2186
J. Opt. Soc. Am. B / Vol. 28, No. 9 / September 2011
Huang et al.
Ultraviolet nonlinear optical crystal: CsBe2BO3F2 Hongwei Huang,1,2 Chuangtian Chen,1,* Xiaoyang Wang,1 Yong Zhu,1 Guiling Wang,1 Xin Zhang,1,2 Lirong Wang,1,2 and Jiyong Yao1 1
Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2 Graduate University of the Chinese Academy of Sciences, Beijing 100190, China *Corresponding author:
[email protected] Received April 29, 2011; accepted July 7, 2011; posted July 12, 2011 (Doc. ID 146821); published August 16, 2011
A new ultraviolet nonlinear optical crystal CsBe2 BO3 F2 has been grown by the flux method with relatively greater size and thickness along the z axis. The crystal structure was determined by single-crystal x-ray diffraction analysis and the space group of it was defined as R32, belonging to the uniaxial class. Optical properties including the ultraviolet absorption edges, refractive indices, phase-matching angles, and effective nonlinear optical coefficients have been systematically determined for the first time. Based on the measured refractive indices, the Sellmeier equations were also fitted. © 2011 Optical Society of America OCIS codes: 160.4330, 190.4400.
1. INTRODUCTION Ultraviolet (UV) nonlinear optical (NLO) materials are of great importance and attracting more interest because the crystals can produce deep-UV (DUV) wavelengths below 300 nm just with a simple second harmonic generation (SHG) method and have merits of the simplicity, stability, better beam quality, and high conversion efficiency. On the other hand, these coherence light sources have played an important role in many advanced technology areas, such as semiconductor photolithography, laser micromachining, material processing, photochemical synthesis, as well as super-high-resolution and angle-resolved photoemission spectrometers [1,2]. It is well known that KBe2 BO3 F2 (KBBF) is a promising DUV NLO crystal [3–5]. However, the crystal is very difficult to grow in more thickness along Z axis because of its strong layer habit; thus, there is still ample scope for developing new DUV NLO crystals. Based on the anionic group theory of the NLO effect in crystals [6,7], we know that the NLO coefficients, birefringence, as well as bandgap of KBBF crystals are mainly determined by the ðBe2 BO3 F2 Þ∞ network structure, while the K þ cation has little effect on the above parameters. As a result, it is conceivable that new DUV NLO crystals can be discovered through the substitution of Rbþ and Csþ for K þ , while the basic framework of the KBBF lattice will be retained in the new crystals. With this approach and through systematic experimental investigations, two new NLO crystals, RbBe2 BO3 F2 (RBBF) and CsBe2 BO3 F2 (CBBF), for DUV harmonic generation have been discovered by our group [8,9]. In 2009, we reported in a systematical way the linear and nonlinear optical properties of an RBBF crystal in the paper [10]. Now the relatively large bulk crystal of CBBF has been successfully grown with the flux method, so in this paper, the basic structure, linear and nonlinear optical properties of the CBBF crystal is discussed in detail.
ometer with graphite-monochromatic Mo Kα radiation (λ ¼ 0:71073 Å). A colorless, transparent crystal with approximate dimensions of 0:1 × 0:1 × 0:2 mm3 was selected for structure determination. The structure was solved with Shelxtl-97 by the direct method and refined by full-matrix least-squares techniques with anisotropic thermal parameters. Baydina [11] first discovered and synthesized the CBBF compound in 1975, which was reported to have a monoclinic structure with C2 space group. As soon as the crystal was grown up by our group, the structure was redetermined. Similar to KBBF [12] and RBBF, the space group of CBBF is defined to be R32, belonging to a trigonal and uniaxial system. As shown in Fig. 1, the uniaxial character of the crystal is obviously observed. The refinement gives lattice parameters (a ¼ b ¼ 4:4391 Å and c ¼ 21:125 Å) and a reliable factor of R. Relevant crystallographic data and the refinement conditions for the Rietveld analysis are listed in Table 1. The atomic coordinates and equivalent isotropic displacement parameters are also given in Table 2. In the CBBF structure, the major building units are planar BO3 and tetrahedral BeO3 F. The O─B─O bond angle is 120° and the B─O bond length is 1:37 Å in the BO3 group. The distances of Be─O and Be─F are 1.63 and 1:53 Å, respectively, in the unit of BeO3 F. The BO3 units are parallel to each other in aligned arrangement and the F atoms are located above and below the Be atoms alternately, which makes adjacent BeO3 F groups spatially reciprocal. Each BO3 group connecting two neighboring BeO3 F groups compose a limitless plane framework of ðBe2 BO3 F2 Þ∞ along the a–b plane, which is the same as the frameworks of KBBF and RBBF. Infinite layers of ðBe2 BO3 F2 Þ∞ join together along the c axis via electrostatic force of the Cs atom and F atom, thus, the crystal displays layer tendency. The structure of CBBF is described in Fig. 2.
