths in the octahedra change from 1.9946 to 2.0664 Å, corresponding to an increase of 3.60%. This octahedral deformation introduces a distortion to the neighboring dodecahedra. It is noteworthy that introducing Sc3+ ions into the octahedral sites can equilibrate the polyhedra with different coordination modes, and partly reduce the chemical stress that is brought about by the maldistribution of the active doping ions, so that the crystal structure becomes much more stable [7, 16, 35]. In comparison to YAG and YGG [1, 34], the dodecahedra in the structure display a large deformation and the polyhedra come much closer to the ideal regular dodecahedral coordination. Due to the dodecahedral and octahedral variations in the crystal structure, the appearance of some additional low intensity peaks away from the Bragg positions in the refinement would be reasonable. The crystal structure modification certainly will also affect the structure of the absorption and emission spectra [36, 37].
Table 1. Atomic positions of YSGG by Rietveld Refinement
Table 2. Bond lengths in YSGG, YGG and YAG
Table 3 shows the XRF analysis results on the Yb:YSGG crystals, indicating that the concentration of elemental Sc remains constant and agrees well with the reported stoichiometric composition in all the as-grown crystals [24]. Furthermore, in the Yb:YSGG crystal, the measured Sc content corresponds well to the Rietveld refinement value in Table 1. The effective segregation coefficient (keff) of elemental Yb and Sc presented in the inset of Fig. 3 is above 1 and around 0.94, respectively. This result suggests that this composition ratio is favorable for obtaining high quality crystals and a homogeneous distribution of the dopants. Some studies, such as on single-phase Nd3Lu2Ga3O12 investigated by Suchow et al, have demonstrated that the smaller rare-earth ions (Lu3+, Yb3+, Tm3+, Er3+, etc.) can successfully be introduced into the octahedral sites in synthetic iron or gallium garnets [38–40]. The Yb + Y values in the chemical formula from the XRF composition data are all greater than 3; therefore, the as-grown crystals should have or antisite defects, where the Y3+ or Yb3+ ions occupy the octahedral site that the Sc3+ or Ga3+ ions randomly occupied, leading to an intrinsic stoichiometric disorder in the as-grown crystals. Based on the results of the static-lattice atomic calculation [41], the smaller the ionic radius of the rare earth, the more easily it enters into the octahedral site. Hence, compared to, antisite defects are more likely to be generated. The phenomenon of producing antisite defects implies that the absorption and emission spectra of the as-grown crystals will be much broader as a result of the partial structural disorder [17, 37].
Table 3. XRF analysis of the Yb:YSGG crystals
3.3 Thermal survey
On the basis of Neumann’s principle, the linear thermal expansion coefficient of cubic Yb:YSGG, as a second rank tensor, has only one independent element. The measured values for the four compositions are shown in Fig. 5 (a). From the drawing, we see that the average linear thermal expansion coefficient is 6.55 × 10−6 K−1 for YSGG and 6.86 × 10−6 K−1 for the 100 at.% Yb3+-doped sample. The values of all the compositions are smaller than that of YAG (8.71 × 10−6 K−1), YGG (8.93 × 10−6 K−1) and GGG (9.20 × 10−6 K−1), the values which were obtained by a linear fit of the lattice expansion data reported by S. Geller, et al [42]. The small thermal expansion coefficient suggests that the Yb:YSGG crystals are not especially sensitive to increasing temperature. They will benefit from this characteristic in the high-power laser field because of a reduction in the thermal lensing effect caused by thermal expansion of the crystal [43]. In addition, the average linear thermal expansion coefficient for the 50 at.% doped crystal reaches a magnitude of 7.10 × 10−6 K−1, which is greater than that of YSGG. This phenomenon is mainly attributed to the structural distortion caused by the Yb3+ doping ions. For 50 at.% doped crystals the degree of disorder reaches the maximum value [44].
Fig. 5 (a) Thermal expansion versus temperature, (b) Specific heat versus temperature, (c)Thermal diffusivity versus temperature, (d) Thermal conductivity versus temperature.
From Fig. 5 (b), it can be seen that the specific heat (Cp) of all Yb:YSGG crystals shows approximately linear increasing as the temperature is elevated. At 300 K, the Cp value of Yb:YSGG decreases from 0.493 J·g−1·K−1 to 0.414 J·g−1·K−1 with the increase of Yb3+ concentration in the range from 0 at. % to 100 at.%. For the same Yb3+ dopant concentration of 10 at.%, the Cp of Yb:YSGG (0.469 J·g−1·K−1) is smaller than the value obtained for Yb:YAG (0.63 J·g−1·K−1), but larger than that of Yb:YGG (0.43 J·g−1·K−1) [13].
