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To achieve the non-critical phase-matching (NCPM) frequency conversions of an Nd:glass laser (1053 nm), a series of GdxY1-xCOB (x = 0.186, 0.156, 0.132, and 0.127) crystals have been grown by the Czochralski pulling method. Using an optical parametric oscillator laser, the room temperature NCPM second-harmonic-generation (SHG) and third-harmonic-generation (THG) wavelengths along the y-axis were determined for different GdxY1-xCOB crystals. By controlling the temperature of the Gd0.132Y0.868COB crystal, the NCPM SHG and THG of the 1053 nm laser were realized at 28 °C and 55 °C, respectively. Correspondingly, the angular acceptance, temperature acceptance, and optical conversion efficiencies were researched with a 1053 nm Nd:YLF laser as the fundamental light source. The gray-track generated during large energy THG is expected to be removed effectively by elevating the crystal temperature.
© 2016 Optical Society of America
1. Introduction
Since the 1990s, more and more attention has been paid to the single-crystal growth, crystal structure, and nonlinear optical (NLO) properties of rare-earth calcium oxyborate crystals (ReCa4O(BO3)3 or ReCOB) with space group Cm [1–11], since these crystals possess a suitable NLO coefficient, high laser damage threshold, large phase-matching range and non-hygroscopicity, as well as stable physical, chemical, and mechanical performance. Furthermore, when Re3+ ions are partly replaced by active ions Nd3+ or Yb3+, these crystals can be used as laser materials and self-frequency-doubling laser materials [12–22]. As congruently melting compounds, these crystals can be grown, by the conventional Czochralski (Cz) pulling method, to large sizes with high optical quality. Among them, YCOB and GdCOB are extensively investigated due to their wide transmission spectra and excellent frequency conversion properties.
In 1999, Furuya et al. showed that GdxY1-xCOB was a substitutional solid solution and that the optical birefringence could be controlled by changing the compositional parameter x [23]. Umemura et al. reported the refractive index formula of GdxY1-xCOB in 2001 [24]. So far, the Non-critical phase-matched (NCPM) second-harmonic-generation (SHG) and third-harmonic-generation (THG) of Nd:YAG lasers (1064 nm) have been demonstrated in different GdxY1-xCOB (0.2 ≤ x ≤ 0.37) crystals [25–29]. The wide waveband NCPM attributes of such crystals, as the optimum PM style in the NLO domain, offer many special advantages, including large angular acceptance, free of beam walk-off, and high utilization of single crystals, and have aroused in people popular and sustained research interest [30–33].
To date, in addition to 1064 nm, 1053 nm has emerged as another important and mature solid-state laser wavelength, which can be obtained from Nd:glass or Nd:YLF crystals. Nd:glass is especially preferred as a working medium for large-energy and high peak power lasers because it offers larger volume, better optical homogeneity, and easier processing and molding than crystal candidates. For these reasons, 1053 nm Nd:glass lasers and their SHG and THG have been popularly used in the largest laser equipment, i.e. inertial confinement fusion (ICF) facilities. In addition, the 1053 nm laser and its frequency conversions have been used in various fields, including scientific research, medicine, biological engineering, photoelectric detection, material analysis, industrial cutting and measuring. Unfortunately, to the NCPM properties of GdxY1-xCOB crystals, all of the previous researches were focused on the 1064 nm laser; no one was examining the important laser wavelength of 1053 nm. Recently, a noncollinear PM scheme based on a KDP crystal was suggested to perform the type-I THG of Nd:glass lasers in ICF facilities, to elevate the angular acceptance, and to make a “prefocusing” design both possible and feasible [34]. In fact, the type-I NCPM THG of 1053 nm in GdxY1-xCOB crystals is also a hopeful technical route for this novel design.
In this paper, we realized type-II NCPM SHG and type-I NCPM THG of a 1053 nm laser in a GdxY1-xCOB (x = 0.132) crystal for the first time. Correspondingly, the acceptance temperature, acceptance angles, and optical conversion efficiencies were investigated. Our research demonstrates that Gd0.132Y0.868COB is a promising NLO crystal for frequency conversion applications of a 1053 nm laser. It should be noted that in this paper the “THG” always means the cascaded “THG”, i.e. the sum-frequency process between the fundamental wave and the SHG wave. It is not the direct THG from the fundamental wave.