3. CRYSTAL GROWTH
2. STRUCTURE Single-crystal x-ray diffraction data for CBBF was collected at room temperature on a Bruker P4 single-crystal diffract0740-3224/11/092186-05$15.00/0
Because of the toxicity of BeO, all of the experiments were performed in a ventilated system. Polycrystalline samples of CBBF were prepared by high-temperature solid-state © 2011 Optical Society of America
Huang et al.
Vol. 28, No. 9 / September 2011 / J. Opt. Soc. Am. B
2187
Table 2. Atomic Coordinates and Overall Isotropic Thermal Displacement Parameter for CBBF Atom Site Parameters Atom
X
Y
Z
Cs F O Be B
0.0000 0.0000 0.3096(10) 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.73493(17) 0.5000 0.8071(4) 0.5000
UðeqÞ
a
0.01123(18) 0.0109(8) 0.0065(10) 0.0082(15) 0.0044(16)
a UðeqÞ is defined as one-third of the trace of the orthogonalized U ij tensor.
Fig. 1. (Color online) Interference pattern of CBBF along the c axis.
reaction. The initial reactants used for this experiment consisting of Cs2 CO3 , BeO, NH4 HF2 , and H3 BO3 are all analytically pure. The chemical equation can be expressed as follows: Cs2 CO3 þ 4BeO þ 2NH4 HF2 þ 2H3 BO3 ¼ 2CsBe2 BO3 F2 þ CO2 ↑ þ 5H2 O↑ þ 2NH3 ↑: The raw materials were mixed in stoichiometric proportions, heated gradually up to 720 °C, and kept at this temperature in air for 48 h. As a result, a single-phase powder of CBBF was obtained, which was examined by powder x-ray diffraction analysis. The CBBF single crystal can be grown by both flux and hydrothermal methods. Recently McMillen et al. [13] succeeded in growing crystals of millimeter size using the hydrothermal method. However, no optical properties of the crystals were reported until now. In our studies, it was grown by the hightemperature flux method. After several self-fluxes were investigated for growing CBBF crystal, finally, the B2 O3 –CsF flux
system was utilized and the spontaneous nucleation technique was used. The flux and CBBF powder mixed in proper proportions were placed into an airtight platinum crucible of 70 mm in height and diameter with a cover to reduce vaporizing, then were heated up gradually to 800 °C, held for at least 50 h in a programmable temperature electric furnace and stirred to ensure that the solution melted completely and mixed homogeneously. After that, the temperature was lowered to the saturation temperature (750 °C) in a day and decreased at a rate of 0:5–2 °C=day to keep the crystals growing. When the final crystallization temperature (650 °C) was reached, the furnace was cooled to room temperature within 2 days. The CBBF crystal was obtained from the solid in the crucible, which was dissolved by dilute acid. As Fig. 3 shows, a single CBBF crystal in regular shape and another cut and polished CBBF crystal thickness of 2:3 mm were obtained (Fig. 3). The specific heat was measured by differential scanning calorimetry (DSC) using the Labsys TG-DTA16 (SETARAM) thermal analyzer (the DSC was calibrated with Al2O3) for temperatures ranging from 25 °C to 500 °C, with a scanning rate of 30 °C= min and 10 °C= min from 500 °C to 1150 °C. Xray diffraction (XRD) is a frequently used method to analyze the type and phase of a crystal. As Fig. 4 shows, the DSC curve shows a single endothermic peak at 950 °C, which may be attributed to the incongruent melting of CBBF, then the incongruent melting behavior was confirmed by comparing the powder XRD diffraction of CBBF crystal with that of melted
Table 1. Crystallographic Data and X-ray Rietveld Refinement for CBBF Chemical formula Formula weight Space group Unit cell parameters
Index ranges θ range No. of reflections No. of refined parameters Final R indices [I > 2σðIÞ] R indices (all data) Goodness-of-fit on F 2 Largest diff peak and hole
CsBe2 BO3 F2 247.73 R32 a ¼ b ¼ 4:4391ð6Þ Å, c ¼ 21:125ð4Þ Å, V ¼ 360:51ð10Þ Å3 α ¼ β ¼ 90°, γ ¼ 120°, Z ¼ 4 −6 ≤ h ≤ 6, −6 ≤ k ≤ 6, −31 ≤ l ≤ 20 2:89°–32:45° 192 22 R1 0.0294 wR2 0.0527 R1 0.0305 wR2 0.0528 0.920 2.345 and −1:244 ðe · Å−3 Þ
Fig. 2. (Color online) (a) Structure of CBBF crystal. (b) Planar lattice of ðBe2 BO3 F2 Þ∞ .
