Deep UV nonlinear optical crystal:

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1 Chen et al. Vol. 26, No. 8/ August 2009/J. Opt. Soc. Am. B 1519 Deep UV nonlinear optical crystal: RbBe 2 BO 3 F 2 Chuangtian Chen, 1, * Siyang Luo, 1,2 Xiaoyang Wang, 1 Guiling Wang, 1 Xiaohong Wen, 1 Huaxing Wu, 1,2 Xin Zhang, 1,2 and Zuyan Xu 1 1 Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , China 2 Graduate University of the Chinese Academy of Sciences, Beijing , China *Corresponding author: cct@mail.ipc.ac.cn Received March 26, 2009; accepted May 27, 2009; posted March 6, 2009 (Doc. ID ); published July 8, 2009 Sizeable crystals of RbBe 2 BO 3 F 2 (RBBF) were obtained by the flux method. The crystal structure was determined by x-ray data and the space group was proven to be R32, belonging to the uniaxial class. The linear and nonlinear optical parameters, including the cutoff wavelength, refractive indices, phase-matching angles, and effective nonlinear optical coefficients were determined for the first time to our knowledge, and then the Sellmeier equations were also constructed. By using an RBBF prism coupling device (PCD), tunable fourthharmonic output from a Ti:sapphire laser and the sixth harmonic of an Nd-based laser were also obtained with relatively high power Optical Society of America OCIS codes: , INTRODUCTION With developments in semiconductor photolithography, laser micromachining, material processing, as well as super-high-resolution and angle-resolved photoemission spectrometers, the need for coherent light wavelengths below 200 nm has become increasingly urgent over the past decade. Although excimer lasers can emit certain discrete coherent wavelengths in the UV and deep UV spectral regions with high average output power, scientists in this area still need compact and efficient solid-state lasers because of their narrow bandwidth, good beam quality, tunability, and relative ease of handling. It is obvious that the best way to produce deep UV coherent light with solid-state lasers is through cascaded frequency conversion using deep UV nonlinear optical (NLO) crystals. Thus the key point in this important area is to discover suitable high-performance NLO crystals. Up to now only KBe 2 BO 3 F 2 (KBBF) has been able to meet these demands to a certain extent [1]. However, it is well known that this crystal is very difficult to grow because of its strong layer tendency; thus, there is still ample scope for developing new deep UV NLO crystals. Based on the anionic group theory of the NLO effect in crystals [2,3], we know that the NLO properties, birefringence, and band gap in KBBF crystals are mainly determined by the Be 2 BO 3 F 2 n lattice structure, while the K + cation has little effect on the above parameters. As a result, it is conceivable that new deep UV NLO crystals can be discovered through the substitution of Rb + and Cs + for K +, while the basic framework of the KBBF lattice will be retained in the new crystals. With this approach and through systematical experimental investigations, two new NLO crystals, RbBe 2 BO 3 F 2 (RBBF) and CsBe 2 BO 3 F 2 (CBBF), for deep UV harmonic generation have been discovered by our group [4,5]. However, up to now only relatively large bulk crystals of RBBF have been successfully grown, so in this paper, only the basic structure as well as the linear and nonlinear optical properties of the RBBF crystal will be discussed. Our results indicate that RBBF is an excellent deep UV NLO crystal. 2. EXPERIMENTAL Polycrystalline RBBF was prepared by a normal solidstate reaction. The chemical equation is as follows: Rb 2 CO 3 +4BeO+2NH 4 HF 2 +2H 3 BO 3 =2RbBe 2 BO 3 F 2 +CO 2 +5H 2 O +2NH 3. The starting compounds, all analytically pure, were mixed homogeneously in stoichiometric proportions, heated gradually up to 700 C, and kept at that temperature in air for 2 3 days. After cooling to room temperature, the solid product was then ground to powder for the preparation of crystal growth. As an important addition, all of the operations had to be performed in a ventilated system to protect the operators because of the toxicity of BeO. A high-temperature flux method was adopted to grow the single crystal in air using a spontaneous nucleation technique. The flux and RBBF powder were mixed in the appropriate molar ratio and placed in a sealed platinum crucible to prevent the raw materials from volatilizing when heated in a furnace to 850 C for 2 days to ensure complete dissolution of the solute. Afterwards, the temperature was lowered to the saturation temperature and kept constant for 20 h to form the initial spontaneous nucleation seed crystals, then reduced at a rate of C/day to maintain growth. After the required crystal size was reached, the temperature was reduced to room temperature within three days. The crystal was obtained after the residues in the crucible were dissolved by dilute acid /09/ /$ Optical Society of America