2. Crystal growth
The type-II NCPM SHG and type-I NCPM THG wavelengths of YCOB crystals were theoretically calculated to be 1032 nm and 1035 nm, respectively. With an increase of the compositional parameter x, the NCPM wavelength of GdxY1-xCOB was also increased. For the 1064 nm laser, the type-II NCPM SHG and type-I NCPM THG were realized at the same compositional parameter, x = 0.28 [26, 27]. Based on the relationship between the NCPM wavelength and the compositional parameter x [26], we speculated that the doping level of Gd3+ ions would be 21% or so for the NCPM SHG of 1053 nm. So the compositional parameter x of the first GdxY1-xCOB crystal was designed to be 21%.
The starting raw materials for crystal growth were high-purity Gd2O3, Y2O3, CaCO3 and H3BO3 powders (99.99%). They were weighed based on the stoichiometric ratio of the composition. Considering the evaporation of B2O3 during solid-state reactions and crystal growth, an excess of H3BO3 (2.0 wt%) was added to the raw materials in order to obtain high-quality crystals. The starting materials were ground, mixed, and then calcined at 1000 °C for 10 h to eliminate the adsorbed water of H3BO3 and to decompose CaCO3 completely. Then the compounds were cooled, ground, mixed, pressed into pieces, and sintered at 1150~1160 °C for 12 h to synthesize the GdxY1-xCOB polycrystalline materials for crystal growth. The heating process was performed in air.
A single crystal of GdxY1-xCOB was grown in a nitrogen atmosphere containing 4 vol% oxygen. An iridium crucible (ϕ60 × 50mm3) was used for crystal growth. The polycrystalline materials were put into it and heated by a 2 kHz low-radio-frequency furnace (TDL-J40 single-crystal-growth furnace). The growth temperature was controlled by a EUROTHERM 3504 controller/programmer. Before seeding, the melt was kept 30 °C above its melting point for at least 2 h to make the melt homogeneous and avoid the formation of polycrystals. A <010>-orientated YCOB crystal bar with dimensions of 4 × 4 × 30 mm3 was used as the seed to pull the crystal from the melt at an appropriate temperature (normally 10~20 °C) higher than the melting point of the melt. The pulling rate varied from 0.5 mm/h to 2 mm/h and the rotation speed was kept between 8 rpm and 12 rpm during the crystal growth. After growth, the crystal was cooled down to room temperature at a rate of 10~50 °C/h.
Using an optical parametric oscillator (OPO) laser, we made preliminary measurements, at room temperature, of the NCPM wavelengths on the y-axis of this crystal, i.e. the nominal Gd0.21Y0.79COB crystal. We found that the NCPM SHG and THG wavelengths were both longer than 1053 nm; thus, we continuously grew three other GdxY1-xCOB crystals with lower, different compositions, under similar growth processes. All of the as-grown GdxY1-xCOB crystals, shown in Fig. 1, were colorless and free of macro defects. No scattering points were observed under the irradiation of a 10 mW, 532 nm green laser pointer. The (01) and (101) facets were always exposed on their appearances. Using Gd0.21Y0.79COB polycrystalline materials as the standard sample, the chemical compositions of the as-grown crystals were measured by using X-ray fluorescence analysis (XRF). The samples for composition analysis were prepared from the corresponding as-grown crystals and ground into fine powders. The XRF results showed that the Gd concentrations were 0.186, 0.156, 0.132, and 0.127, respectively. Comparing with the original composition of raw materials, the segregation coefficients k of Gd and Y ions were calculated to be on the order of 0.88 and 1.03 in the Gd0.186Y0.814COB crystal, respectively. Y3+ ions were easier than Gd3+ ions to incorporate into the crystal lattice, which coincides with the previous report [28].
Fig. 1 Photographs of the as-grown crystals of (a) Gd0.186Y0.814COB, (b) Gd0.156Y0.844COB, (c) Gd0.132Y0.868COB and (d) Gd0.127Y0.873COB.