2188
J. Opt. Soc. Am. B / Vol. 28, No. 9 / September 2011
Huang et al.
Fig. 5. Transmittance of CBBF crystal in the UV region.
By using a right-angle prism with apex angle 30° made from CBBF crystal, the refractive index data have been measured at 11 wavelengths ranging from the UV to IR region by means of the minimum deviation angle method. The experimental setup was a goniometer–spectrometer system. (SpectroMaster UVVIS-IR, Trioptics, Germany). Table 3 lists the measurement results. The Sellmeier equations can be obtained by fitting the refractive index data, as follows: Fig. 3. (Color online) (a) Single CBBF crystal in relatively regular shape with size 30 × 30 × 2 mm3 . (b) Another cut and polished CBBF crystal with thickness of 2:3 mm.
residues. Therefore, large crystals of CBBF must be grown with flux and below the decomposition temperature.
4. LINEAR OPTICAL PROPERTIES A CBBF sample with dimensions of 5 × 5 × 2:33 mm3 was prepared for transmittance measurement. The UV and IR transmittance spectrums were recorded using a McPherson VUVas2000 and Bio-Rad FTS-60V spectrometer, respectively, as shown in Figs. 5 and 6. It can be seen that the cutoff wavelength on the UV side is located at 151 nm, while on the IR side it is about 3700 nm.
Fig. 4. DSC curve of CBBF crystal.
0:0091453 − 0:0101828λ2 λ2 − 0:0126509 0:0070027 n2e ¼ 2:0802682 þ 2 λ − 0:0109331 − 0:0047423λ2 ðunits of λ are in μmÞ:
n2o ¼ 2:2562126 þ
ð1Þ
By using these Sellmeier equations we can calculate the refractive indices of the crystal. Figure 7 and Table 3 show the measured and calculated refractive indices. It can be seen that the theoretical values agree well with the experimental data. In order to further validate the accuracy of the Sellmeier equations, the phase-matching characteristics of CBBF were also investigated, and the phase-matching angles for type-I SHG were determined for the fundamental wavelengths from 1080 to 470 nm. The measured and calculated phase-matching
Fig. 6. Transmittance of CBBF crystal in the IR region.
Huang et al.
Vol. 28, No. 9 / September 2011 / J. Opt. Soc. Am. B
Table 3.
2189
Measured and Calculated Refractive Indices of CBBF with Δ as the Absolute Value of the Difference between the Measured and Calculated Values. no
ne
Wavelength (μm)
Exp
Cal
Δ
Exp
Cal
Δ
0.2537 0.363 0.4047 0.4358 0.480 0.5461 0.5875 0.6438 0.7065 0.8521 1.014
1.5596139 1.5269022 1.5215640 1.5184957 1.5152763 1.5115366 1.5099998 1.5082333 1.5065752 1.5037796 1.5014915
1.5596108 1.5269598 1.521515 1.5184898 1.5151886 1.5116582 1.5099995 1.5081867 1.5065785 1.5038019 1.5014791
0.000003 0.000058 0.000049 0.000006 0.000088 0.000120 0.000000 0.000047 0.000003 0.000022 0.000012
1.4869543 1.4620046 1.4578738 1.4555058 1.4530541 1.4501353 1.4489877 1.4476837 1.4464750 1.4444665 1.4430305
1.4869514 1.4620518 1.4578429 1.4555038 1.4529568 1.4502513 1.448994 1.4476379 1.4464594 1.4445127 1.4430094
0.000003 0.000047 0.000031 0.000002 0.000097 0.000120 0.000006 0.000046 0.000016 0.000046 0.000021
angles are shown in Table 4 and Fig. 8. It can be seen that they agree well. The Sellmeier equations indicate that it is possible to achieve SHG phase matching in CBBF down to the wavelength of 201 nm, which indicates that it may have promising applications in the UV region.
