BaCaBO3F: A nonlinear optical crystal investigated for UV light generation

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2508–2512

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BaCaBO3F: A nonlinear optical crystal investigated for UV light generation K. Xu , P. Loiseau, G. Aka Laboratoire de Chimie de la Matie`re Condense´e de Paris, CNRS-UMR 7574, Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France

a r t i c l e in f o

a b s t r a c t

Article history: Received 15 May 2008 Received in revised form 11 December 2008 Accepted 22 January 2009 Communicated by V. Fratello Available online 1 February 2009

A single crystal of BaCaBO3F (BCBF) has been grown by the Czochralski pulling method in air. This compound was investigated for nonlinear optical applications especially in the ultraviolet. This BCBF material has a hexagonal structure with P6¯2m space group (Z=3) [D.A. Keszler, A. Akella, K.I. Schaffers, T. Alekel, Mater. Res. Soc. Symp. Proc. 329 (1994) 15]: its cell parameters are a=9.0489(8) A˚ and c=4.3257(4) A˚. Its basic structural unit is [BO3]3 [D.A. Keszler, A. Akella, K.I. Schaffers, T. Alekel, Mater. Res. Soc. Symp. Proc. 329 (1994) 15] group. The UV absorption edge was measured at 210 nm. The refractive indices were measured by the minimum deviation technique and fitted to the Sellmeier equations. In this article, second harmonic generation (SHG) and third harmonic generation (THG) phase-matching angle calculations are discussed. & 2009 Elsevier B.V. All rights reserved.

PACS: 81.10.Fq 42.70.Mp 42.65.Kg Keywords: A2. Czochralski B1. Barium calcium borate fluoride B2. Nonlinear optic materials B2. Second harmonic generation B2. Third harmonic generation

1. Introduction 1.1. Interests in the research of UV (ultraviolet) lasers The photon energy in the UV region is sufficient to induce bond-breaking in many materials. In addition, the UV light can be tightly focused. So, UV laser light is considered as a clean energy source for the synthesis and processing of materials: micromachining, microdrilling, surface cleaning and stripping of plastics, ceramics and metals. Until now, high-power UV output was only obtained using raregas halide excimer lasers (e.g. XeCl, KrF and ArF). However, these lasers present some disadvantages: they have large dimensions, use toxic gases, possess high-voltage gaseous discharges and need regular maintenance. Therefore, it is necessary to substitute compact solid-state lasers for rare-gas halide excimer lasers.

1.2. Obtaining of solid-state UV lasers by means of nonlinear optics In order to generate UV light, cascade frequency conversion from an IR laser using nonlinear optical (NLO) crystals is an Corresponding author. Tel.: +33 1 53 73 79 24; fax: +33 1 46 34 74 89.

E-mail address: [email protected] (K. Xu). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.126

effective technique. For example, the fundamental wavelength of Nd: YAG (Y3Al5O12) lasers (l=1064 nm) is converted to visible light 2o at l=532 nm by second harmonic generation (SHG), and then to UV wavelengths 3o, 4o, 5oy at l=355, 266, 213 nmy by third, fourth, fifthy harmonic generations, respectively. The performance of high-power solid-state UV lasers depends on NLO crystals employed for frequency conversion. Most of the NLO crystals currently used for UV light generation are borate crystals because B–O bonds have relatively high resistance against laserinduced damage and high transparency in the UV region. Many borate crystals have been studied, for instance: BBO (b-BaB2O4) [2], LBO (LiB3O5) [3], CBO (CsB3O5) [4], CLBO (CsLiB6O10) [5], KBBF (KBe2BO3F2) [6], SBBO (SrBe2BO7) [7], KAB (K2Al2B2O7) [8], LB4 (Li2B4O7) [9] and so on. Four main basic structure units are encountered in these kinds [B3O7]5 [BO3]3 and [BO4]5. According to of crystals: [B3O6]3 , , the ‘‘anionic group theory’’ [10], which introduces a relationship between the macroscopic optical properties and microscopic structural characteristics of borate crystals, the [B3O6]3 anionic group has a larger second-order susceptibility w(2) than the three other borate groups. As regards the NLO effective coefficients, the [B3O6]3 group is the most suitable for the construction of a NLO material. But crystals based on [B3O6]3 groups (e.g. BBO [2]) are characterized by a UV absorption edge at a longer wavelength and a greater walk-off angle, which is due to the perfect planar