3. Dependence of the NCPM wavelength on crystal composition
Four samples prepared from different GdxY1-xCOB crystals (x = 0.186, 0.156, 0.132, and 0.127) were used for experimental determination of the NCPM SHG and THG wavelengths at room temperature. All the samples were oriented along their refractive index principal axes (x, y and z). They were cut in the y-direction (which was the NCPM direction of a 1 μm laser) and their end faces were polished. The fundamental light source was an OPO tunable laser system (Opolette HE 355 II) with an emission wavelength range of 410~2400 nm. The wavelength tuning step was 1 nm, the pulse width was 5 ns, and the repetition rate was 20 Hz. The pulse energy was tunable in a range of 0~5 mJ at different wavelengths. For the measurement of THG wavelengths, a 6.7 mm-long KTP cut along the type-II PM direction (90°, 32.8°) was used as the SHG crystal, and a filter with transmittance (T) = 50% at 351 nm, T < 0.1% at 1053 and 526 nm was placed between the crystal sample and the probe of the energy meter. At 24 °C, the type-II NCPM SHG and type-I NCPM THG wavelengths for different y-cut GdxY1-xCOB crystals were determined by tuning the OPO wavelength to find which one could yield the maximum frequency conversion signal. The measured results are listed in Table 1.
Table 1. NCPM SHG and THG wavelengths for different y-cut GdxY1-xCOB crystals.
Correspondingly, the dependence of the NCPM wavelength on the compositional parameter x was plotted in Fig. 2. It could be seen that the NCPM SHG and THG wavelengths grew in an approximately linear manner with the increase of the compositional parameter x. At a room temperature of 24 °C, the Gd0.132Y0.868COB crystal realized the NCPM SHG of 1053 nm, while none of the four crystals realized the NCPM THG of 1053 nm. According to the results of linear fitting, the compositional parameter x (i.e. the proportion of Gd3+ ions) should be ~0.141 for the type-I NCPM THG of 1053 nm at this temperature, as indicated by the red circle in Fig. 2.
Fig. 2 Dependence of the NCPM wavelengths on the values of the compositional parameter x at room temperature.
4. Experiments with a 1053 nm laser
As shown in Table 1, the NCPM SHG and THG wavelengths of the Gd0.132Y0.868COB crystal were 1053 nm and 1051 nm respectively; thus, among our samples, this crystal was optimum for the NCPM frequency conversion of a 1053 nm laser. Using this crystal, the NCPM SHG and THG experiments of 1053 nm were conducted successively. The THG experimental setup is shown in Fig. 3. The fundamental light source was a Nd:YLF laser with a wavelength of 1053 nm, a 5 mm diameter near-Gaussian beam, a pulse width of 50 ps, and repetition rate of 1 Hz. A half-wave plate was used to adjust the direction of linear polarization. The fundamental energy was monitored by the sampling system composed of a beam splitter (partially reflective at 1053 nm) and an energy calorimeter. A KDP crystal was used as the frequency doubler. The Gd0.132Y0.868COB crystal was placed in a copper cube whose temperature was controlled accurately (accuracy ± 0.1 °C). The copper cube was sealed to prevent thermal diffusion with quartz glasses covering the entrance and exit faces. This temperature-controlling device was fixed on a motorized rotary stage with a precision of 0.001°. In this way, both the temperature acceptance and the angular acceptance of the Gd0.132Y0.868COB crystal could be measured. The output multiple-wavelength lasers were dispersed by a quartz prism and the THG energy was detected by another energy calorimeter.
Fig. 3 Experimental setup for the THG of the1053 nm laser (1 Nd:YLF laser, 2 half-wave plate, 3 beam splitter, 4 KDP crystal, 5 Gd0.132Y0.868COB crystal, 6 quartz prism and 7, 8 probe of energy calorimeter).
4.1 Type-II NCPM SHG
By removing the half-wave plate 2 and the frequency doubler 4 in Fig. 3, the type-II NCPM SHG properties of the Gd0.132Y0.868COB crystal were investigated. The optimum NCPM temperature for 1053 nm was 28 °C. Figure 4(a) shows the variation of the SHG signal with the temperature detuning under the fixed fundamental energy, where the full-width at half-maximum (FWHM) temperature bandwidth was 15 °C. Considering the crystal length was 2.9 cm, the temperature acceptance bandwidth △TL should be 43.5 °C·cm. Maintaining the crystal temperature at 28 °C, the external acceptance angle was measured by monitoring the normalized SHG signal as the sample was rotated in the polarization plane of the fundamental wave (i.e. the bisection plane of the z-axis and x-axis). From the results, shown in Fig. 4(b), the FWHM acceptance angle was determined to be on the order of 95 mrad, which was equivalent to an external angular acceptance bandwidth of 162 mrad·cm1/2. It was 23 times larger than the external angular acceptance bandwidth of the YCOB crystals, which was found to be 6.8 mrad·cm1/2 for a type-II critical SHG at 1064 nm in (112°, 81.3°)-cut [10].
