Growth and optical properties of (K0.62Na0.38)2Al2B2O7 crystal

Optical Materials 34 (2012) 1575–1578

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Growth and optical properties of (K0.62Na0.38)2Al2B2O7 crystal Zhenxiong Wu a,b, Yinchao Yue a, Lirong Wang a,b, Guiling Wang a, Zhanggui Hu a,⇑ a

Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 6 November 2011 Received in revised form 21 March 2012 Accepted 24 March 2012 Available online 19 April 2012 Keywords: A1.Top seeded solution growth A2.K2Al2B2O7 B1.Nonlinear optical crystals B2. Fourth harmonic generation

a b s t r a c t Large bulk of (K1 xNax)2Al2B2O7 crystal with reactants of K2CO3 and Na2CO3 in a molar ratio of 1:1 was grown by a top seeded solution growth (TSSG) method. The flux systems for the growth of (K1 xNax)2Al2B2O7 single crystal were investigated. (K1 xNax)2Al2B2O7 single crystal could be easily grown from NaF + LiF flux system with the molar ratio of NaF: LiF = 1:1. The X-ray powder diffraction showed that (K1 xNax)2Al2B2O7 crystal adopted the same structure type of KABO. The molecular formula of the asgrown crystal was (K0.62Na0.38)2Al2B2O7 determined by ICP-AES. The transmittance of (K0.62Na0.38)2Al2B2O7 crystal increased below 300 nm compared with those of fewer amounts of Na+ substitution KABO crystals. The conversion efficiency of the fourth harmonic generation with a specimen 6.74 mm in length reached 20.1% for a picosecond mode-locked Nd:YAG laser. With an input power of 26.8 W, a 353 mW average output power was obtained through fourth harmonic generation of an nanosecond Q-switched Nd:YAG laser. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In 1998, K2Al2B2O7 (KABO) as a new nonlinear optical crystal was reported by Hu et al. [1]. The material crystallizes in the space group P321 (Z = 3) with cell parameters a = b = 8.55800(2) Å, c = 8.45576(3) Å [2]. The nonlinear coefficient d11 of the KABO crystal is 0.45 pm/V, which is larger than the KDP d36 of 0.38 pm/ V [3]. Furthermore, it possesses good radiation resistance to UV light. No evidence of photorefractive effect was observed during the SHG experiment of KABO down to the UV region. b-BaB2O4 (BBO) [4] crystal has a high NLO coefficient, however, it has a large walk-off angle, small angular bandwidth and significant photorefractive effect, which make it unfavorable for generating high output power at 266 nm. CsLiB6O10(CLBO) [5] crystal has a moderate NLO coefficient and good properties but heavy hygroscopicity. Compared with these two crystals, KABO crystal is free of moisture and possesses smaller walk-off and larger acceptance angles than BBO crystals, stable chemical–physical properties and good mechanical properties. All of these merits indicate that KABO may be a competitive candidate for fourth harmonic generation of Nd3+ doped lasers, such as Nd:YAG, Nd:YLF and Nd:YVO4, etc. K2Al2B2O7 (KABO) is a new nonlinear optical crystal capable of laser harmonic generation in the UV range. However, the KABO crystal has a serious problem concerning abnormal absorptions in the 200–300 nm regions that greatly reduces the conversion ⇑ Corresponding author. Tel.: +86 10 82543721; fax: +86 10 82543709. E-mail addresses: [email protected], [email protected] (Z. Hu). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.03.023

efficiency of fourth harmonic generation of Nd-based lasers and 193 nm sum-frequency output and limits its application in spite of its excellent nonlinear properties [6]. In order to improve quality of KABO crystals and study other properties of KABO crystals, many researches about the structure and growth of doped KABO crystals have been carried on. The structure of Na-doped KABO has been studied by Meng et al. [7] and He et al. [8] and large bulk Na-doped KABO crystals have been grown in our previous work [9]. The growth and properties of Fe-doped KABO crystals have been studied by Wang et al. [10,11] In this paper, based on our previous work, we doped more Na+ in the growth system and used new flux system to grow (K1 xNax)2Al2B2O7 crystals in which there were more amounts of Na+ substituting K+ in the crystal lattice. Furthermore, the structure characteristics, optical transmittance have been measured and the generation of 266 nm radiation based on the second harmonic of 532 nm using the as-grown (K1 xNax)2Al2 B2O7 crystal was also reported. 2. Growth of (K1 xNax)2Al2B2O7 crystal

2.1. Flux selection Since KABO decomposes below its melting point, it is necessary to use a suitable flux to grow this crystal. Compared with the application of a variety of growth methods and fluxes reported by some authors [12–19], NaF has been found to be a better flux and has been used to grow a bulk crystal with dimensions of 50  20  17 mm3