2 1520 J. Opt. Soc. Am. B/ Vol. 26, No. 8/ August 2009 Chen et al. Fig. 1. Large single crystal of RBBF with a transparent area greater than mm 2. A Bruker P4 single-crystal diffractometer with monochromatic Mo K radiation = Å was used to determine the structure of the RBBF crystal. The measurement was made at 20±1 C using a high-opticalquality RBBF crystal mm 3 in size. The structure was then solved and refined by using fullmatrix least-squares refinement on F 2 with Shelxl-97 software. The transmittance spectrum of the crystal on the UV side was performed on a spectrophotometer (VUVas2000, McPherson). The transmittance spectrum in the infrared was performed on a spectrophotometer (FTS-60V, Bio- Rad). The refractive indices of the crystal were determined by the minimum deviation angle technique using an RBBF right-angle prism with a precision goniometer spectrometer (SGo1.1, Veb Freiberger Prazisionsmechanik). Details of the measurement of the refractive indices can be found in [6]. The Maker fringes technique was used to determine the d 11 coefficient. A Q-switched Nd:YAG laser (Spectra- Physics, Model Pro 230) at nm with a pulse width of 10 ns and 10 Hz repetition rate was used as the fundamental light source. The second-harmonic signal from the sample crystals was selectively detected by a photomultiplier tube (Hamamatsu, Model R105) and averaged by a fast-gated integrator and boxcar averager (Stanford Research Systems), then recorded. Fig. 2. Interference pattern of RBBF along the c axis. 3. CRYSTAL GROWTH AND STRUCTURE In 1975, Baydina [7] first synthesized the compound, and then recently MCMillen et al. [8] succeeded in growing crystals of mm size using the hydrothermal method. However, so far no optical properties of the crystal have been reported. Experimentally, RBBF crystals can now be grown by both flux and hydrothermal methods. However, the first sizeable crystal was grown in our group by the former method, which is convenient because the crystal decomposes above 900 C before melting at about 1007 C. Figure 1 shows a picture of an as-grown RBBF single crystal of high quality that has a large transparent area greater than mm 2 and a thickness of 1.2 mm. Its synthesis and growth are described in the experimental section. Baydina et al. [7] originally reported the structure of RBBF as being in the C2 space group. After obtaining the single crystal we redetermined the structure on the basis of the x-ray data. Similar to KBBF [9], the space group of RBBF proved to be R32 [point group D3(32)], belonging to the uniaxial class, with lattice constants a=b = Å and c= Å (see Table 1). The space group has been further confirmed by observations of the interference pattern, which shows explicitly uniaxial characteristics (Fig. 2). The basic building units of RBBF are BO 3 3 and BeO 3 F 5 polyhedra. The B O distances of the BO 3 3 Table 1. Crystallographic Data of RBBF a Positional Parameters Atom x y z U eq b Rb (17) F (12) (5) O (6) (6) (5) B (8) Be (2) (7) a Space group, R32 trigonal system, D 3 ; cell parameters, a=b= Å, c = Å, z=3; index ranges, 7 h 7, 7 k 7, 31 l 31; range of, ; no. of observations of peaks with intensities I 2, 323; R indices, R 1 =0.0293, wr 2 =0.0735; largest peak in final difference map, e.a 3. b U eq is defined as one third of the trace of the orthogonalized U ij tensor. Fig. 3. (a) Crystal structure of RBBF. (b) Two-dimensional network structure of Be 2 BO 3 F 2.

