Optical Materials 35 (2013) 2376–2381

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Optical Materials

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Study on optical weak absorption of borate crystals ⇑ Xiaomao Li a,b, Zhanggui Hu a, , Yinchao Yue a, Xuesong Yu c, Zheshuai Lin a, Guochun Zhang a a Key Lab of Functional Crystals and Technology, Technical Institute of Physics and Chemistry, Chinese Academy Sciences, Beijing 100190, China b University of the Chinese Academy of Sciences, Beijing 100049, China c China Center for Information Industry Development, Beijing 100846, China article info abstract

Article history: Borate crystal is an important type of nonlinear optical crystals used in frequency conversion in all-solid- Received 15 March 2013 state . Especially, LiB3O5 (LBO), CsB3O5 (CBO) and CsLiB6O10 (CLBO) are the most advanced. Although Received in revised form 18 June 2013 5À these borate crystals are all constructed by the same anionic group-(B3O7) , they show different nonlin- Accepted 19 June 2013 ear optical properties. In this study, bulk weak absorption values of three borate crystals have been stud- Available online 27 August 2013 ied at 1064 nm by a photothermal common-path interferometer. The bulk weak absorption values of them along [100], [010] and [001] directions were obtained, respectively, to be approximately Keywords: 17.5 ppm cmÀ1, 15 ppm cmÀ1 and 20 ppm cmÀ1 (LBO); 80 ppm cmÀ1, 100 ppm cmÀ1 and 40 ppm cmÀ1 Nonlinear optical crystal (CBO); 600 ppm cmÀ1, 600 ppm cmÀ1 and 150 ppm cmÀ1 (CLBO) at 1064 nm. The results showed an obvi- Borate crystal Weak absorption ous discrepancy of the values of these crystals along three axis directions. A correlation between the bulk weak absorption property and crystal intrinsic structure was then discussed. It is found that the bulk weak absorption values strongly depend on the interstitial area surrounded by the B–O frames. The inter- stitial area is larger, the bulk weak absorption value is higher. Ó 2013 Published by Elsevier B.V.

5À 1. Introduction are constructed by the same anionic group-(B3O7) , which is shown in Fig. 1. Although these crystals are all built up by the same anionic

In nonlinear optical materials, borate crystals, such as LiB3O5 group, they still show different optical properties. Particularly, it is (LBO), CsB3O5 (CBO) and CsLiB6O10 (CLBO), have been known to known that, for CLBO crystal, there is a considerable discrepancy be significant for conversion of all-solid-state lasers. LBO and of its laser-induced damage threshold (LIDT) between [100] and CBO crystals were first discovered by Chen et al. and Wu et al., [001] directions [18]. It results from the significantly different struc- respectively, in 1987 and 1993 [1–3]. Then, CLBO crystal was dis- ture between two directions [19]. The results demonstrated that the covered by Mori et al. in 1995 [4,5]. These borate crystals have intrinsic structure of the crystals is able to make the optical proper- good characters of the nonlinear optical crystals (the wide angular, ties different. The weak absorption is another important optical spectral and temperature bandwidths, a large nonlinear coefficient, property of the nonlinear optical crystals. Its value is able to directly a small walk-off angle and so forth) and a comprehensive applica- reflect the absorption capacity and the sensitivity degree of the crys- tion in green- laser systems [6–11]. Therefore, they have tals for a certain wavelength. This tightly relates to the applications been investigated over the past two decades. of the crystals. Until now, study on the weak absorption of borate LBO and CBO belong to the biaxial optical class and the ortho- crystals has been seldom published except that Nikolov et al. re- rhombic with space group Pn21a and P212121, respectively ported the weak absorption values of LBO crystal in 2011 [20] and [12,13]. CLBO is a tetragonal crystal with space group I-42d. It be- Shanshan Liu et al. from our lab reported that of CBO crystal in longs to the uniaxial optical class. The unit cell parameters of these 2012 [21]. The results reflected the weak absorption property of crystals are as follows: LBO (a = 8.46 Å, b = 7.38 Å, c = 5.13 Å, two crystals merely using for SHG (LBO) and THG (CBO), respec- a = b = c =90°), CBO (a = 6.21 Å, b = 8.521 Å, c = 9.17 Å, a = b = c = tively. Here, we focused on the weak absorption property of the bo- 90°) and CLBO (a = b = 10.494 Å, c = 8.939 Å, a = b = c =90°) [14– rate crystals along different axis directions. We expected to know 16]. In all these borate crystals, the dielectric axes coincide with whether or not there is a relationship between the optical weak the crystallographic axes. Their relationship can be shown by (X, absorption property and the crystalline structure. Y, Z) ? (a, c, b) (LBO), (X, Y, Z) ? (c, a, b) (CBO), (X, Z) ? (a, c) In this work, an investigation of the relationship between the (CLBO), respectively [17]. The basic structures of three crystals weak absorption property and the intrinsic structures of LBO, CBO and CLBO crystals has been carried out. The bulk weak absorp- tion values of three borate crystals along [100], [010] and [001] ⇑ Corresponding author. Tel.: +86 10 82543721. directions were studied at 1064 nm for the first time by a photo- E-mail address: [email protected] (Z. Hu). thermal common-path interferometer. A correlation between the

