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 $300\mathrm{nm}$ 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 ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ (KBBF) is a promising DUV NLO crystal [3, 4, 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 $({\mathrm{Be}}_2{\mathrm{BO}}_3{\mathrm{F}}_2{)}_{\infty }$ network structure, while the $K^{+}$ 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 ${\mathrm{Rb}}^{+}$ and ${\mathrm{Cs}}^{+}$ for $K^{+}$, 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, ${\mathrm{RbBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ (RBBF) and ${\mathrm{CsBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ (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.

2. STRUCTURE

Single-crystal x-ray diffraction data for CBBF was collected at room temperature on a Bruker P4 single-crystal diffractometer with graphite-monochromatic Mo $K\alpha$ radiation ($\lambda =0.71073\AA$). A colorless, transparent crystal with approximate dimensions of $0.1\times 0.1\times 0.2{\mathrm{mm}}^3$ 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 ($a=b=4.4391\AA$ and $c=21.125\AA$) 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 param eters are also given in Table 2.

In the CBBF structure, the major building units are planar ${\mathrm{BO}}_3$ and tetrahedral ${\mathrm{BeO}}_3\mathrm{F}$. The $\mathrm{O}─\mathrm{B}─\mathrm{O}$ bond angle is $120°$ and the $\mathrm{B}─\mathrm{O}$ bond length is $1.37\AA$ in the ${\mathrm{BO}}_3$ group. The distances of $\mathrm{Be}─\mathrm{O}$ and $\mathrm{Be}─\mathrm{F}$ are 1.63 and $1.53\AA$, respectively, in the unit of ${\mathrm{BeO}}_3\mathrm{F}$. The ${\mathrm{BO}}_3$ 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 ${\mathrm{BeO}}_3\mathrm{F}$ groups spatially reciprocal. Each ${\mathrm{BO}}_3$ group connecting two neighboring ${\mathrm{BeO}}_3\mathrm{F}$ groups compose a limitless plane framework of $({\mathrm{Be}}_2{\mathrm{BO}}_3{\mathrm{F}}_2{)}_{\infty }$ along the $a–b$ plane, which is the same as the frameworks of KBBF and RBBF. Infinite layers of $({\mathrm{Be}}_2{\mathrm{BO}}_3{\mathrm{F}}_2{)}_{\infty }$ 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

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 reaction. The initial reactants used for this experiment consisting of ${\mathrm{Cs}}_2{\mathrm{CO}}_3$, BeO, ${\mathrm{NH}}_4{\mathrm{HF}}_2$, and ${\mathrm{H}}_3{\mathrm{BO}}_3$ are all analytically pure. The chemical equation can be expressed as follows:

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 high-temperature flux method. After several self-fluxes were investigated for growing CBBF crystal, finally, the ${\mathrm{B}}_2{\mathrm{O}}_3–\mathrm{CsF}$ 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 $70\mathrm{mm}$ in height and diameter with a cover to reduce vaporizing, then were heated up gradually to $800°\mathrm{C}$, held for at least $50\mathrm{h}$ 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 ($750°\mathrm{C}$) in a day and decreased at a rate of $0.5–2°\mathrm{C}/\text{day}$ to keep the crystals growing. When the final crystallization temperature ($650°\mathrm{C}$) 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 $2.3\mathrm{mm}$ 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 $25°\mathrm{C}$ to $500°\mathrm{C}$, with a scanning rate of $30°\mathrm{C}/\min$ and $10°\mathrm{C}/\min$ from $500°\mathrm{C}$ to $1150°\mathrm{C}$. X-ray 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 $950°\mathrm{C}$, 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 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 $5\times 5\times 2.33{\mathrm{mm}}^3$ 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, 6. It can be seen that the cutoff wavelength on the UV side is located at $151\mathrm{nm}$, while on the IR side it is about $3700\mathrm{nm}$.

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 UV-VIS-IR, Trioptics, Germany). Table 3 lists the measurement results. The Sellmeier equations can be obtained by fitting the refractive index data, as follows:

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 $470\mathrm{nm}$. The measured and calculated phase-matching 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 $201\mathrm{nm}$, 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 $d_{ij}$ coefficients, i.e., $d_{11}$ and $d_{14}$. The matrix form of the coefficients can be written as follows:

Theoretical calculation and experiments both reveal that $d_{14}$ is very small. On the other hand, the effective deff coefficients of CBBF are expressed as follows:

We see that the $d_{14}$ coefficient does not enter into the $d_{\text{eff}}$ coefficients; thus, it is only $d_{11}$ that needs to be determined. This has been precisely measured by the Maker fringes technique with a $10\times 10\times 1.7{\mathrm{mm}}^3$ 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 (Spectra- Physics, 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. ${\mathrm{KH}}_2{\mathrm{PO}}_4$ (KDP) was often used as a standard nonlinear optical crystal in the measurement of nonlinear optical coefficients. A $1.80\mathrm{mm}$ 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 $d_{11}$ coefficient of CBBF and that for the $d_{36}$ coefficient of KDP, the former can be exactly deduced to be $d_{11}=0.5\mathrm{pm}/\mathrm{V}$ (if $d_{36}(\mathrm{KDP})=0.39\mathrm{pm}/\mathrm{V}$ 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 ${\mathrm{B}}_2{\mathrm{O}}_3–\mathrm{CsF}$ flux system by spontaneous crystallization with greater size thickness of more than $2\mathrm{mm}$ for the first time. The structure was 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 param eters, including cutoff wavelength, refractive indices, phase-matching 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).

Table 1. Crystallographic Data and X-ray Rietveld Refinement for CBBF

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Table 2. Atomic Coordinates and Overall Isotropic Thermal Displacement Parameter for CBBF

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Table 3. Measured and Calculated Refractive Indices of CBBF with Δ as the Absolute Value of the Difference between the Measured and Calculated Values.

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Table 4. Phase-matching Angles for Type-I SHG with CBBF ^a

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Fig. 1 Interference pattern of CBBF along the c axis.

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Fig. 2 (a) Structure of CBBF crystal. (b) Planar lattice of $({\mathrm{Be}}_2{\mathrm{BO}}_3{\mathrm{F}}_2{)}_{\infty }$.

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Fig. 3 (a) Single CBBF crystal in relatively regular shape with size $30\times 30\times 2{\mathrm{mm}}^3$. (b) Another cut and polished CBBF crystal with thickness of $2.3\mathrm{mm}$.

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Fig. 4 DSC curve of CBBF crystal.

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Fig. 5 Transmittance of CBBF crystal in the UV region.

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Fig. 6 Transmittance of CBBF crystal in the IR region.

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Fig. 7 Refractive indices dispersion curve. Circles and points, experimental values; curves, fits given by the Sellmeier equation.

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Fig. 8 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.

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Fig. 9 Maker fringes of the $d_{11}$ coefficient of CBBF. Solid curve, experimental Maker fringe of $d_{11}$; dashed curves, fitted fringes and envelope.

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1. 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). [CrossRef]   [PubMed]  

2. 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 ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ superconductor by laser-based angle-resolved photoemission spectroscopy,” Phys. Rev. Lett. 100, 107002 (2008). [CrossRef]   [PubMed]  

3. 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). [CrossRef]   [PubMed]  

4. C. T. Chen, G. L. Wang, X. Y. Wang, and Z. Y. Xu, “Deep-UV nonlinear optical crystal ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$—discovery, growth, optical properties and applications,” Appl. Phys. B 97, 9–25 (2009). [CrossRef]  

5. 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 ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ crystal,” Opt. Lett. 28, 254–256 (2003). [CrossRef]   [PubMed]  

6. 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). [CrossRef]  

7. Z. S. Lin, Z. Z. Wang, C. T. Chen, S. K. Chen, and M. H. Lee, “Mechanism for linear and nonlinear optical effects in ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ crystal,” Chem. Phys. Lett. 367, 523–527 (2003). [CrossRef]  

8. X. H. Wen, “Study on the growth and properties of the nonlinear optical crystals: MBBF ($\mathrm{M}=\mathrm{Na}$, K, Rb, Cs),” Ph.D. dissertation (Chinese Academy of Sciences, 2006).

9. 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).

10. 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: ${\mathrm{RbBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$,” J. Opt. Soc. Am. B 26, 1519–1525 (2009). [CrossRef]  

11. I. A. Baydina, V. V. Bakakin, L. P. Bacanova, and N. A. Pal’chik, “X-ray structural study of borato-fluoroberyllates with the composition ${\mathrm{MBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ ($\mathrm{M}=\mathrm{Na}$, K, Rb, Cs),” Zh. Strukt. Khim. 16, 963–965 (1975). [CrossRef]  

12. L. Mei, X. Huang, Y. Wang, Q. Wu, B. Wu, and C. Chen, “Crystal structure of ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$,” Z. Kristallogr. 210, 93–95 (1995). [CrossRef]  

13. C. D. McMillen and J. W. Kolis, “Hydrothermal crystal growth of ${\mathrm{ABe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ ($\mathrm{A}=\mathrm{K}$, Rb, Cs, Tl) NLO crystals,” J. Cryst. Growth 310, 2033–2038 (2008). [CrossRef]  

14. 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 ${\mathrm{Li}}_x{\mathrm{Cs}}_{(1-x)}{\mathrm{B}}_3{\mathrm{O}}_5$ crystals,” J. Opt. Soc. Am. B 24, 2877–2882 (2007). [CrossRef]