5. NONLINEAR OPTICAL PROPERTIES Similar to KBBF and RBBF in the space group R32, CBBF also has only two nonzero dij coefficients, i.e., d11 and d14 . The matrix form of the coefficients can be written as follows: 2
d11 dij ¼ 4 0 0
−d11 0 0
0 d14 0 0 0 0
0 −d14 0
3 0 −d11 5: 0
ð2Þ
Theoretical calculation and experiments both reveal that d14 is very small. On the other hand, the effective deff coefficients of CBBF are expressed as follows: deff ¼ d11 cos θ cos 3Φ; type I deff ¼ d11 cos2 θ sin 3Φ; type II:
ð3Þ
We see that the d14 coefficient does not enter into the deff coefficients; thus, it is only d11 that needs to be determined.
Fig. 7. (Color online) Refractive indices dispersion curve. Circles and points, experimental values; curves, fits given by the Sellmeier equation.
This has been precisely measured by the Maker fringes technique with a 10 × 10 × 1:7 mm3 c-cut crystal plate (the arrangement of the axes can be found in Ref. [6]). A detailed description of the experimental setup can be found in Ref. [14]. A flash-lamp-pumped Q-switched Nd:YAG laser (SpectraPhysics, Model Pro 230) was used as the fundamental light source. The signal was received by a photomultiplier tube (Hamamatsu, R105) and averaged by a boxcar. KH2 PO4 (KDP) was often used as a standard nonlinear optical crystal Table 4. Phase-matching Angles for Type-I SHG with CBBF a Fundamental Wavelength (nm)
SHG Wavelength (nm)
470 490 510 520 530 532 540 680 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 1000 1020 1040 1060 1064 1080
235 245 255 260 265 266 270 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 532 540
Phase-matching angle (deg) Exp
Cal
Δ
58.03 54.60 51.37 50.11 48.81 48.59 47.86 36.11 35.06 34.04 33.16 32.30 31.49 30.79 30.13 29.48 28.95 28.40 27.93 27.46 27.02 26.61 26.21 25.89 25.59 25.29 25.02 24.97 24.75
58.29 54.53 51.39 49.99 48.68 48.43 47.47 36.01 34.92 33.91 32.99 32.14 31.35 30.62 29.95 29.33 28.75 28.22 27.72 27.27 26.84 26.45 26.09 25.76 25.46 25.18 24.93 24.88 24.70
−0:26 0.07 −0:02 0.12 0.11 0.16 0.39 0.10 0.14 0.13 0.17 0.16 0.14 0.17 0.18 0.15 0.20 0.18 0.21 0.19 0.18 0.16 0.12 0.13 0.13 0.11 0.09 0.09 0.05
a Exp, measured angles; Cal, angle calculated using the Sellmeier equations; Δ, difference between the measured and calculated values.
2190
J. Opt. Soc. Am. B / Vol. 28, No. 9 / September 2011
Huang et al.
determined by single-crystal x-ray diffraction and the space group was proven to be R32, just like KBBF. Thermal analysis gives CBBF an inconsistent melting point. The optical parameters, including cutoff wavelength, refractive indices, phasematching angles, and the effective NLO coefficients were determined, from which the Sellmeier equations for CBBF have also been constructed. All of the results indicate that CBBF is a promising UV NLO crystal. Future efforts will be devoted to the growth of large crystals.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) under grant 50590402, and the National Basic Research Project of China (2010CB630701). Fig. 8. (Color online) Type-I SHG phase-matching angles versus fundamental wavelength for CBBF in the whole spectral region. Solid line, curve calculated from the Sellmeier equations; points, data from the experiments.
REFERENCES 1.
2.
3. 4. 5.
6. Fig. 9. (Color online) Maker fringes of the d11 coefficient of CBBF. Solid curve, experimental Maker fringe of d11 ; dashed curves, fitted fringes and envelope.
in the measurement of nonlinear optical coefficients. A 1:80 mm thick (110)-cut KDP was prepared as the reference crystal. The experimental fringes are shown in Fig. 9 (black lines), in which the fitted fringes and envelops on the basis of the Maker fringe theory are also included (dashed lines). Figure 9 shows clearly that the theoretical Maker fringes coincide with the experimental curve very well. Through comparison between the fringe envelope of the d11 coefficient of CBBF and that for the d36 coefficient of KDP, the former can be exactly deduced to be d11 ¼ 0:5 pm=V (if d36 ðKDPÞ ¼ 0:39 pm=V is adopted), which is comparable to that of KBBF [4] and RBBF [10].