The thermal diffusion coefficient (d) of Yb:YSGG is also a second-rank tensor, analogous to the thermal expansion coefficient. As shown in Fig. 5 (c), It can be seen that the d value of all Yb:YSGG crystals decreases with increasing temperature. As a laser crystal, the thermal conductivity (k) is especially important. The k of Yb:YSGG is determined by using the formula:
where ρ, Cp, and d denote the density, the specific heat, and the thermal diffusion coefficient, respectively. Figure 5 (d) shows that the k value exhibits a tendency of decreasing with rising temperature. In addition, At 300 K, with a dopant concentration between 0 at.% to 50 at.% the k value decreases from 6.543 W·m−1·K−1 to 3.708 W·m−1·K−1, while for the 100 at.% sample (YbSGG) it increases to 4.425 W·m−1·K−1. Because there are no free electrons in the insulator Yb:YSGG crystal, its thermal conductivity is mainly influenced by the variation in the mean free path of phonons. Hence the thermal conductivity can also be written as:where ν is the sound velocity, which is considered as a constant, and L is the phonon mean free path. Due to the relatively large mass difference between Sc3+ ions and Ga3+ ions in the octahedral sites, a certain degree of disorder will exist in the YSGG crystal. Structural disorder can increase phonon scattering among crystal lattices, and lead to a reduction of the mean free path of phonons. Therefore, YSGG shows a lower k magnitude than that of YAG (10.3 W·m−1·K−1) and YGG (9 W·m−1·K−1) [45]. In addition, for the 50 at.% Yb3+-doped crystal the degree of disorder in the dodecahedral sites reaches the maximum value, so it shows the lowest k in the all as-grown crystals.3.4 Optical and spectroscopic characterization
As displayed in Fig. 6, the ultraviolet (UV) absorption edge of YSGG is about 260 nm. It should be noted that the Yb:YSGG crystal exhibits quite remarkable optical transmission properties and is suitable for application as a laser host material over the range of 260 nm to 2500 nm, except for absorption at the energy transition of Yb3+: 2F7/2→2F5/2. In the visible light region, there are no absorption peaks caused by Yb2+ ions or color centers [31, 46], which are always introduced in the CZ method. These results manifest the high optical quality of the Yb:YSGG crystals and the superiority of the OFZ method.
Figure 7 presents the absorption of Yb:YSGG crystals over the wavelength range from 800 to 1100 nm at RT. In the previous structural refinement, it can be clearly seen that the deviation magnitude between the two different Y–O bonds in YSGG is 0.0126 Å. Compared to the values in YGG (0.0894 Å) and YAG (0.1306 Å) [1, 34], the geometrical shape of the dodecahedra in YSGG has obviously changed a great deal. So the Yb3+ crystal field in Yb:YSGG is expected to be quite different from the field in Yb:YAG or Yb:YGG. For Yb:YSGG crystals, the central wavelength for the strongest absorption band is located around 930 nm; this value compares to 940, 970, and 971 nm for Yb:YAG, Yb:YGG, and Yb:GGG, respectively [47–49]. It should be noted that such large bandwidth (about 30 nm) is quite suitable and desirable for efficient laser diode (LD) pumping. In addition, From Table 4, we can see that for the different Yb3+-component crystals, the absorption cross-section (σabs) basically remains unchanged around 930 nm. Apart from the wide absorption range, absorption around 970 nm, the so-called zero phonon line, is also relatively high. The σabs at the zero phonon line is slightly changed for different Yb3+ dopant concentrations. The maximum and minimum σabs values at the zero phonon line are exhibited by the 15 at.% (0.49 × 10−20 cm2) and 100 at.% (0.31 × 10−20 cm2) samples, respectively. Detailed values for the other are shown in Table 4.