ARTICLE IN PRESS K. Xu et al. / Journal of Crystal Growth 311 (2009) 2508–2512

2. Experimental section and results 2.1. Synthesis and crystal growth The starting materials used for this experiment consist of BaCO3 (499.9%), CaCO3 (99.95%), B2O3 (99.98%) and BaF2 (99%). The two-step solid-state reaction proceeds as follows: BaCO3 þ 2CaCO3 þ B2 O3 ¼ BaCa2 ðBO3 Þ2 þ 3CO2 m BaCa2 ðBO3 Þ2 þ BaF2 ¼ 2BaCaBO3 F In the first step, polycrystalline samples of BaCa2(BO3)2 were prepared by mixing together stoichiometric ratios of raw materials BaCO3, CaCO3 and B2O3 and heating at 1000 1C for 18 h. In the second step, BaF2 was added to the BaCa2(BO3)2 powder: this mixture was compressed into rod, and then the rod was sintered at 1000 1C for 12 h in air. The polycrystalline charge obtained was examined by X-ray powder diffraction, which shows that a single BCBF phase was formed. The thermal behaviour of BCBF (Fig. 1) was characterized by DTA using a NETZSCH TGDTA 413 system. BCBF was heated in a platinum crucible, in argon, from 22 to 1400 1C at a rate of 10 1C/min, and cooled down to room temperature at a rate of 10 1C/min. Upon heating, the DTA curve shows a single

heating

120 60 DTA (μv)

hexagonal structure of the [B3O6]3 structural unit. The LB4 crystal, which is based on the [BO4]5 group, has the shortest UV absorption edge. However, its NLO effective coefficient is the weakest [9]. So, it seems that the compromises between the UV absorption edge and NLO effective coefficient are found for both [B3O7]5 and [BO3]3 anionic groups. The LBO, CBO and CLBO materials composed of the [B3O7]5 basic structural unit, show interesting properties only for UV light generation, but not for deep UV application since their SHG cut-offs are located at 277, 273 and 237 nm, respectively. In fact, three-dimensional endless (B3O7)n-N networks are not preferred for producing large birefringence. Thus, it is small birefringence that leads to shifting the SHG cut-off towards the red. Finally, the [BO3]3 anionic group is the most suitable basic structural unit of NLO crystals for UV and deep UV light generation. In order to possess a relatively large birefringence and high dij coefficients, a borate crystal must have coplanar and dense [BO3]3 groups in its structure. Borate crystals containing the [BO3]3 anionic group such as KBBF and SBBO tend to grow in a layered structure. So these crystals are difficult to grow and are mechanically fragile. In addition, Be is very toxic. This is why KAB, which possesses a similar structure to KBBF and SBBO, was developed. KAB has better mechanical properties than the two previous materials. Until now, this material was considered as the best choice for UV light generation at 193 nm. However, the growth of large KAB crystals remains challenging to perform. Therefore, we would like to research a new nonlinear optical crystal composed of [BO3]3 groups, whose crystal growth is much easier to carry out. In 1994, Keszler [1] first published that BCBF has a hexagonal structure with P6¯2m space group (Z=3). Cell parameters of BCBF are: a=9.0489(8) A˚ and c=4.3257(4) A˚. Its basic structural unit is [BO3]3. Then, Schaffers et al. [11] grew a single crystal of Yb3+: BaCaBO3F by the Czochralski method and reported its spectroscopic properties. Recently, Zhang et al. [12,13] studied the crystal growth of BCBF and characterized the linear and nonlinear optical properties of this material. BCBF crystals with large size were grown by the Kyropoulos method, but these crystals suffer from inclusions. In this paper, the BCBF crystal is grown by the Czochralski method [14]. Crystal quality and optical properties are compared with previous work [12,13].

2509

0 -60 -120

cooling

-180 500

600

700

800 900 1000 1100 1200 1300 1400 Temperature (°C)

Fig. 1. DTA of the as-grown BaCaBO3F single crystal (heating and cooling at 10 1C/min).

Fig. 2. Photograph of BCBF crystals grown by the Czochralski pulling method.

endothermic peak located at 1096 1C, which is attributed to the congruent melting of BCBF. During the cooling process, there is only a single exothermic peak at 1028 1C related to the crystallisation of this compound. The crystal growth of BCBF was carried out by the Czochralski pulling method in air. A Pt wire was used as seed. The growth rate was 0.2 mm/h with rotation rate of 20 rpm. Single crystals were grown with sizes up to diameter=14 mm and length=35 mm (Fig. 2). The as-grown crystals were of very high optical quality without inclusions, which shows that the BCBF single-crystal growth by the Czochralski method gives way to better crystal quality in comparison with the Kyropoulos method. The measured Mohs hardness of BCBF is 5.

2.2. Structure By using a Siemens D5000 diffractometer with Co Ka radiation, the crystal, grown by the CZ method, was identified as pure BCBF by X-ray powder diffraction (XRD) and its pattern was refined by the Rietveld method. The observed, calculated and difference plots of the X-ray diffraction data are shown in Fig. 3. Table 1 shows the data collection and the refinement conditions for the Rietveld analysis.

