Recent Advances in Crystal Growth in China: Laser, Nonlinear Optical

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DOI: 10.1021/cg100911s

Published as part of the Crystal Growth & Design 10th Anniversary Perspective.

2010, Vol. 10 4672–4681

Recent Advances in Crystal Growth in China: Laser, Nonlinear Optical, and Ferroelectric Crystals Ning Ye,* Chaoyang Tu, Xifa Long, and Maochun Hong Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China Received July 8, 2010; Revised Manuscript Received September 6, 2010

ABSTRACT: Crystal growth in China has a long history and is developing rapidly at the present time. In the past decade, significant efforts have been directed toward the research of structure-property relationships and high quality crystal growth, which has brought to fruition many fields covering nonlinear optical, laser, ferro-/piezo-electric, and wide band gap semiconductor crystals. In this contribution, selected research work accomplished by Chinese scientists in crystal engineering and growth is reviewed.

1. Introduction Dating back to more than 1000 years far before the establishment of modern sciences, China has a very long history in crystal growth, involving for example brine condensation and vermilion refinement. From the early 1960s, the increasing demands for modern science and technology on functional crystals boosted crystalline research, leading to a continuous boom in bulk crystal growth and new crystal discoveries in China. Successful bulk crystal growth includes, for example, the hydrothermal growth of quartz, flux growth of YAl3(BO3)4 (YAB) or KTiOPO4 (KTP) series, Czochralski (CZ) growth of garnet or LiNbO3 (LN) series, Bridgman growth of Bi4Ge3O12 (BGO) or halides, and aqueous growth of KH2PO4 (KDP) series. Various crystal growth techniques have fully emerged since then, affecting a characteristic community on crystal growth research in China. On the other hand, great achievements have been made in the exploration of new functional crystals coupled with studies on relationships between crystal structures and optical properties. Among those, the discoveries of a series of nonlinear optical (NLO) borate crystals, for example, BaB2O4 (BBO), LiB3O5 (LBO), and KBe2BO3F2 (KBBF), pioneered the new optical crystals exploration and gave a great impetus to the development of laser technology in the UV region. In the past decade, efforts were directed continuously toward the research of structure-property relationships and high quality crystal growth, which has brought to fruition many fields covering NLO, laser, scintillator, ferro-/piezo-electric, and wide band gap semiconductor crystals. In this contribution, selected research work accomplished by Chinese scientists on crystal design and growth is reviewed. 2. NLO Crystals By means of frequency conversion, NLO crystals function to generate tunable laser beams covering various optical *To whom correspondence should be addressed. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 10/06/2010

spectra regions. Currently, commercially available crystals are capable of harmonic generation in a region from UV to near-IR. Significant efforts have been made to grow high quality crystals to improve their laser performance in practical applications on one hand, and on the other hand, to discover new crystals to extend spectra coverage into the deep-UV and mid-IR regions. 2.1. Growth of KBe2BO3F2 Crystals. KBe2BO3F2 (KBBF) is known as the only material that can achieve direct frequency doubling below 200 nm.1 It was first synthesized by Botsanova et al.,2 and its single crystal structure was first reported in symmetry space group C2 in the late 1960s.3 About 30 years later, the discovery of KBBF’s excellent NLO properties in the deep-UV region stimulated intensive studies on its growth and properties.4-6 The structure is redetermined as belonging to space group R32.7 A KBBF crystal of 20  20  2 mm3 in size was obtained by spontaneous nucleation in a flux.8 It is rather difficult to obtain a crystal thicker than 2 mm due to the layer growth habit in a high temperature flux system, which causes a slow growth rate in the [0001] direction. Chen’s group proposed a prism coupling technique to achieve phase matching with a thin KBBF slab grown by flux method.4,9 The generation of nanoseconds radiation at 177.3 nm with a maximum average power of 34.7 mW by second harmonic generation (SHG) in a 2.06mm-thick KBBF crystal was reported, which was pumped with a homemade 4.2 W nanoseconds Nd:YAG laser at 355 nm operating at 10 kHz and 49 ns.10 KBBF crystals with dimensions up to 12  10  6 mm3 were grown by the hydrothermal method for the first time11 (Figure 1). Their growth habit is significantly improved under hydrothermal conditions in comparison with the high temperature flux method. As-grown crystals have well-defined isometric morphology of the rhombohedral system, clearly exhibiting {0001}, rhombohedral {0112} and {1011} facets. However, the quality is not as high as desired, which is confirmed by the fact that the SHG conversion efficiency is r 2010 American Chemical Society

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Figure 2. Hydrothermally grown KTP crystals.17