Fig. 4 Tuning curves of the NCPM SHG signal for the 1053 nm laser in the Gd0.132Y0.868COB crystal: (a) the temperature tuning curve and (b) the angular tuning curve.
When the Gd0.132Y0.868COB crystal was fixed at the optimum NCPM conditions, i.e. zero temperature and angular detunings, the SHG energy was measured by adjusting the fundamental energy. The SHG output energy was 1.8 mJ at 9.6 mJ fundamental energy, corresponding to an optical conversion efficiency of 18%. The relatively low efficiency originated from the small effective NLO coefficient (d31 ≈−0.3 pm/V [35]) for this PM style in GdxY1-xCOB crystals; it could be improved by optimizing the experimental conditions. To avoid crystal damage at high power intensity when the fundamental energy was elevated, we used another 1053 nm Nd:YLF laser as the fundamental light source to perform the NCPM SHG experiment for the second time. Its pulse width was 1 ns, and the repetition rate was 1 Hz. Using a beam compression system, which was composed of two plane-convex lens (f = 125 mm and f = 50 mm, respectively), the original 7 mm diameter of the fundamental beam was compressed to a diameter of 2.8 mm. Comparing with the above Nd:YLF laser with a 5 mm diameter and a 50 ps pulse width, the present light source could supply 3 times the brightness at a much lower the peak power. In this way, the fundamental energy could be increased substantially without the danger of damage to the NLO crystal, and both the SHG energy and the optical conversion efficiency showed remarkable improvements. As shown in Fig. 5, the type-II NCPM SHG output reached 15.4 mJ when the fundamental energy was 35 mJ, corresponding to an optical conversion efficiency of 44%. This conversion efficiency reached the obtained levels for a type-II NCPM SHG of 1064 nm in GdxY1-xCOB, which were 36.7% [25] and 43% [26] respectively.
4.2. Type-I NCPM THG
Using the experimental setup presented in Fig. 3, we performed the NCPM THG experiment with a 1053 nm laser and a Gd0.132Y0.868COB crystal, where the quartz half-wave plate was used to adjust the polarization of the fundamental wave; correspondingly, the THG output could be optimized. It should be noted that in this experiment the SHG process was critical. The process was performed by a type-II PM KDP crystal (cut angle = (59°, 0°), length = 1cm, and effective nonlinear coefficient = 0.34 pm/V). Only the THG process was non-critical and was performed by the type-I PM Gd0.132Y0.868COB crystal (cut angle = (90°, 90°), length = 2.9 cm, and effective nonlinear coefficient = 0.59 pm/V [35]). In the initial run of the experiment, in order to satisfy the PM condition, the polarization of the fundamental wave was adjusted to the bisection plane of the o-light and e-light of the KDP crystal, while the z-axis of the Gd0.132Y0.868COB crystal was in alignment with the e-light of the KDP crystal (i.e. the x-axis was in alignment with the o-light of the KDP crystal). During the experiment, the half-plate 2 in Fig. 3 was rotated to adjust the polarization of the fundamental wave to achieve the maximum output of the cascaded THG. The generated THG laser from the Gd0.132Y0.868COB crystal was divided from the remained fundamental and second harmonic lasers by a quartz prism and detected by an energy calorimeter.
From Table 1, we know that at an ambient temperature of 24 °C, the type-I NCPM THG wavelength of the Gd0.132Y0.868COB crystal was 1051 nm. By properly elevating the crystal temperature, it could be adjusted to 1053 nm. At 24 °C, the type-I THG PM direction of 1053 nm in xy plane deviated the y-axis for an exterior angle of 8.2°, i.e. Δϕext = 8.2°. The type-I THG PM direction of 1053 nm in zy plane deviated the y-axis for an exterior angle of 5.0°, i.e. Δθext = 5.0°. Based on the crystal refractive index, the internal deviating angles were about 4.8° and 2.9°, respectively. By increasing the crystal temperature to 55 °C, the THG PM angle was adjusted to the y-axis, i.e. the direction of (θ, ϕ) = (90°, 90°), which meant that the NCPM THG was realized. Figure 6 shows the variation of the THG signal with the temperature detuning, where the FWHM temperature bandwidth was found to be 10 °C. Considering the crystal length of 2.9 cm, the temperature acceptance bandwidth △TL was 29 °C·cm.
Fig. 6 Temperature tuning curve of the NCPM THG signal for the 1053 nm laser in the Gd0.132Y0.868COB crystal.
