Optical Materials 43 (2015) 6–9

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Optical Materials

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Growth and nonlinear optical properties of Zn-doped LiB3O5 crystals

http://dx.doi.org/10.1016/j.optmat.2015.02.0130925-3467/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +86 10 82543721; fax: +86 10 82543709.E-mail address: hu@mail.ipc.ac.cn (Z. Hu).

Lei Yang a,b, Yinchao Yue a, Qian Mao a,b , Xiaomao Li a,b, Zhanggui Hu a,⇑a Beijing Center for Crystal Research and Development, Key Lab of Functional Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy ofSciences, Beijing 100190, Chinab University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 January 2015Received in revised form 10 February 2015Accepted 11 February 2015Available online 5 March 2015

Keywords:Top-seeded solution growthLBOOptical homogeneityNonlinear optical crystals

Zn-doped LiB3O5 (LBO) single crystals with high quality were successfully grown from theLi2O–MoO3–ZnF2 ternary system by the top-seeded solution growth method. The suitable region forLBO crystal growth was investigated by growth experiments, as well as viscosity and volatility measure-ments, which confirmed that the optimal molar ratio of Li2O:MoO3:ZnF2 was 1:1.5:0.2. The second-har-monic generation efficiency of Zn-doped LBO crystal increased by 16% compared with that of the LBOcrystals grown from the MoO3 flux. The optical homogeneity was at 10�6 cm�1. Optical absorption atthe critical wavelengths of 1064 nm was measured to be 15 and 18 ppm cm�1, respectively.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction LBO single crystals have been obtained with the method of top-

LiB3O5 (LBO) crystals have been successfully commercialized andwidely used in high-power solid-state lasers for harmonic gen-eration and optical parametric processes as an excellent nonlinearoptical crystal [1–3]. The basic structural unit of LBO is the(B3O7)5� group [4–6]. Which results the formation of an endlesshelix of (B3O7)5�

n ? 1 extended along the c-axis, and staggered atan angle of nearly 45� to each other. LBO possesses a relatively largeeffective nonlinear optical coefficient, a high laser damage threshold(25 GW/cm2 for 0.1 ns pulses at 1064 nm), and a wide optical trans-parency range (150–3200 nm) [6]. It has been considered as one ofthe most reliable nonlinear optical crystals for the second harmonicgeneration (SHG) of 1064 nm lasers [3]. To meet the demands forLBO crystals in defense and energy aspects, large-sized LBO is apromising material for the frequency multiplier of drivers in thefield of laser fusion instead of KDP/DKDP. In addition, the growthof large-sized crystals can greatly reduce the costs of productionand processing with high utilization from a business standpoint.Sizable LBO crystals have been grown. However, high viscositywas a major limiting factor in obtaining large LBO crystals that hin-ders mixing and mass transport in the melt [7]. This phenomenonleads to long duration and unstable growth (spontaneous nucle-ation, inclusions, etc.) of the interface. Therefore, further explo-ration to optimize the flux, reduction in melt viscosity, andincreased mobility to accelerate solute transport is the key to realizethe rapid growth of large-sized LBO crystals.

seeded solution growth (TSSG) technology successfully in recentdecades. Since the end of the 1980s, most LBO crystals have beengrown from the melt of B2O3 self-flux within a long period time.In 1989, C. Chen [6] obtained 30 � 30 � 15 mm3 LBO crystals in a£40 � 40 platinum crucible. In 1992, Ukachi [8], grew20 � 20 � 15 mm3 LBO crystals from 90–94% B2O3 melt. As a flux,MoO3 has been applied widely in growing LBO crystals in recentyears. In 1999, Pylneva et al [9] grew 100 � 82 � 45 mm3 LBO crys-tals from MoO3-flux melt. In 2010, Kokh et al. [10] obtained >1.3 kglarge-sized LBO crystals with sizes of 100 � 82 � 45 mm3 using themethod of heat-field symmetry control. In 2011, large LBO crystalsgrown at the 2 kg level were reported [11,12].