Figure 1. Hydrothermally grown KBBF crystals.11

1 or 2 orders of magnitude smaller than that of crystals grown by the flux method.12 For example, the conversion efficiency of the fourth harmonic generation of an Nd:YAG laser (10 ps, 10 Hz) is about 30%; meanwhile, for a crystal grown by the hydrothermal method, it is only about 0.12% with the same laser system, suggesting certain structural defects in the KBBF crystals grown by the hydrothermal method. It remains challenging on the preparation of KBBF crystals with high optical quality and in large sizes. 2.2. Hydrothermal Growth of Gray-Track Resistance KTiOPO4. KTiOPO4 (KTP) is one of the most important NLO crystals, and in particular, the best frequency-doubling material for Nd:YAG lasers. The hydrothermal method was first employed to grow KTP crystals since it was invented early in the 1970s.13 In the 1980s, the flux method was introduced by a number of Chinese scientists, and the method was quickly found to cost less and be more efficient than existing methods.14 However, KTP crystals grown from flux suffered from the incorporation of transition metal impurities during the growth process, resulting in a lower damage threshold and gray-track under a high power laser.15,16 Hydrothermal crystals, on the other hand, are grown in an insulated system, so impurity contaminants are avoided. Thus, the approach was reconsidered to obtain gray-track resistant KTP crystals. A KTP single crystal up to 26  83  5 mm3 in size and 132 g in weight was achieved by the hydrothermal method (Figure 2),17 whose transition metal impurities were less than 6 ppm. The gray-track resistance of a KTP crystal was measured by exposing the crystal under a high power green laser. The bulk absorption of another detecting laser beam was monitored and recorded to evaluate the gray-track behaviors. The results show that the hydrothermal KTP crystal possesses higher optical quality and greater ability against gray-track, and consequently, is suitable for uses in high power applications. 2.3. BaTeMo2O9, a New Mid-IR NLO Crystal. BaTeMo2O9 (BTM) was first reported by Halasyamani as an NLO material containing d0 transition metal cations (Mo6þ) and nonbonded electron pairs (Te4þ), which are susceptible to the second-order Jahn-Teller effect.18 It crystallizes in a non-centrosymmetric monoclinic crystal system with space group P21. Large BTM single crystals were grown from a TeO2-MoO3 system by the flux method (Figure 3),19,20 with

Figure 3. Flux-grown of BTM crystals.19,20

an optical transparent range from 0.4 to 5.2 μm. The determined dispersions of refractive indices show that the BTM crystal is optically negative biaxial and is phasematchable for frequency conversions in the mid-IR region. BTM has comparable and even larger piezoelectric constants (d34 = 30.25 pC/N, s44 = 36.46 pm2/N) than LiTaO3 and SiO2 do, suggesting that it is not only a potential NLO crystal in mid-IR but also a very promising piezoelectric crystal at room temperature.21 2.4. BaGa4S7, a New Mid-IR NLO Crystal with High Laser Damage Threshold. The generation of a high-power tunable laser in the range of 3-20 μm, especially in band II (3-5 μm) and band III (8-14 μm) of three atmospheric transparent windows, has become the research focus of the IR laser technology. It is particularly challenging to have suitable NLO materials working in those two bands, which require both high NLO coefficients and high laser damage threshold simultaneously. The major shortcoming in currently available IR crystals, however, is their low laser-induced damage threshold due to their narrow band gap, limiting their uses in optical parametric oscillator (OPO) and high power laser output. Recently, a new family of ternary chalcogenides containing Li element, such as LiGaS2 and LiInS2, was discovered.22 Owing to their wider band gap, such types of materials, containing alkaline or alkaline-earth metals, have a higher laser-induced damage threshold than that of chalcopyrite semiconductors. BaGa4S7 (BGS) was formerly synthesized by Eisenmann et al., and its single-crystal structure was reported as belonging to Pmn21.23 Crystal boules up to Φ 10  20 mm3 in dimension were grown by the Bridgman-Stockbarger technique (Figure 4).24 Their linear and nonlinear optical properties are measured, revealing a wide optical transparent region (350 nm to 13.7 μm), high secondorder susceptibility coefficients (d33 = 12.6 pm/V), and high