Keeping the crystal temperature at 55 °C, and the y-axis as the central direction, we measured the external acceptance angle by monitoring the variation of the THG output signal with the angular detuning. The crystal sample was rotated around the x-axis in the zy plane (θ direction), and around the z-axis in the xy plane (ϕ direction), respectively. The results are shown in Fig. 7, where the FWHM acceptance angles (external) were found to be 120 mrad in the θ direction and 69 mrad in the ϕ direction, respectively. These values corresponded to angular acceptance bandwidths of ΔθL1/2 = 204 mrad·cm1/2 and ΔϕL1/2 = 117 mrad·cm1/2, respectively.
Fig. 7 Angular tuning curves of the NCPM THG signal for the 1053 nm laser in the Gd0.132Y0.868COB crystal: (a) θ direction and (b) ϕ direction.
Maintaining the crystal temperature and orientation at the optimum NCPM conditions, the THG energy and conversion efficiency were measured by adjusting the fundamental wave energy. As shown in Fig. 8, the THG output was 2.8 mJ when the fundamental energy was 9.4 mJ, corresponding to an optical conversion efficiency of 29.8%. The relatively large effective NLO coefficient and the long interaction distance (2.9 cm) were the principal reasons for the high efficiency NCPM THG.
In Fig. 8, it can be observed that the THG conversion efficiency became saturated when the fundamental pulse energy was larger than 8 mJ. At a 10.5 mJ fundamental energy, the THG conversion efficiency even presented an obvious decline; correspondingly, the growth of the THG output energy slowed. This phenomenon originated from the appearance of the gray-track, which had been found previously during the high power THG of 1064 nm in GdxY1-xCOB [29, 36]. Figure 9(a) presents the photograph of the gray-track damage in our Gd0.132Y0.868COB crystal. The damage, a brown trace along the beam path, was not induced by a fundamental or second harmonic laser. This problem was explained by the intrinsic absorption of the Y2+ color center, which was induced by oxygen defects in the crystal [36]. It reduced the transmission of the crystal from the ultraviolet to the visible range and caused degradation of THG output as we observed in our experiment. Furuya et al. have proven that crystals grown in air ambient have greater damage threshold than those grown in an Ar atmosphere because of the reduction of oxygen defects [36]; at the same time, they found that the gray-track could be removed by crystal annealing above 150 °C for more than 20 h [29]. We used a similar method to handle the damaged Gd0.132Y0.868COB crystal, and obtained the same result. After thermal annealing at 150 °C for 20 h in a Muffle furnace, the THG-induced gray-track had been removed successfully, as shown in Fig. 9(b). The NCPM temperature of GdxY1-xCOB can be controlled by adjusting the Gd/Y composition. By reducing properly the composition of Gd3+ ions, NCPM THG will be realized at temperatures greater than 150 °C. In this way, it can be expected that the THG energy and conversion efficiency of a 1053 nm laser will be greatly improved without the formation of gray-track, just as with the method that Furuya et al. utilized [29].
Compared with popular NLO crystals, such as BBO and KTP, the unique advantage of GdxY1-xCOB crystals is NCPM, which lies along one of the optical principal axes of a low-symmetric NLO crystal. As a special PM style, its angular acceptance is much greater than other PM styles, and there is no beam walk-off. Thus, NCPM is also called the “Optimum PM”. Although the effective nonlinear optical coefficient of a GdxY1-xCOB crystal is smaller than those of BBO and KTP, it can be compensated for by a long crystal length (the benefit of no beam walk-off), or by utilizing a focused fundamental beam (the benefit of a large angular acceptance bandwidth). GdxY1-xCOB crystals can be grown by the Cz pulling method, which means that large size, high-quality single crystals can be obtained easily in a short period of time. Besides, the GdxY1-xCOB crystal is grown along y-direction, i.e. the NCPM direction that we needed. It elevates the utilization of the as-grown crystal and, at the same time, provides convenience for crystal processing. In short, the NCPM style of the GdxY1-xCOB crystal is favorable for stable, high-efficiency frequency conversion, as long as the crystal length is increased or the fundamental laser beam is focused. At the same time, the advantages in crystal growth and processing are also helpful for future practical applications.