0925-3467/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.optmat.2013.06.039 X. Li et al. / Optical Materials 35 (2013) 2376–2381 2377

(PCI) manufactured by Stanford photon-solution company in US [22], which includes a probe laser (633 nm), a chopper and a lock-in amplifier. The test procedure is described briefly as follows: First of all, the crystal sample had to be fixed on a 3D motorized translation stage which is in the middle of the PCI system. The pump beam was modulated by a chopper with a frequency of 380 Hz and became a periodic light signal. Furthermore, the light beam focused on a point with the probe beam via several optical components. The power of the pump beam is monitored by a pow- ermeter. The angle between the pump and probe beams is 0.12 rad. The pump beam diameter is approximately 55 lm. Additionally, the sample held by the 3D motorized translation that is controlled by the computer gradually crossed the focus along the pump beam transmission direction. The available probe signals received by a

5À photodetector were filtrated by the lock-in amplifier and then sent Fig. 1. The (B3O7) anionic group. to the computer directly. Finally, the weak absorption values of the samples were acquired by the data being processed by the Table 1 computer. Specimens used in the experiment.

Samples Size (a  b  c) Quantity 3. Results and discussion CLBO 4  4  72 CBO 4  4  6.5 2 CBO 4  4  31The theory of weak absorption has been described in previous LBO 4  4  6.5 3 research [23]. For CLBO crystal, the crystalline structure along [100] direction is completely the same with that along [010] Unit of the sample’s size: mm3. direction (shown in Fig. 3(a and b)). Thus, we only studied the for- mer direction as an example here. The structure along [001] direc- tion is also shown in Fig. 3(c). The bulk weak absorption values at bulk weak absorption value and the crystal intrinsic structure was 1064 nm of CLBO samples in [100] and [001] directions are pre- then discussed. sented in Figs. 4 and 5, respectively. The two peaks reflect the weak absorption of two end surfaces of the sample. It is considered that 2. Experiments they attribute to the tiny defects or contamination on the surfaces. The stable and homogenous area between the two peaks shows the 2.1. Sample preparation bulk weak absorption of the sample. The value in middle of the fig- ure was chosen as the bulk weak absorption value of the sample. In this work, eight LBO, CBO and CLBO specimens were investi- Thereby, the bulk weak absorption values in [100] and [001] gated. These borate crystals were grown by our own laboratory. directions are nearly 600 ppm cmÀ1 and 150 ppm cmÀ1, respec- The specimens of one type crystal were originated from the same tively. The figures clearly show that the bulk weak absorption va- rough crystal grown by the flux method. All of them were pro- lue in [100] direction is four times higher than that in [001] cessed under the same condition. The details of them are given direction. It illustrates that CLBO crystal possesses a much stronger by Table 1. absorption capacity in [100] direction than [001] direction. It can be deduced that the following reasons probably result in such a 2.2. Experimental apparatus and test procedure considerable difference of the bulk weak absorption values be- tween two directions. The apparatus used for the weak absorption investigation is First of all, it is known that a large vacant space formed by two presented in Fig. 2. The apparatus consists of a computer, a pump isolated Cs+ cations and the B–O frames can be easily found from laser (1064 nm) and a photothermal common-path interferometer the projection of CLBO crystal structure onto (100) plane (shown

Fig. 2. Scheme of the PCI equipment. 2378 X. Li et al. / Optical Materials 35 (2013) 2376–2381

Fig. 4. The bulk weak absorption behavior of CLBO crystal along [100] direction.

Fig. 5. The bulk weak absorption behavior of CLBO crystal along [001] direction.

Fig. 6. The bulk weak absorption behavior of CBO crystal along [100] direction.

Fig. 3(c)). The interstitial areas in two directions are theoretically obtained to be approximately 33.5 Å2 and 20.8 Å2 by a crystalline structure model [24]. When a high power pump beam crosses along both directions, the larger interstice is much easier to be ex- panded and distorted. It is attributed to the thermophysical prop- erty of the crystal, such as thermal diffusion, thermal conductivity, photo-thermal coefficient and so forth. Especially, the thermal expansion capacity of CLBO crystal presented in Ref. [25] supports the different degree of the sensitive extension in two directions. Fig. 3. The projections of CLBO crystal structure onto (a) (100) plane, (b) (010) Thus, the weak absorption property which is mainly caused on ba- plane and (c) (001) plane. sis of the photo-thermal effect shows much stronger in [100] direction. Additionally, the huge interstice whose size is nearly in Fig. 3(a)). Comparatively, there is a smaller interstice in [001] three times larger than that of H2O molecule provides enough direction (a vacant space in middle of the diagram shown in spaces for infiltration of these molecules [26]. In this case, the X. Li et al. / Optical Materials 35 (2013) 2376–2381 2379

Fig. 7. The bulk weak absorption behavior of CBO crystal along [010] direction.