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Figure 4. BGS crystals grown by the Bridgman method.24

laser damage threshold (1.2 J/cm2 at 1.064 μm and a 15 ns pulse width). Such feature promises BGS practical applications in high power frequency conversion in the mid-IR region. 2.5. Halides, Series of Crystals Grown from Solution with High Laser Damage Threshold. In the past decade or so, a number of researchers have investigated the feasibility of ternary halides as mid-IR NLO materials because their large band gap is potentially beneficial to the improvement of laser damage threshold. CsGeX3 (X = Cl, Br, and I) was the first halides series discovered as potential candidates in this regard.25,26 Consequently, several halides containing asymmetric metal-halide polyhedron and coordination units with lone-pair cations have been grown as IR NLO materials. CsCdBr3 was synthesized initially to adopt a centrosymmetric space group P63/mmc.27 A new form of CsCdBr3 crystals with a non-centrosymmetric structure P63mc was prepared from an aqueous solution containing the mixture of CdI2 and CsBr, although the effect of additional I- on the phase transition was not demonstrated in the paper.28 The CsCdBr3 crystal exhibits an SHG effect of about twice as large as that of KDP, and a very wide transparent window covering from 300 nm to the mid-IR region. SbF3 crystallizes orthogonally in space group C2cm and has a net polarization along the [100] direction. Its spontaneous polarization emanates from the favorable alignment of the lone pair of electrons in Sb3þ. An SbF3 single crystal with the size of 10  3  2 mm3 was grown by slow solvent evaporation in aqueous solution at a constant temperature.29 The intensity of SHG is about 5.8 times as large as that of KDP. The SbF3 crystal is transparent in the range of 0.29-12 μm. Cs2Hg3I8 was first reported by Fedorov et al. who prepared it in an aqueous solution.30 A large single crystal with the size of 25  14  5 mm3 was grown by slow solvent evaporation in acetone at a constant temperature by Qin (Figure 5).31 The intensity of the SHG effect is similar to that of KTP, and the effect is phase-matchable. The compound is transparent in the range of 0.5-25 μm. NaSb3F10 was first reported by Fourcade et al. in 1975.32 A single crystal with the size of 12  10  8 mm3 was obtained by slow solvent evaporation in water at a constant temperature.33 NaSb3F10 belongs to hexagonal structure with non-centrosymmetric space group P63. Similar to that of SbF3, the overall polarization originates from the approximate parallel alignment of the lone pair electrons. NaSb3F10 exhibits a phase-matchable SHG effect 3.2 times as strong as that of KDP. It is thermally stable up to 220 °C and transparent in the range of 0.25-7.8 μm with a band gap about 5.0 eV. Experimental measurement indicates that its laser induced damage threshold is about 1.3 GW/cm2, a value higher than any reported IR NLO crystals.

Figure 5. A Cs2Hg3I8 crystal grown from acetone.31

Figure 6. Hydrothermally grown ZnO crystals.36

2.6. Growth of High Quality ZnO Crystals with the Hydrothermal Method. ZnO single crystals can be grown via the chemical vapor transport (CVT) method, pressurized melt method, flux method, or hydrothermal method. The hydrothermal method, benefiting from a relatively low growth temperature and an approximate thermodynamic equilibrium growth condition, allows the production of ZnO crystals with low defect density and high crystallinity.34,35 However, the disadvantage of the conventional hydrothermal method lies in the basic mineralizers. Under basic solution, metal impurities from autoclave parts can be corroded easily and subsequently incorporated into the lattice of ZnO crystals. A new hydrothermal scheme was proposed based on the chemical potential principle to suppress certain impurities from incorporating into ZnO

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Figure 7. Crystals52 of Nd3þ:GdAl3(BO3)4 (a)46 and Er3þ:Yb3þ:YAl3(BO3)4 (b).52