5. Conclusion
The NCPM SHG and THG of a 1053 nm laser were realized in GdxY1-xCOB crystals for the first time. With the Gd0.132Y0.868COB crystal, a type-II NCPM SHG of 1053 nm was achieved at 28 °C, giving out a temperature acceptance bandwidth of 43.5 °C·cm, an angular acceptance bandwidth of 162 mrad·cm1/2, and an optical conversion efficiency of 44%. With the same crystal, a type-I NCPM THG of 1053 nm was achieved at 55 °C, and the corresponding temperature acceptance bandwidth, angular acceptance bandwidth, and optical conversion efficiency were 29 °C·cm, 204 (ΔθL1/2) and 117 (ΔϕL1/2) mrad·cm1/2, and 29.8%, respectively. Our research indicated that GdxY1-xCOB is a promising NLO crystal for NCPM frequency conversions of a 1053 nm laser. In addition, considering large crystal size and other advantages of NCPM, this crystal presents clear possibilities for use in optical parametric chirped-pulse amplification, master oscillator power-amplifier and “prefocusing” laser systems.
Acknowledgments
This work was supported by the National Natural Science Foundations of China (Grant No. 61178060, 51202129 and 91022034), and the Shandong Provincial Natural Science Foundation for Distinguished Young Scholar, China (JQ201218).
References and links
1. T. N. Khamaganova, V. K. Trunov, and B. F. Dzhurinskii, “Crystal structure of calcium samarium oxyborate Sm2Ca8O2(BO3)6,” Russ. J. Inorg. Chem. 36(4), 855–857 (1991).
2. R. Norrestam, M. Nygren, and J. O. Bovin, “Structural investigations of new calcium-rare earth (R) oxyborates with the composition Ca4RO(BO3)3,” Chem. Mater. 4(3), 737–743 (1992). [CrossRef]
3. A. B. Ilyukhin and B. F. Dzhurinskii, “Crystal structure of binary oxoborates LnCa4O(BO3)3,” Russ. J. Inorg. Chem. 38(6), 847–850 (1993).
4. G. Aka, A. Kahn-Harari, F. Mougel, D. Vivien, F. Salin, P. Coquelin, P. Colin, D. Pelenc, and J. P. Damelet, “Linear- and nonlinear-optical properties of a new gadolinium calcium oxoborate crystal, Ca4GdO(BO3)3,” J. Opt. Soc. Am. B 14(9), 2238–2247 (1997). [CrossRef]
5. M. Iwai, T. Kobayashi, H. Furuya, Y. Mori, and T. Sasaki, “Crystal growth and optical characterization of rare-earth (Re) calcium oxyborate ReCa4(BO3)3 (Re=Y or Gd) as new nonlinear optical material,” Jpn. J. Appl. Phys. 36(2), L276–L279 (1997). [CrossRef]
6. C. Reuther, R. Möckel, M. Hengst, J. Götze, A. Schwarzer, and H. Schmidt, “Growth and structure of Ca4La[O|(BO3)3],” J. Cryst. Growth 320(1), 90–94 (2011). [CrossRef]
7. J. J. Adams, C. A. Ebbers, K. I. Schaffers, and S. A. Payne, “Nonlinear optical properties of LaCa4O(BO3)3.,” Opt. Lett. 26(4), 217–219 (2001). [CrossRef] [PubMed]
8. H. D. Jiang, D. W. Li, K. Q. Zhang, H. Liu, and J. Y. Wang, “Optical and thermal properties of nonlinear optical crystal LaCa4O(BO3)3,” Chem. Phys. Lett. 372(5-6), 788–793 (2003). [CrossRef]
9. Y. Q. Liu, L. Wei, F. P. Yu, Z. P. Wang, Y. G. Zhao, S. Han, X. Zhao, and X. G. Xu, “Crystal growth and efficient second-harmonic-generation of the monoclinic LaCa4O(BO3)3 crystal,” CrystEngComm 15(30), 6035–6039 (2013). [CrossRef]
10. Y. Liu, H. Qi, Q. Lu, F. Yu, Z. Wang, X. Xu, and X. Zhao, “Type-II second-harmonic-generation properties of YCOB and GdCOB single crystals,” Opt. Express 23(3), 2163–2173 (2015). [CrossRef] [PubMed]