In the present work, the composite flux system of Li2O–mMoO3–ZnF2 was studied, and a metal fluoride component wasintroduced into the flux system. Results of our investigations onthe growth of LBO crystals were presented [13,14]. High-qualityLBO crystals were successfully grown from the Li2O–mMoO3–ZnF2 ternary system by TSSG. The volatility and viscosity of thegrowth system were investigated. Moreover, the volatility of theternary system was greatly reduced compared with MoO3 melt.The crystals were characterized by inductive coupled plasma emis-sion spectrometer (ICP), transmittance spectroscopy, opticalhomogeneity, optical absorption, and powder SHG effect. ICP ana-lysis of the obtained LBO crystal of solutions 1, 2 and 3 showed theZn2+ in the LBO crystals grown from the flux of Li2O–MoO3–ZnF2,and its UV-cutoff wavelength was near 160 nm of the transmissionspectrum. Furthermore, its second-harmonic generation efficiencyincreased compared with LBO crystals grown from MoO3 flux. Thisresult showed that Li2O–mMoO3–ZnF2 was an excellent flux, and

Table 1The results of LBO crystals grown from solutions with different compositions.

No. Li2O:MoO3:ZnF2 (molar ratio) Saturation T (�C) Volatility (%) Size (mm3), weight (g) Results

1 1:1.5:0.1 745 0.71 39 � 36 � 25 mm3, 28.2 g High quality2 1:1.5:0.2 743 0.88 42 � 33 � 21 mm3, 20.3 g High quality3 1:1.5:0.5 752 0.94 28 � 22 � 16 mm3, 26.7 g Cracking4 1:1.5:0 743 0.53 30 � 24 � 18 mm3, 26.7 g High quality

L. Yang et al. / Optical Materials 43 (2015) 6–9 7

that Zn-doped LBO single crystals were obtained from them for thefirst time.

2. Experiments

The growth procedure was as follows: High-purity reagentsLi2CO3, H3BO3, MoO3 and ZnF2 were weighed accurately at appro-priate ratios, mixed uniformly in the agate mortar and then meltedin a £80 � 70 mm3 platinum crucible. The mixture was then heat-ed to 40 �C above the melting point in a resistance-heated furnaceand maintained under stirring for 48 h with a platinum stirrer toensure completely melting of the solution and homogeneouslymixing. The saturation temperature was determined exactly byprevious seeding measurements and maintained at this tem-perature for a few days until no further growth was detected.Afterward, a seed oriented along the [001] direction was intro-duced and kept in contact with the melt surface until slight disso-lution was observed at a temperature of 10 �C higher than thesaturation temperature. Then decreased the temperature to thesaturation point at a rate of 30 �C/h, followed by cooling slowlyand carefully with a cooling rate of 0.1–0.5 �C/day until the endof growth. The growing crystal was rotated at a rate of 20–50 rpm with the rotation direction inverted every 3 min. Whengrowth was finished, pulled the crystal out of the solution surfaceslowly and then cooled the whole furnace to room temperature ata rate of 15–25 �C/h.

The volatility of solutions with various compositions was sur-veyed in the experiments by a DVII + Pro viscometer. The obtainedcrystals were characterized with elemental analysis through ICP-AES. At room temperature, the transmittance spectra of the LBOcrystals were recorded with a McPherson VUVaS2000 spectropho-tometer within the wavelength range of 120–380 nm and theirpowder SHG determination was carried out with fundamental fre-quency of the rays of a 1064 nm Nd:YAG Q-switched laser. Opticalhomogeneity was evaluated by a Wyko RTI 4100 laser interfer-ometer at 633 nm. Optical absorption was investigated by the pho-to-thermal common path interferometer (PCI).

3. Results and discussion

3.1. Crystal growth

Solutions with different molar ratios of Li2O–MoO3–ZnF2 wereprepared in a platinum crucible in various growth experimentsfor LBO growth. The growth results of various solution composi-tions are summarized in Table 1, LBO crystals with good

Fig. 1. Crystals of No. 1, 2 and 3 LBO (No. 3

morphology were found to be obtained from solutions 1 and 2 withdifferent composition by seed-submerged growth. However, theLBO crystal cracked into two pieces when the molar ratio ofLi2O:MoO3:ZnF2 reached 1:1.5:0.5.