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6000 Yobs Ycal Yobs-Ycal Positions of the Bragg reflections

Intensity

4000

2000

Table 2 Atomic coordinates and overall isotropic thermal displacement parameter for BCBF. Atom

Site

X

Y

Z

B (A˚2)

Ba Ca B1 B2 O1 O2 F

3g 3f 2c 1a 6j 6i 3g

0.2870(1) 0.6132(4) 0.6667 0 0.5048(7) 0.140(1) 0.659(1)

0 0 0.3333 0 0.1909(8) 0 0.057(2)

0.5 0 0 0 0 0.135(2) 0.5

0.715(2) 0.715(2) 0.715(2) 0.715(2) 0.715(2) 0.715(2) 0.715(2)

0 Table 3 Interatomic distances (A˚) in BCBF.

-2000 40

80

120

2θ (°) Fig. 3. Observed, calculated X-ray powder diffraction patterns, and difference profiles for the prepared single crystal BaCaBO3F. (lKa1(Co)E1.789 A˚; lKa2(Co)E1.792 A˚)

Table 1 Powder X-ray Rietveld refinement for BCBF. Chemical formula Formula weight Space group a (A˚) c (A˚) Z Dcalc (g/cm3) X-ray radiation Monochromator 2y range (1) Step width (1) No. of points Counting time (s per step) Temperature (K) No. of reflections No. of refined parameters

Reliability factors RWP (%) RP (%) RBragg (%) RF (%)

w2

BaCaBO3F 255.214 P6¯2m 9.0515(1) 4.3263(6) 3 4.15 Co Ka Graphite 11.99–131.00 0.01 11901 26.5 293 192 22

15.4 11.9 6.4 4.7 2.3

The experimental results of lattice parameters (a=9.052 A˚, c=4.326 A˚) are in good agreement with the cell parameters published by Keszler (a=9.049 A˚, c=4.326 A˚) [1]. The refinement gives reliability factors of RBragg=6.4 and RF=4.7 with w2=2.3. The refined atomic coordinates and interatomic distances are summarized in Tables 2 and 3. For the refinement, we used an overall isotropic displacement parameter B for all the elements since the quality of the X-ray diffraction data was not good enough to reach satisfactory values of individual B. Nevertheless, attempts to refine individual B factors revealed that fluorine vacancies could be formed because of an abnormal increase of its B value. Fig. 4 shows the crystal structure of BCBF and the coordination environments of Ba and Ca atoms in BCBF calculated from our structural refinement. Ba is surrounded by 3 fluorine and 8 oxygen atoms. The average Ba–O and Ba–F distances are, respectively, 2.80 and 3.09 A˚. The immediate neighbourhood of Ca is composed of 5 oxygen (average Ca–O distance: 2.41 A˚) and 2 fluorine atoms (average Ca–F distance: 2.22 A˚).

Ba–O(1)  2 Ba–O(1)  2 Ba–O(2)  4 Ba–F Ba–F Ba–F

2.854(3) 2.854(5) 2.749(9) 3.14(1) 3.39(1) 2.73(1)

Ca–O(1)  2 Ca–O(1)  2 Ca–O(2) Ca–F  2

2.376(6) 2.486(6) 2.31(1) 2.15(2)

B(1)–O(1)  3

1.386(8)

B(2)–O(2)  3

1.393(6)

There are only [BO3]3 groups in this material with a mean B–O distance of 1.39 A˚. The relative orientations of these [BO3]3 groups in the (0 0 1) plane are presented in Fig. 5. This arrangement of [BO3]3 groups is favourable for producing a moderate to large birefringence [15]. 2.3. Ultraviolet spectrum measurement The optical transmission spectrum of BCBF was measured at room temperature using a Cary/5E/UV–vis–NIR (Varian) spectrophotometer. The UV absorption edge is at 210 nm (Fig. 6), which is slightly smaller than the result obtained (220 nm) in previous work [12,13]. This edge is unexpectedly high as BCBF is built from [BO3]3 group and contains fluorine, which should lead to a deep transparency in the UV below 190 nm. This feature is probably due to the presence of defects in the prepared crystal. 2.4. Refractive index measurement By using the minimum deviation method, refractive indices were measured at ambient temperature in the wavelength range between 266 and 1064 nm with a prism cut (apex angle=59.691) and oriented from the CZ boule. BCBF is a negative uniaxial optical crystal [11,13]. The birefringence for light at 589 nm is 0.05. Sellmeier equations determined from measurements (Table 4) are shown in Eq. (1). n2o ¼ 2:71455 þ n2e ¼ 2:55994 þ

0:01871

l2  0:01730 0:01599 2

l  0:01752

2

 0:00906l

2

 0:00528l

(1)

ARTICLE IN PRESS K. Xu et al. / Journal of Crystal Growth 311 (2009) 2508–2512

2511

Ba Ca

Ba

O F B Ca

Ba site

Ca site

Fig. 4. (a) Crystal structure of BCBF and (b) polyhedral coordination of Ba and Ca atoms in BCBF.