11. Y. Q. Liu, F. P. Yu, Z. P. Wang, S. Hou, L. Yang, X. G. Xu, and X. Zhao, “Bulk growth and nonlinear optical properties of thulium calcium oxyborate single crystals,” CrystEngComm 16(30), 7141–7148 (2014). [CrossRef]
12. H. J. Zhang, X. L. Meng, P. Wang, L. Zhu, X. S. Liu, X. M. Liu, Y. Y. Yang, R. H. Wang, J. Dawes, J. Piper, S. Zhang, and L. Sun, “Growth of Yb-doped Ca4GdO(BO3)3 crystals and their spectra and laser properties,” J. Cryst. Growth 222(1-2), 209–214 (2001). [CrossRef]
13. H. J. Zhang, X. Meng, L. Zhu, C. Wang, R. Cheng, W. Yu, S. Zhang, L. Sun, Y. Chow, W. Zhang, H. Wang, and K. S. Wong, “Growth and laser properties of Nd:Ca4YO(BO3)3 crystal,” Opt. Commun. 160(4-6), 273–276 (1999). [CrossRef]
14. S. J. Zhang, Z. X. Cheng, J. R. Han, G. Y. Zhou, Z. S. Shao, C. Q. Wang, Y. Chow, and H. H. Chen, “Growth and investigation of efficient self-frequency-doubling NdxGd1− xCa4O(BO3)3 crystal,” J. Cryst. Growth 206(3), 197–202 (1999). [CrossRef]
15. H. Yu, N. Zong, Z. Pan, H. Zhang, J. Wang, Z. Wang, and Z. Xu, “Efficient high-power self-frequency-doubling Nd:GdCOB laser at 545 and 530 nm,” Opt. Lett. 36(19), 3852–3854 (2011). [CrossRef] [PubMed]
16. Y. Lu and G. Wang, “Growth and spectroscopic properties of Nd3+:LaCa4O(BO3)3 crystals,” J. Cryst. Growth 253(1-4), 270–273 (2003). [CrossRef]
17. H. J. Zhang, X. L. Meng, C. Q. Wang, L. Zhu, X. S. Liu, C. M. Dong, R. P. Cheng, X. M. Liu, R. H. Wang, and S. J. Zhang, “Growth and self-frequency doubling laser output of Nd:Ca4Gd0.275Y0.725O(BO3)3 crystal,” J. Cryst. Growth 210(4), 815–818 (2000). [CrossRef]
18. A. Aron, A. Kahn-Harari, D. Vivien, F. Druon, F. Balembois, P. Georges, A. Brun, N. Lenain, and M. Jacquet, “Spectroscopic properties and laser performances of Yb:YCOB and potential of the Yb:LaCOB material,” Opt. Mater. 16(1), 181–188 (2001). [CrossRef]
19. D. Vivien, F. Mougel, F. Augé, F. Balembois, G. Lucas-Leclin, P. Georges, A. Brun, P. Aschehoug, J. M. Benitez, and N. Le Nain, “Nd:GdCOB: overview of its infrared, green and blue laser performances,” Opt. Mater. 16(1-2), 213–220 (2001). [CrossRef]
20. F. Druon, F. Augé, F. Balembois, P. Georges, A. Brun, A. Aron, F. Mougel, G. Aka, and D. Vivien, “Efficient, tunable, zero-line diode-pumped, continuous-wave Yb3+:Ca4LnO(BO3)3 (Ln= Gd, Y) lasers at room temperature and application to miniature lasers,” J. Opt. Soc. Am. B 17(1), 18–22 (2000). [CrossRef]
21. C. Q. Wang, H. J. Zhang, X. L. Meng, L. Zhu, Y. Chow, X. S. Liu, R. Cheng, Z. Yang, S. J. Zhang, and L. K. Sun, “Thermal, spectroscopic properties and laser performance at 1.06 and 1.33 μm of Nd:Ca4YO(BO3)3 and Nd:Ca4GdO(BO3)3 crystals,” J. Cryst. Growth 220(1-2), 114–120 (2000). [CrossRef]
22. F. Mougel, F. Augé, G. Aka, A. Kahn-Harari, D. Vivien, F. Balembois, P. Georges, and A. Brun, “New green self-frequency-doubling diode-pumped Nd:Ca4GdO(BO3)3 laser,” Appl. Phys. B 67(5), 533–535 (1998). [CrossRef]
23. H. Furuya, M. Yoshimura, T. Kobayashi, K. Murase, Y. Mori, and T. Sasaki, “Crystal growth and characterization of GdxY1−xCa4O(BO3)3 crystal,” J. Cryst. Growth 198, 560–563 (1999). [CrossRef]