Table 4. σabs (λZl) and σem (λpeak) for Yb:YSGG Crystals
Based on the reciprocity method [50], the stimulated emission cross-section (σem) of the transition for Yb3+ from the 2F5/2 level to the 2F7/2 level is obtained by the following equation:
where σabs (λ) is the absorption cross-section at wavelength λ, where Zl (1.375) and Zu (1.385) are the lower and upper manifold partition functions, respectively [51]. The quantity h is the Planck constant, k is the Boltzmann constant, c is the velocity of light, and λZl is the zero phonon line wavelength. The calculated emission spectrum is shown in Fig. 8. The similar spectral distribution for different Yb3+-doped crystals in the absorption and emission spectrum suggests that the Yb3+ crystal field is almost independent of the Yb3+ concentration. The strongest emission peak (λpeak) is located around 1026 nm for all Yb3+-doped crystals. Compared to that of Yb:YAG (1030nm) [50], the λpeak shows a blue-shift. The largest observed value of σem is in the 10 at.% crystal at λpeak with a magnitude of 1.52 × 10−20 cm2 and a full-width at half-maximum (FWHM) of 11.4 nm. This σem value is lower than those of Yb:YAG (2.03 × 10−20 cm2), Yb:GGG (2.0 × 10−20 cm2), and Yb:YGG (2.56 × 10−20 cm2) [13, 49]. Although a smaller σem value is detrimental for high power lasers, it suggests that the Yb:YSGG crystal has good energy storage capacity in another aspect, so that Yb:YSGG can be regarded as a promising Q-switching laser material. As shown in Table 4, for the YbSGG sample, although its σem is only 1.46 × 10−20 cm2, the FWHM at λpeak is 13.1 nm, which is larger than that of Yb:YAG (10 nm), Yb:GGG (10 nm), and Yb:YGG (11 nm) [13, 49]. Yb:YAG and Yb:YGG possess excellent mode-locking properties, and a Kerr-lens mode-locked Yb:YAG laser and passive mode-locked Yb:YGG laser using a semiconductor saturable absorber mirror (SESAM), which produce pulses as short as 200 fs and 245 fs, respectively, have been reported recently [52, 53]. So we assume that large region around λpeak for the YbSGG crystal is very favorable for the generation of ultrashort pulses, and the pulse should be smaller than that of Yb:YAG and Yb:YGG.The RT fluorescence lifetime (τ) is also an important parameter for judging the energy storage capacity of Yb3+ ions in hosts. As Fig. 9 shows, in crystals with a dopant concentration between 5 at.% to 15 at.%, the RT fluorescence lifetime increases, while for the 25 at.% to 100 at.% samples it drops dramatically. The reduction mainly arises because the Yb–Yb separation distance becomes shorter with decreasing lattice constant, increasing the density of quenching centers in the highly doped crystals, and leading to energy migration among the Yb3+ ions [9, 31]. As displayed in the inset of Fig. 9, the value of 1.06 ms for the 5 at.% Yb3+-doped sample, although slightly smaller than that of Yb:YGG (1.1 ms), is still larger than that of Yb:YAG (0.95 ms) or Yb:GGG (0.8 ms) [36, 49, 54]. For the high Yb3+-doped crystals, the short fluorescence lifetime is not favorable for storing the thermal population of the upper laser level and strong re-absorption at the emission wavelength also can weaken the output laser intensity. The thin-disk laser is particularly well suited for quasi-three-level systems. It can provide a high pump-power density without introducing a large thermal lensing effect, thus overcoming the disadvantages of the ytterbium laser and obtaining outstanding laser performance [55]. This thin-disk concept has been extensively used in the high-power and ultrafast laser fields, such as with Yb:YAG and Yb:CaGdAlO4 [56, 57]. In addition, the cw laser performance of the 5 at.% Yb3+-doped crystal was also studied in the plano-concave resonator, which we have used in the previous work [51]. The most efficient laser operation is obtained with the output coupler at T = 3%, generating an output power of 5.46 W at an absorbed pump power of 9.51 W, and corresponding to optical-to-optical and maximum slope efficiencies of 57.4% and 72.9%, respectively.
Fig. 9 RT fluorescence lifetime of Yb:YSGG. (Inset) Fluorescence lifetime value for different dopant concentrations.
4. Conclusions
Cubic Yb:YSGG garnet crystals were successfully grown by the OFZ method. The phase purity and cell parameter variation were investigated by XRPD, and the crystal composition was measured by XRF. The YSGG crystal structure was also qualitatively studied by employing Rietveld refinement with TOPAS software. The results of thermal survey show that the thermal conductivity declines from 6.543 W·m−1·K−1 for YSGG to 3.708 W·m−1·K−1 for 50 at.% Yb3+-doped crystals, but rises to 4.425 W·m−1·K−1 for the YbSGG. The systematically spectroscopic investigations suggest that the Yb3+ crystal field of Yb:YSGG is different from that of Yb:YAG, Yb:YGG, and Yb:GGG, and it is almost independent of the Yb3+ dopant concentration. In addition, the thin-disk laser is a good solution for the fluorescence quenching in high Yb3+-doped crystals. All these results indicate that Yb:YSGG single crystals hold promise for application in the high-power and ultrafast laser fields.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51202127, 51272131, 51025210, and 51021062).
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