6. CONCLUSION As a member of the KBBF family, a new UV NLO crystal, CBBF, was successfully grown from the B2 O3 –CsF flux system by spontaneous crystallization with greater size thickness of more than 2 mm for the first time. The structure was
7. 8. 9.
10. 11.
12. 13. 14.
T. Kiss, F. Kanetaka, T. Yokoya, T. Shimojima, K. Kanai, S. Shin, Y. Onuki, T. Togashi, C. Zhang, C. T. Chen, and S. Watanabe, “Photoemission spectroscopic evidence of gap anisotropy in an f -electron superconductor,” Phys. Rev. Lett. 94, 057001 (2005). W. Zhang, G. Liu, L. Zhao, H. Y. Liu, J. Q. Meng, X. L. Dong, W. Lu, J. S. Wen, Z. J. Xu, G. D. Gu, G. L. Wang, Y. Zhu, H. B. Zhang, Y. Zhou, X. Y. Wang, Z. X. Zhao, C. T. Chen, Z. Y. Xu, and X. J. Zhou, “Identification of a new form of electron coupling in the Bi2 Sr2 CaCu2 O8 superconductor by laser-based angle-resolved photoemission spectroscopy,” Phys. Rev. Lett. 100, 107002 (2008). T. Kanai, X. Y. Wang, S. Adachi, S. Watanabe, and C. T. Chen, “Watt-level tunable deep ultraviolet light source by a KBBF prism-coupled device,” Opt. Express 17, 8696–8703 (2009). C. T. Chen, G. L. Wang, X. Y. Wang, and Z. Y. Xu, “Deep-UV nonlinear optical crystal KBe2 BO3 F2 —discovery, growth, optical properties and applications,” Appl. Phys. B 97, 9–25 (2009). T. Togashi, T. Kanai, T. Sekikawa, S. Watanabe, C. T. Chen, C. Zhang, Z. Xu, and J. Wang, “Generation of vacuum-ultraviolet light by an optically contacted, prism-coupled KBe2 BO3 F2 crystal,” Opt. Lett. 28, 254–256 (2003). C. T. Chen, N. Y. J. Lin, J. Jiang, W. R. Zeng, and B. C. Wu, “Computer-assisted search for nonlinear optical crystals,” Adv. Mater. 11, 1071–1078 (1999). Z. S. Lin, Z. Z. Wang, C. T. Chen, S. K. Chen, and M. H. Lee, “Mechanism for linear and nonlinear optical effects in KBe2 BO3 F2 crystal,” Chem. Phys. Lett. 367, 523–527 (2003). X. H. Wen, “Study on the growth and properties of the nonlinear optical crystals: MBBF (M ¼ Na, K, Rb, Cs),” Ph.D. dissertation (Chinese Academy of Sciences, 2006). C. T. Chen, X. H. Wen, R. K. Li, and C. Q. Zhang, “Fluoroberyllium borate nonlinear optical crystals and their growth and applications,” China patent ZL 200510088739.3(CN 100526521C) (2006). C. T. Chen, S. Y. Luo, X. Y. Wang, G. L. Wang, X. H. Wen, H. X. Wu, X. Zhang, and Z. Y. Xu, “Deep UV nonlinear optical crystal: RbBe2 BO3 F2 ,” J. Opt. Soc. Am. B 26, 1519–1525 (2009). I. A. Baydina, V. V. Bakakin, L. P. Bacanova, and N. A. Pal’chik, “X-ray structural study of borato-fluoroberyllates with the composition MBe2 BO3 F2 (M ¼ Na, K, Rb, Cs),” Zh. Strukt. Khim. 16, 963–965 (1975). L. Mei, X. Huang, Y. Wang, Q. Wu, B. Wu, and C. Chen, “Crystal structure of KBe2 BO3 F2 ,” Z. Kristallogr. 210, 93–95 (1995). C. D. McMillen and J. W. Kolis, “Hydrothermal crystal growth of ABe2 BO3 F2 (A ¼ K, Rb, Cs, Tl) NLO crystals,” J. Cryst. Growth 310, 2033–2038 (2008). X. Zhang, X. A. Wang, G. L. Wang, Y. C. Wu, Y. Zhu, and C. T. Chen, “Determination of the nonlinear optical coefficients of the Lix Csð1−xÞ B3 O5 crystals,” J. Opt. Soc. Am. B 24, 2877–2882 (2007).