3 Chen et al. Vol. 26, No. 8/August 2009/J. Opt. Soc. Am. B 1521 groups in the structure are uniform and equal to 1.37 Å, and the O B O bond angle is exactly 120. The Be O and Be F bond lengths are 1.64 and 1.52 Å, respectively. The B and O atoms are located in the same plane perpendicular to the c axis, while the Be atoms with Be F bonds parallel to the c axis are alternately above and below the plane at a distance of Å. Each BO 3 3 group joins two other adjacent BeO 3 F 5 groups in the same direction to form an infinite lattice sheet of Be 2 BO 3 F 2 along the a b plane, which is the same as the framework of KBBF. The distance between neighboring layers is up to 6.59 Å, but there are only weak Rb F interactions to bind them. Therefore, the structure exhibits a layering tendency along the c axis, which makes it difficult to grow thick crystals there. The structure of RBBF is depicted in Fig. 3. The hardness of RBBF is 2.9 on the standard Mohs hardness scale, which is much softer than BBO 4.0 and LBO 6.0 but harder than KBBF 2.6. The RBBF crystal is highly stable in air and even in hot water at 100 C or in acids such as HNO 3 and HCl. Fig. 4. Fig. 5. Transmittance of RBBF crystal in the UV region. Transmittance of RBBF crystal in the IR region. 4. LINEAR OPTICAL PROPERTIES As shown in Fig. 4, the cutoff wavelength of the crystal on the UV side is located at 160 nm. The transmittance spectrum in the infrared regions is shown in Fig. 5, where we can see that the cutoff wavelength is 3550 nm. By using a right-angle prism with an apex angle of made from a 2.2 mm thick RBBF crystal, nine refractive indices have been measured in the visible region. The data are listed in Table 2. However, these refractive indices are by no means enough to fit the Sellmeier equations of the crystal, because RBBF possesses a wide phase-matching wavelength range in the UV region. It is therefore necessary to use phase-matching angles, in the UV spectral region particularly, combined with refractive index data to fit the Sellmeier equations. Table 3 lists the type I phase-matching angles of the crystal in the wavelength range from deep UV to near infrared. From these data, the Sellmeier equations can be obtained by fitting the refractive indices and type I second-harmonic generation (SHG) phase-matching angles listed in Tables 2 and 3, as follows: n 2 o = is in m. n 2 e = By using these Sellmeier equations, we can calculate the refractive indices of RBBF crystals within an accuracy of four significant figures. Figure 6 and Table 2 show the measured and calculated refractive indices. It can be seen that the theoretical values agree well with the experimental data. The measured and calculated phasematching angles are also shown in Fig. 7 and Table 3, 1 Table 2. Measured and Calculated Refractive Indices of RBBF with as the Absolute Value of the Difference Between the Measured and Calculated Values Wavelength (nm) n e Cal Exp Cal Exp n o

4 1522 J. Opt. Soc. Am. B/ Vol. 26, No. 8/ August 2009 Chen et al. Table 3. Phase-matching Angles for Type I SHG with RBBF a Phase-matching angle (deg) Fundamental Wavelength (nm) SHG Wavelength (nm) Exp Cal a Exp, measured angles; Cal, angle calculated using the Sellmeier equations;, difference between the measured and calculated values.

5 Chen et al. Vol. 26, No. 8/August 2009/J. Opt. Soc. Am. B 1523 Fig. 6. Dispersion of refractive indices. The triangles are experimental data. The curves are calculated from the Sellmeier equations (1). Fig. 9. Maker fringes of the d 11 coefficient of RBBF. Solid curve, experimental Maker fringe (type-i) of d 11 ; dashed curves, theoretical fringe and theoretical envelope. d11 d11 0 d d 14 d Theoretical and experiment calculation both reveal that d 14 is very small. On the other hand, the effective d eff coefficients of RBBF are as follows: d 11 cos cos 3 type-i, d 11 cos 2 sin 3 type-ii. 3 Fig. 7. Type I SHG phase-matching angles versus fundamental wavelength for RBBF in the whole spectral region. Solid line, curve calculated from the Sellmeier equations; circles, data from the experiments. which indicate that it is possible to achieve SHG phasematching down to 170 nm. Thus RBBF has a wide phasematching range, particularly in the deep UV range. 5. NONLINEAR OPTICAL PROPERTIES Similar to KBBF in the space group R32, RBBF 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: We can see that the d 14 coefficient does not contribute to the d 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 mm 3 c-cut crystal plate (the arrangement of the axes is shown in Fig. 8). Figure 9 shows the Maker fringes, where the dashed curves represent the theoretical fringes and envelope based on the refractive indices calculated from the Sellmeier equations [Eq.(1)]. 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 RBBF and that for the d 36 coefficient of KDP as a reference, for the former we can deduce exactly that d 11 = 0.45±0.01 pm/v (if d 36 KDP =0.39 pm/v is adopted), which is comparable to that of KBBF [10]. As with KBBF [11], the as-grown RBBF is still too thin to be cut along the phase-matching direction for producing deep UV harmonic generation below 200 nm. To solve this problem, we also adopt the special prism coupling device RBBF-PCD [12]. Figure 10 shows this sandwich structure in which the interfaces between the fused silica Fig. 8. Arrangement of the sample axes for the determination of the d 11 coefficient of RBBF. Fig. 10. RBBF. Schematic of the special prism coupling device with