In the growth experiments, seed crystals cut along the [001]directions were used. The crystals grown along the [010] and[001] directions were very easy to crack. The rotation rates variedfrom 25 rpm at the initial growth to 40 rpm at the last stage ofgrowth. The growth period was about 8–15 days. High-qualityLBO crystals were obtained. In the case of solution 3, we foundLBO crystals cracked into two parts. In addition, the crystal waspure without inclusions. Generally, the obtained LBO crystals hadno obvious infections like inclusions, as illustrated in Fig. 1.

3.2. Volatility and viscosity

Volatility and viscosity are two important factors affecting crys-tal growth, and are both closely related to composition. High vis-cosity can make mass transfer difficult and induce spontaneousnucleation. Therefore, a solution with low viscosity and volatilityis expected to grow high-quality crystals. Consequently, the vis-cosity and volatility of solutions with different compositions weremeasured under the same experimental conditions to explore theLi2O–MoO3–ZnF2 ternary system. The starting materials Li2CO3,H3BO3, and MoO3 were weighed at corresponding ratios inTable 1, homogeneously mixed, melted in a £70 � 60 mm3 plat-inum crucible and accurately weighed for the initial weight ofgrowth material. When LBO growth finished, we weighed theweight of the growing raw materials again after supplying thematerials in accordance with the stoichiometric crystals. The initialweight was divided by the change in weight was the volatility, andthe order of volatility from low to high was solution 1, solution 2,and solution 3, as shown in Table 1. Volatility increased with theintroduction of more ZnF2. Volatility can give rise to an undesirablesupersaturated part near the surface and induced unstable growth.Thus, too much fluoride was detrimental (Li2O:ZnF2 P 1:0.5) tocrystal growth.

A high-temperature viscometer equipped with a small platinumcylinder for viscous damping was calibrated using water–glycerolsolutions at room temperature. The molybdenum oxide system offlux served as the standard solution. For each solution, viscositywas measured at different temperatures, and the results are pre-sented in Fig. 2. At the same temperature, viscosity stronglydepended on the ZnF2 content. This result indicated that theincrease in ZnF2 could significantly reduce the viscosity of the melt.

LBO grown from solution 3 cracked).

Fig. 2. Viscosities of solutions with different compositions. Fig. 4. Powder SHG effect of LBO crystals.

8 L. Yang et al. / Optical Materials 43 (2015) 6–9

Compared with solution 4, the viscosity of the other three solu-tions rather drastically changed because of ZnF2. Although the vis-cosity of solution 3 was lower than that of the other solutions, itsvolatility was higher and its crystal cracked. As a result, solution2 was selected as the optimal system for LBO crystal growth.Therefore, the ZnF2 flux could reduce the viscosity of the solutionsbecause the boron–oxygen network was destroyed.

3.3. Crystal characterization

3.3.1. ICP-AES analysisTo determine whether impurity ions were mixed into the crys-

tal, elemental analysis was carried out by ICP-AES. Results showedthe presence of zinc ion in the crystal. The arrangement in LBOcrystal is known to be relatively close, and entry is difficult forthe general ion. Thus, this study was the first to obtain dopedLBO crystals. The molar content of zinc in LBO grown from solution2 was about 0.0481% and even 0.0795% for LBO crystals grownfrom solution 3.

3.3.2. Transparency spectrumGiven that Zn ions crystallized in the growth process, the cutoff

wavelength was influenced. To further confirm whether Zn2+ exist-ed in the as-grown LBO crystals, transparency spectra were record-ed within the range of 120–380 nm. For this measurement, asample with a thickness of 4.5 mm was cut from the as-grown

Fig. 3. Ultraviolet transmittance spectra of LBO crystals.

LBO crystal and polished to optical grade. Fig. 3 shows the trans-mission spectra of LBO crystals in the UV region. The UV cutoffwavelength was 160 nm, and a 10 nm red shift was observed forthe cutoff wavelength, which was further evidence of the presenceof zinc ions in LBO crystals.