crystals by employing new mineralizers (Figure 6).36 ZnO crystals grown by this method have a unique electrical character of higher carrier mobility with the presence of high carrier concentration. No visible emission bands are observed in the PL spectrum at 10 K. The reason that the new hydrothermal method yields low resistance ZnO crystals is interpreted as a much higher concentration of H impurity and a much lower concentration of Li impurity incorporated into the ZnO lattice, compared to samples prepared by the conventional hydrothermal method. 3. Laser Crystals In the past decade, laser crystals were extensively studied in China. The laser hosts used were spread all over borates, aluminates, tungstates, molydbates, vanadates, and halides. Rare earth ions, such as Nd3þ, Yb3þ, Er3þ, Tm3þ, Ho3þ, and Dy3þ, and transition ions, such as Cr3þ, are widely used as the active centers for generations ranging from visible to mid-IR. Such works in China for outstanding laser crystals are introduced in detail as follows. 3.1. Borates. As a laser host, borate possesses favorable chemical and physical characteristics and a higher damage threshold. In particular, borate has a higher NLO efficiency resulting from its B-O structure. After being doped with active ions, borate can serve as a self-frequency conversion multifunction laser medium. Re3þ:RAl3(BO3)4 crystals are usually grown by the flux method because they decompose before melting. The crystals of Nd3þ:GdAl3(BO3)4, Yb3þ:GdAl3(BO3)4, Yb3þ:YAl3(BO3)4, Er3þ:Yb3þ:GdAl3(BO3)4, and Er3þ:Yb3þ:YAl3(BO3)4 with sizes up to 20-30 mm were grown from a K2Mo3O10-B2O3 solvent by the research groups of Tu,37-46 Huang,47-49 and Wang,50-52 respectively (Figure 7). Generations ranging from violet to IR by self-frequency conversion are obtained in Nd3þ: GdAl3(BO3)4 (NGAB) crystal using a dye laser as a pumping source. Furthermore, the red-blue-green tricolor generations are also achieved synchronously with an NGAB crystal. A laser output of 6.4 mW at 532 nm is obtained using a continuous wave (CW) Ti:sapphire tunable laser as a pumping source. Quasi-continuous-wave output powers of 1.8 W at 1.5-1.6 μm with a slope efficiency of 19% and 0.78 W with a slope efficiency of 14% are achieved in diode-pumped c-cut and a-cut Er3þ: Yb3þ: GAB crystals, respectively.49 Quasi-continuous-wave output power of 2.0 W at 1.5-1.6 μm with a slope efficiency of 21% is also obtained in a hemispherical cavity in an Er3þ: Yb3þ:YAB crystal.48 The output of 1.1 W and 700 mW with the typical differential slopes 54% and 30%, respectively, for the

o- and e-waves are achieved in a Yb3þ:GAB crystal. 1.1 W CW green output from a diode-end-pumped self-frequency-doubled (SFD) Yb:YAB laser, with a diode-to-green optical conversion efficiency of 10%, was reported.50 The output laser in the 1120-1140 nm range is demonstrated pumped by an InGaAs laser. The slope efficiency is 3.8%, and the self-frequencydoubling output power of 1 mW is achieved.51 A Cr3þ:LaSc3(BO3)4 (LSB) crystal with dimension Φ 25  35 mm3 was obtained using the CZ method in 2002.53-55 The absorption and photoluminescence spectra are studied. The lifetime of Cr3þ:LSB is 15 μs. The photoluminescence spectrum of Cr3þ:LSB via 4T2 f 4A2 transition is a broadband emission from 740 to 1280 nm centered near 960 nm at room temperature. The absorption and photoluminescence spectra reveal that in Cr3þ:LSB, Cr3þ ions occupy weak crystal field sites. Re3þ:Ca4RO(BO3)3 (Re = Nd, Yb; R = Y, Gd) crystals with large sizes and good optical quality were grown also using the CZ method from Wang’s group.56-58 The groundstate energy-level splitting is up to 1012 cm-1 and the measured emission lifetime of Yb3þ is 2.65 ms. Polarized laser oscillation with E//x in Yb3þ:YCa4O(BO3)3 is obtained at 1085.6 nm using a coupler of 0.5% transmission, yielding an output power of 7.3 W with an optical-to-optical efficiency of 63%. Higher output coupling (>2%) leads to polarized oscillation with E//z at laser wavelengths 2000 pC/N), large longitudinal electromechanical coupling factors (k33 > 92%), high dielectric constants, low dielectric losses, and exceptionally high strain levels (>1%) compared to d33 ≈ 400 - 600 pC/N, strain ≈0.1%, and k33 = 70-75% in PZT-based piezoceramics.103,104 The growth of these single crystals is challenging due to the complex multicomponent systems. So far, a number of works has been carried out on the growth of PMN-PT single crystals by various methods. Following intensive research and development efforts, large and high-quality single crystals of PMN-PT and PZN-PT have been successfully grown. Shanghai Institute of Ceramics, CAS, took a leading role in growing PMNT single crystals by the Bridgman technique based on the understanding of the features in the PMN-PT system and thermal stability of the PMNT crystals.105