Fig. 8. The bulk weak absorption behavior of CBO crystal along [001] direction.

H2O molecules are much easier to be introduced into the crystal along the a-axis rather than the c-axis. It has been studied that these H2O molecules existing in the channel play an important role in increasing the absorption at near IR spectra range [27]. There- fore, we attribute the high weak absorption value to the large interstitial areas in [100] direction. It is obvious that the great dis- crepancy of the bulk weak absorption values of CLBO crystals in two directions mainly result from their totally different crystalline structure. The bulk weak absorption values of CBO samples in three axis directions are shown in Figs. 6–8. It is presented that the values in [100], [010] and [001] directions are approximately 80 ppm cmÀ1, 100 ppm cmÀ1 and 40 ppm cmÀ1, respectively. The highest value is 2.5 times higher than the lowest one. While there is also a difference among these values, the differential is compar- atively lower than that of CLBO samples. The projections of CBO crystal structure onto (100) plane, (010) plane and (001) plane are presented, respectively, in Fig. 9 (a–c). It is found that there are also apparent vacancies, which are similar with the vacant spaces of CLBO crystal, formed by two isolated Cs+ cations and the B–O frames along [100] and [010] directions. Differently, there is a reticular formation along [001] direction. According to the crystalline structure model, it was estimated that the interstitial areas are about 13.5 Å2, 17.2 Å2 and 7.9 Å2 in three directions, respectively. It is evident that the interstitial area in [001] direction is smaller than that in either [100] or [010] direc- Fig. 9. The projections of CBO crystal structure onto (a) (100) plane, (b) (010) plane and (c) (001) plane. tion. According to the thermophysical properties of CBO crystal [28] and the discussion about CLBO crystalline structure above, the reticular formation with the smaller interstice is fairly more lower than that in either a-axis or b-axis. Furthermore, a previous difficult to be expanded and distorted than that happens in other study has demonstrated that the H2O molecules prefer to infiltrate two directions when they are all irradiated by the same pump into CBO crystal along [010] direction [29]. Even though the inter- beam. Thus, the bulk weak absorption value in c-axis is obviously stitial areas are highly approached to each other, the weak 2380 X. Li et al. / Optical Materials 35 (2013) 2376–2381

Fig. 10. The bulk weak absorption behavior of LBO crystal along [100] direction.

Fig. 11. The bulk weak absorption behavior of LBO crystal along [010] direction.

Fig. 12. The bulk weak absorption behavior of LBO crystal along [001] direction. Fig. 13. The projections of LBO crystal structure onto (a) (100) plane, (b) (010) plane and (c) (001) plane. absorption value in [010] direction is still little higher than that in [100] direction. Therefore, the results interpret the peculiar struc- are extremely compact that are like tight reticular formation. + ture of CBO crystal also results in the differentials of bulk weak Although there is a small vacancy formed by two isolated Li cat- absorption values in three axis directions. ions and B–O frames along [001] direction, its interstitial area is The bulk weak absorption values of LBO samples in [100], [010] still small. The interstitial area in three axis directions can also 2 2 2 and [001] directions are about 17.5 ppm cmÀ1, 15 ppm cmÀ1 and be theoretically calculated to be 3.6 Å , 5.8 Å and 7.1 Å , respec- 20 ppm cmÀ1, respectively (shown in Figs. 10–12). Contrary to the tively. Different from the outcome of CLBO and CBO crystals, these results of CLBO and CBO samples, the bulk weak absorption values areas are greatly approached to others. It indicates that the photo- of LBO samples in these directions are almost equivalent. thermal effect made by the pump beam in three axis directions can LBO crystal belongs to a unique spatial arrangement of endless be extremely similar. In addition, there are rarely impurities or helices. The unique crystalline skeleton is extremely compact water molecules, which are considered to be one of the most sig- [30,31]. The projections of LBO crystal structure onto (100) plane, nificant culprits for increasing the weak absorption values, existing (010) plane and (001) plane are presented in Fig. 13(a–c). We can in these directions of the LBO crystal because of its peculiar consid- find that the crystalline structure along [100] and [010] directions erably compact structural pattern. Therefore, it is conceivable that X. Li et al. / Optical Materials 35 (2013) 2376–2381 2381

Table 2 Details of bulk weak absorption values and interstitial area of the borate crystals in three axis directions.

CLBO CBO LBO Axis direction Value Interstitial area Axis direction Value Interstitial area Axis direction Value Interstitial area [100] 600 33.5 [100] 80 13.5 [100] 17.5 3.6 [010] 600 33.5 [010] 100 17.2 [010] 15 5.8 [001] 150 20.8 [001] 40 7.9 [001] 20 7.1

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