3.3.3. Powder SHG effectThe LBO crystal samples were ground into powders with parti-

cle sizes of less than 50, 50–61, 61–74, 74–98, 98–125, 125–150 lm, 150–200, 200–300 and 300–450 lm through a standardmolecular sieve. Their powder SHG determination was carriedout with the fundamental frequency of the rays of 1064 nmNd:YAG Q-switched laser. At the same time, we took the LBO crys-tals grown from MoO3 system (No. 4) as reference samples forcomparison. Fig. 4 shows the powder SHG effect of LBO crystals.

The powder SHG intensity increased with increased particlesize. The curve tended to be smooth and reached saturation whenparticles were >300 lm. These results indicated that the as-grownLBO crystals can achieve phase matching at 1064 nm wavelength.Furthermore, the second-harmonic generation efficiency of LBOcrystals grown from solutions 1 and 2 increased by 16% and 10%,respectively, compared with LBO crystals grown from the flux ofsolution 4 (MoO3), thereby providing further evidence of the pres-ence of zinc ions in LBO crystals.

3.3.4. Optical homogeneityA 5 � 5 � 2.5 mm3 plate was cut from the as-grown crystal and

then polished for the measurement. To determine its optical homo-geneity, we used a Wyko RTI 4100 laser interferometer with a light

Fig. 5. Bulk weak absorption value of No. 1 LBO crystal at 1064 nm.

Fig. 6. Bulk weak absorption value of No. 2 LBO crystal at 1064 nm.

Fig. 7. Bulk weak absorption value of No. 4 LBO crystal at 1064 nm.

Table 2Bulk weak absorption values of LBO crystals.

No. Bulk weak absorption value at 1064 nm (ppm cm�1)

1. LBO 152. LBO 254. LBO 18

L. Yang et al. / Optical Materials 43 (2015) 6–9 9

source of He–Ne laser of wavelength 633 nm, and its incident beamlaser was parallel to the crystal optical axis. The root-mean-square(RMS) error of refractive index gradient was in the order of10�6 cm�1, which indicated excellent optical quality of the growncrystal. The zinc ions in crystal had no negative influences on thequality and were equal to the standard of commercial LBO crystal.

3.3.5. Optical absorptionThe PCI system was an improvement of the thermal lensing

technique for the detection of weak absorption values of variouscrystals. After the crystal was irradiated by a pump beam, therefractive index was changed in the heated area. Thus, the probebeam experienced phase distortion in this area. The periodic phasedistortion signal of the probe beam was detected with a photode-tector after an aperture. With interferometric sensitivity, phasedistortion of the probe beam caused by the pump beam heatingcould be transformed into corresponding perturbations of the

probe beam intensity by PCI, which is widely used as a sensitivetool for detecting weak absorption.

The optical weak absorption values of LBO crystals at the criticalwavelengths of 1064 nm were measured. The measured sampleswere about 5 � 5 � 7 mm3 in size. The results are shown inFigs. 5–7. The bulk weak absorption values of LBO crystal grownfrom the Li2O–MoO3–ZnF2 ternary system were comparable withthose of the commercially available LBO crystal. The spikes in thefigures reflected the absorption of the crystal surface because somepoints on the surface had much more absorption defects thanothers. The bulk weak absorption value was marked with a red lineand the values are listed in Table 2.

4. Conclusions

In summary, Zn-doped LiB3O5 single crystals were successfullygrown from the Li2O–MoO3–ZnF2 ternary system by the top-seeded solution growth method. The optimal molar ratio ofLi2O:MoO3:ZnF2 for LBO crystal growth was experimentally foundto be 1:1.5:0.2. The elements Mo and Zn were detected in the LBOcrystals grown from the Li2O–mMoO3–ZnF2 flux, and its UV cut offwavelength was near 160 nm of the transmission spectrum. Thesecond-harmonic generation efficiency of Zn-doped LBO crystalincreased by 16% compared with that of the LBO crystals grownfrom the MoO3 flux. The optical homogeneity was 10�6 cm�1,and the optical absorption at the critical wavelengths of 1064 nmwas about 15 and 18 ppm cm�1, respectively. These values werecomparable with those of commercially available LBO crystal.Zn-doped LBO crystal were thus demonstrated to be a promisingnonlinear optical material for UV generation, but further studyon the doping mechanism is required.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Nos. 51132005, 91122023 and 51202259).

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