Figure 14. An as-grown PMN-0.31PT single crystal by the TSSG method.108

PMNT crystals grown using this technique are large in size and excellent in piezoelectric properties. For the growth of PZN-PT crystals, it is necessary to use an excess of PbO as a flux to prevent the formation of a pyrochlore phase. PZNT(91/9) crystals with dimensions of 30 mm in diameter and 25 mm in length were grown by a modified vertical Bridgman method using PbO as a flux in 2002106 (Figure 13). Ferroelectric single crystals were recently grown successfully by the TSSG technique developed.107 Compared to other methods, the TSSG technique offers many advantages in growing single crystals with good quality, high compositional homogeneity, and controlled morphology. The results are significant since composition variations from the phase segregation have been a major hurdle in the development of PMN-PT single crystals in commercial applications. The TSSG method has proven to be a viable alternative for the growth of piezocrystals with homogeneous composition and controlled (001)-faced morphology. Figure 14 shows a PMN-0.31PT single crystals grown by the method.108 The rhombohedral-tetragonal phase transition is detected at TMPB = 108 °C in the poled-sample with a Curie temperature TC (Tmax) = 135 °C. The piezoelectric coefficient d33 is found to be 2200 pC/N, and the longitudinal electromechanical coupling factor k33 reaches 92%. 4.2. Ternary Ferroelectric Crystals. PMNT and PZNT crystals near the MPB range show low Curie temperatures TC (about 150 °C for PMNT and 170 °C for PZNT), and even lower depoling temperatures Trt (75-95 °C), as a result of the rhombohedral-tetragonal phase transition in the MPB region. Such drawback seriously limits their applicability in the high temperature range. Another drawback of the PMNT and PZNT crystals is their low coercive field (Ec = 2-3 kV/cm), which causes the crystals very easy to depole and thereby their piezoelectric performance to degrade. Therefore, new piezo- and ferroelectric single crystals with high performance and high TC are desired. During the past several years, some other relaxor-based ferroelectric crystal systems have been investigated, such as Pb(Yb1/2Nb1/2)O3PbTiO3 (PYNT),109,110 Pb(In1/2Nb1/2)O3-PbTiO3 (PINT),111,112 and Pb(Sc1/2Nb1/2)O3-PbTiO3 (PSNT),113 which, to some extent, exhibit a higher Curie temperature. However, progress in the growth of these crystals has been slow to date, mostly due to the difficulties in controlling proper composition. In order to solve this issue, researchers have tried to modify the PMNT and PZNT crystals by doping a small amount of other ions with the intention to increase the morphotropic phase transition

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be expected. Many opportunities remain for the discovery of new and improved materials to meet future performance requirements. Acknowledgment. This work was supported by the National Science Foundation of China (Nos. 90922035, 50872132, and 61078076).

References

Figure 15. An as-grown PIMNT crystal by the vertical Bridgeman method.117

temperature Trt and also Ec.101,107 But the results are not particularly promising as the effects of chemical modification may vary depending upon the systems and dopants. In recent years, great attention has been directed toward a few ternary solid solution systems in order to overcome the disadvantages of the binary PMN-PT system, including, for example, Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIMNT),114,115 and Pb(Sc 1 / 2 Nb 1 / 2 )O 3 -Pb(Mg 1 / 3 Nb 2 / 3 )O 3 -PbTiO 3 (PSMNT);116 certain promising piezoelectric properties are achieved with a higher Curie temperature. These ternary systems combine the advantages of two respective morphotropic phase boundaries and offer a higher degree of structural instabilities and thus more enhanced piezoproperties. Of particular interest is the PIN-PMN-PT system, whose ternary MPB is located between PIN/PT = 63/37 and PMN/PT = 68/32, both of which were grown successfully by a modified Bridgman technique. Figure 15 reveals a 45 mm-diameter as-grown crystal boule of PIN-PMNPT.117 The characterization of the crystal confirms the advantages initially expected: TRT g 1195 °C and Ec > 7 kV/cm, while the piezoelectric performance rivals that of PMN-PT crystals. In addition, the temperature dependences of the dielectric and piezoelectric properties and the coercive field are more stable. For some other ternary systems, such as PSN-PMN-PT, PYN-PMN-PT, PMN-PZ-PT, they remain quite difficult to grow by the vertical Bridgeman method due to higher temperature and incongruency. Under this circumstance, the TSSG technique emerges as an important approach to obtain solid solution crystals. For example, the crystals of PYN-PMN-PT (15/53/32) were grown by the TSSG technique. The piezoelectric coefficient d33, longitudinal electromechanical coupling factor k33, and coercive field for the as-grown crystals are 1800 pC/N, 90%, and 7 kV/ cm, respectively. Compared to PMNT and PZNT developed thus far, the PYMNT ternary crystals show a higher Curie temperature (205 °C) and a much larger coercive field. These improved properties make the PYN-PMN-PT ternary crystals promising for future transducer applications. 5. Conclusions The past decade has seen important progress in the development of new crystals and improvement of crystal quality in China. With continued interest in deep-UV, mid-IR, ultrafast, widely tunable laser sources, and electromechanical sensors, further development of new functional crystals can

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