24. N. Umemura, H. Nakao, H. Furuya, M. Yoshimura, Y. Mori, T. Sasaki, K. Yoshida, and K. Kato, “90° phase-matching properties of YCa4O(BO3)3 and GdxY1-xCa4O(BO3)3,” Jpn. J. Appl. Phys. 40(1), 596–600 (2001). [CrossRef]
25. Z. P. Wang, X. G. Xu, K. Fu, R. B. Song, J. Y. Wang, J. Q. Wei, Y. G. Liu, and Z. S. Shao, “Non-critical phase matching of GdxY1-xCa4O(BO3)3 (GdxY1-xCOB) crystal,” Solid State Commun. 120(9-10), 397–400 (2001). [CrossRef]
26. A. Zoubir, J. Eichenholz, E. Fujiwara, D. Grojo, E. Baleine, A. Rapaport, M. Bass, B. Chai, and M. Richardson, “Non-critical phase-matched second harmonic generation in GdxY1-xCOB,” Appl. Phys. B 77(4), 437–440 (2003). [CrossRef]
27. M. Yoshimura, H. Furuya, T. Kobayashi, K. Murase, Y. Mori, and T. Sasaki, “Noncritically phase-matched frequency conversion in GdxY1-xCa4O(BO3)3 crystal,” Opt. Lett. 24(4), 193–195 (1999). [CrossRef] [PubMed]
28. S. J. Zhang, Z. X. Cheng, S. J. Zhang, J. R. Han, L. K. Sun, and H. C. Chen, “Growth and noncritical phase-matching third-harmonic-generation of GdxY1− xCa4O(BO3)3 crystal,” J. Cryst. Growth 213(3-4), 415–418 (2000). [CrossRef]
29. H. Furuya, H. Nakao, I. Yamada, Y. F. Ruan, Y. K. Yap, M. Yoshimura, Y. Mori, and T. Sasaki, “Alleviation of photoinduced damage in GdxY1-xCa4O(BO3)3 at elevated crystal temperature for noncritically phase-matched 355-nm generation,” Opt. Lett. 25(21), 1588–1590 (2000). [CrossRef] [PubMed]
30. L. Gheorghe, P. Loiseau, G. Aka, and V. Lupei, “Growth and type-I noncritical phase-matching second-harmonic-generation of Gd1− xRxCa4O(BO3)3 (R 3+= Sc 3+ or Lu 3+) crystals,” J. Cryst. Growth 294(2), 442–446 (2006). [CrossRef]
31. L. Gheorghe, V. Lupei, P. Loiseau, G. Aka, and T. Taira, “Second-harmonic generations of blue light in nonlinear optical crystals of Gd1−xLuxCa4O(BO3)3 and Gd1− xSc xCa4O(BO3)3 through noncritical phase matching,” J. Opt. Soc. Am. B 23(8), 1630–1634 (2006). [CrossRef]
32. M. T. Andersen, J. L. Mortensen, S. Germershausen, P. Tidemand-Lichtenberg, P. Buchhave, L. Gheorghe, V. Lupei, P. Loiseau, and G. Aka, “First measurement of the nonlinear coefficient for Gd1-xLuxCa4O(BO3)3 and Gd1-xScxCa4O(BO3)3 crystals,” Opt. Express 15(8), 4893–4901 (2007). [CrossRef] [PubMed]
33. L. Gheorghe, P. Loiseau, and G. Aka, “Four hundred-nanometer blue-violet light production by type-I noncritical phase-matching second-harmonic generation in Gd1− xRxCa4O(BO3)3 (R= Lu, Sc): Crystal growth and nonlinear characterization,” Opt. Mater. 32(10), 1283–1285 (2010). [CrossRef]
34. J. Chen, Y. Zheng, N. An, and X. Chen, “Noncollinear third-harmonic generation with large angular acceptance by noncritical phase matching in KDP crystal,” Opt. Lett. 40(19), 4484–4487 (2015). [CrossRef] [PubMed]
35. M. V. Pack, D. J. Armstrong, A. V. Smith, G. Aka, B. Ferrand, and D. Pelenc, “Measurement of the χ(2) tensor of GdCa4O(BO3)3 and YCa4O(BO3)3 crystals,” J. Opt. Soc. Am. B 22(2), 417–425 (2005). [CrossRef]
36. H. Furuya, H. Nakao, K. Kawamura, Y. K. Yap, M. Yoshimura, Y. Mori, and T. Sasaki, “Dependence of gray-track threshold of GdYCOB on the crystal growth atmosphere,” J. Cryst. Growth 229(1-4), 265–269 (2001). [CrossRef]