6 1524 J. Opt. Soc. Am. B/ Vol. 26, No. 8/ August 2009 Chen et al. NLO crystal that can produce deep UV harmonic generation below 200 nm through a simple SHG method. The sixth harmonic of an Nd-based laser can also be produced using an RBBF-PCD device. For example, with a nanosecond, 10 khz, 355 nm laser, a maximum output power of 10.8 mw at nm has been obtained recently using an RBBF-PCD device composed of fused silica and CaF 2 prisms cut both at an angle of 75 and a crystal of dimensions mm 3. Figure 12 shows the output power curves at nm as a function of the fundamental wavelength power. Fig. 11. Tunable fourth-harmonic generation of a Ti:sapphire laser versus fundamental wavelength with an RBBF-PCD device. Squares, output power of SHG generated through BBO; dots, output power of fourth-harmonic generation produced by RBBF-PCD. (or calcium fluoride crystal) and RBBF are totally optically contacted. Using this RBBF-PCD device, tunable fourth-harmonic generation of Ti:sapphire lasers has been successfully realized. In the experiment, a femtosecond Ti:sapphire laser (Chameleon-ultra II, Physicalspectral, 150 fs, 80 MHz) is used for the fundamental wavelengths. One BBO crystal produces the SHG of the tunable fundamental wavelengths from 930 to 720 nm. Then, an RBBF-PCD with crystal dimensions of mm 3 is used to produce fourth-harmonic generation over the entire SHG wavelength range. Figure 11 shows the tunable fourth- and second-harmonic power output curves as a function of fundamental wavelengths. Within the tunable deep UV range, the maximum power output is 44.1 mw at nm when the relative SHG power at 405 nm is 2.08 W. From 185 to 200 nm, the power output can be maintained at over 12 mw. Therefore, in addition to KBBF, RBBF is currently another 6. CONCLUSION In this paper we report, to our knowledge, a novel deep UV NLO crystal RBBF, which can produce harmonic generation below 200 nm through a simple SHG method. Thermal analysis indicates that RBBF is an incongruent melting compound, so the crystal can be grown by the flux method. Bulk crystal as large as mm 3 (1.2 mm along the c axis) can now be successfully grown. The x-ray data show that RBBF has the same space structure as KBBF. The linear and nonlinear optical parameters, including the cutoff wavelength, refractive indices, phase-matching angles, and the effective NLO coefficients have been determined for the first time to our knowledge, from which the Sellmeier equations have also been constructed. By using an RBBF-PCD device, tunable fourthharmonic output from a Ti:sapphire laser and the sixth harmonic of an Nd-based laser have also been obtained with relatively high power. These data show that RBBF is an excellent deep UV NLO crystal. Further efforts to grow larger bulk crystals and obtain even higher output powers are underway. ACKNOWLEDGMENTS This work was supported by the State Key Program for Basic Research of China grant 2004CB and the National Natural Science Foundation of China (NSFC) grant Fig nm output power as a function of 355 nm fundamental power with an RBBF-PCD. REFERENCES 1. C. T. Chen, Z. Y. Xu, D. Q. Deng, J. Zhang, and G. K. L. Wong, The vacuum ultraviolet phase-matching characteristics of nonlinear optical KBe 2 BO 3 F 2 crystal, Appl. Phys. Lett. 68, (1996). 2. 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. (Weinheim, Ger.) 11, (1999). 3. Z. S. Lin, Z. Z. W, C. T. Chen, S. K. Chen, and M. H. Lee, Mechanism for linear and nonlinear optical effects in KBe 2 BO 3 F 2 crystal, Chem. Phys. Lett. 367, (2003). 4. X. H. Wen, Ph.D. Dissertation (Institute of Physics and Chemistry, Chinese Academy of Sciences, 2006), China. 5. C. T. Chen, X. H. Wen, R. K. Li, and C. Q. Zhang, China Patent CN A, C. T. Chen, G. L. Wang, X. Y. Wang, Y. Zhu, Z. Y. Xu, T. Kanai, and S. Watanabe, Improved Sellmeier equations and phase-matching characteristics in deep-ultraviolet region of KBe 2 BO 3 F 2 crystal, IEEE J. Quantum Electron. 44, (2008). 7. I. A. Baydina, V. V. Bakakin, L. P. Bacanova, and N. A. Pal chik, X-ray structural study of borato-fluoroberyllates

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