Transmittance (%)

20

15

10

5

Fig. 5. [BO3]3- groups of BaCaBO3F in the (0 0 1) planes.

0 The Sellmeier coefficients for BCBF crystals grown by the Czochralski method are almost the same as the results of BCBF crystals grown by the Kyropoulos method reported in [12,13]. However, our results are more reliable in the UV range because the refractive indices were examined not only in the visible range but also in the UV range (355 and 266 nm).

2.5. Calculation of phase-matching angle for SHG and THG From the Sellmeier equations, the limits of types I and II PM wavelengths for SHG are 622 and 855 nm respectively (Fig. 7). The calculated type I and type II PM angles based on the refractive index data for SHG of Nd:YAG laser @ 1064 nm are, respectively, 36.91 and 56.01. From the expression for deff (deff=d22 cos y sin 3F) in uniaxial crystals of 62 m point group, the best efficiency for SHG is expected for F=301. From the Sellmeier equations given in Eq. (1), the calculation shows that BCBF can achieve third harmonic generation (THG) by sum frequency generation (1064+532-355 nm). The shortest

200

250 300 Wavelength (nm)

350

Fig. 6. Transmission spectrum of the prepared BCBF crystal.

fundamental wavelength is about 887 nm for type I (ooe) THG configuration. For type II THG configuration, these fundamental wavelengths are about 1050 nm (eoe) and 1549 nm (oee) (Fig. 8). Fig. 8 indicates that the PM angles for type I and II (eoe) THG at 1064 nm are y=55.51 and 80.51, respectively.

3. Conclusion and perspective A single crystal of BaCaBO3F was grown by the Czochralski method from a congruent melt in air. The crystals obtained are chemically stable and are not hygroscopic. The structure of BCBF was identified by XRD and its refinement was performed with the Rietveld method. According to the results of the refinement,

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Table 4 Refractive indices of the BCBF crystal. no (exp.)

no (calc.)

ne (exp.)

ne (calc.)

0.2660 0.3547 0.4358 0.4678 0.4811 0.5086 0.5461 0.5780 0.6439 0.7376 0.7453 0.7869 0.8252 0.8761 0.9271 0.9447 1.0640

1.7504 1.6988 1.6795 1..6750 1.6732 1.6703 1.6669 1.6646 1.6608 1.6568 1.6566 1.6551 1.6542 1.6531 1.6519 1.6517 1.6496

1.7504 1.6988 1.6796 1.6749 1.6733 1.6703 1.6669 1.6645 1.6607 1.6569 1.6566 1.6553 1.6543 1.6530 1.6520 1.6516 1.6496

1.6911 1.6454 1.6283 1.6242 1.6228 1.6202 1.6171 1.6151 1.6118 1.6084 1.6084 1.6073 1.6064 1.6056 1.6046 1.6042 1.6024

1.6911 1.6453 1.6284 1.6243 1.6228 1.6202 1.6172 1.6151 1.6118 1.6086 1.6083 1.6073 1.6064 1.6054 1.6045 1.6042 1.6026

3000 Type I Type II

2500

Fundamental λ (nm)

l (mm)

Fundamental λ (nm)

Type I-ooe Type II-eoe Type II-oee

2500

2000

1500

1000

500 30

40

50

60 θ (°)

70

80

90

Fig. 8. Phase-matching angle for type I and II THG as a function of the wavelength of the fundamental wave.

performance of Yb3+: BCBF [12], however no self frequency doubling process is reported. Therefore, Yb or Nd: BCBF could be promising materials for practical applications, able to generate visible laser light by a self frequency doubling process with only one single crystal.

2000

Acknowledgement 1500

This work was supported by the National Center for Scientific Research of France.

1000

References

500 20

30

40

50

60

70

80

90

θ (°) Fig. 7. Phase-matching angle for type I and II SHG as a function of the wavelength of the fundamental wave.

fluorine vacancies are probably formed in the structure of BCBF. So, single-crystal X-ray diffraction will be carried out in order to confirm this supposition. Its UV absorption edge occurs at 210 nm. Type I and II PM angles based on the Sellmeier equations for SHG of Nd:YAG laser are 36.91 and 56.01, respectively. According to these calculations, BCBF allows UV generation at 355 nm by THG of a Nd:YAG laser operating at 1064 nm. The phase-matching angle is 55.51 for type I and 80.51 for type II. Its effective NLO coefficient deff for type I SHG of Nd:YAG laser is 0.23 pm/V [13]. In addition, BCBF presents a suitable accommodation site for Yb3+ or Nd3+ ions. Schaffers et al. have investigated the infrared laser

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