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successfully using an improved top-seeded solution growth (TSSG) method. Here we also tried to use flux to grow (K1 xNax)2Al2B2O7 crystal by TSSG method. The polycrystalline (K1 xNax)2Al2B2O7 samples were synthesised from analytical reagent grade K2CO3, Na2CO3, Al2O3, H3BO3 in a proper molar ratio of 1:1:2:4. First NaF flux was investigated for growing (K1 xNax)2Al2B2O7 crystal. The mole ratio of (K1 xNax)2Al2B2O7: NaF in the growth system was 1:2 that was the same as KABO crystal growth. However, the surface of the solution solidified, when the temperature of the solution was decreased to 860 °C. So the (K1 xNax)2Al2B2O7 crystal growth was unsuccessful. Then, the similar experiment was performed using LiF as flux. The result showed that LiF was unfavorable for the growth of (K1 xNax)2Al2B2O7 crystal. Third, NaF + LiF flux system with a molar ratio of (K1 xNax)2Al2B2O7: (NaF + LiF) = 1:2 was adopted to grow (K1 xNax)2Al2B2O7 crystal. When NaF had a lower proportion than LiF in the growth solution, such as NaF: LiF = 1:5, the crystal growth was at relatively low temperature and the crystal was hexagonal morphology using the KABO seed oriented in [0 0 1] direction. When the proportion of NaF was higher than LiF, such as NaF: LiF = 3:2, the solution had a high saturation temperature and the crystal grown along [0 0 1] seed orientation exhibited triangular morphology. The changes of the morphology were due to the polarity of the solution with different proportion of NaF and LiF. Unfortunately, the crystals were very thin and their qualities were also very poor. Finally, when the amount of NaF was the same as LiF in the mixed flux, large bulk of (K1 xNax)2Al2B2O7 crystal could be grown from the solution at an appropriate temperature. 2.2. Top-seed growth of (K1 xNax)2Al2B2O7 crystal KABO crystals were grown using a top-seeded slow cooling technique with a resistance-heated furnace. The starting materials were prepared from analytically pure chemicals of K2CO3, Na2CO3,

Al2O3, H3BO3, NaF, LiF in the molar ratio of 1:1:2:4:2:2, and then were mixed together and ground in an agate mortar. It was added to a platinum crucible with the dimensions of u 90  70 mm3. The platinum crucible was placed inside a preheated (1050 °C) muffle furnace and the powder melted rapidly. The (K1 xNax)2Al2B2O7 compound was synthesised in situ when the solution formed. This process was repeated until there was enough amount of charge in the crucible for crystal growth. When the crucible was enough charged, it was put into a furnace that was similar in design to that reported previously [20] and the temperature was raised to 950 °C at a rate of 20 °C/h. The solution was stirred for 24 h with a platinum stirrer to assure that it was completely homogeneousness. The saturation point was tested repeatedly with seeds. The saturation point was usually determined to be in the range of 745– 755 °C which was much lower than that of KABO crystals grown from NaF flux system. A seed along the [1 1 0] direction which was the best growth direction for KABO crystal [21] was introduced above at a temperature 10 °C higher than the saturation point. Then the temperature was decreased to the saturation point in 30 min in order to melt the surface of the seed crystal. The temperature was kept at the saturation point for 24 h and then decreased at a rate lying in the range of 0.05–0.2 °C/d, and the growing crystal were rotated at a rotation speed between 20 and 50 rpm with inversion at every 90 s. After 30–40 days, the grown crystal was slowly drawn out of the surface of the solution and then cooled down to room temperature at a rate of 20 °C/h. We were able to obtain large bulk of crystal with the dimensions of 70  30  12 mm3 (as shown in Fig. 1a) through doing this process repeatedly. The crack was forming along the upside of the crystal and the fractions missed in the top right corner because the crystal dropped into the crucible when the furnace cooled down to room temperature. However, the quality of the crystal was not very good, macroscopic bulk defects such as cracks and inclusions of flux can be observed by eye.

Fig. 1. Photography of (K0.62Na0.38)2Al2B2O7 crystal: (a) (K0.62Na0.38)2Al2B2O7 crystal was grown from mixed flux with NaF: LiF = 1:1 (mole ratio), (b) transparent portion cut from (K0.62Na0.38)2Al2B2O7 crystal, (c) optically polished crystal specimen with dimension of 3.5  3.2  6.74 mm3.

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3. Results and discussion

Intensity (arb.unit)

K2 Al2 B2 O7

K0.8 Na0.2

The as-grown K1-x Na x

2

K0.6 Na0.4

20

30

40

Al2 B2 O7

Al2 B2 O7 crystal (x=0.38)

K0.4 Na0.6

10

2

50

2

2

Al2 B2 O7

Al2 B2 O7

60

70

Fig. 2. Powder X-ray diffraction patterns of the as-grown (K1 xNax)2Al2B2O7 crystal and of the Polycrystalline K2Al2B2O7, (K0.8Na0.2)2Al2B2O7, (K0.6Na0.4)2Al2B2O7 and (K0.4Na0.6)2Al2B2O7.

80

Transmittance (%)

The structure of (K1 xNax)2Al2B2O7 crystal was determined by Xray powder diffraction using a Bruker D8 Advance X-ray diffractometer equipped with CuKa (Ka1 and Ka2) radiation. The angular range of 2h was 10°–70°, the scanning step width was 0.02°, and the scan rate was 1 sec/step. Polycrystalline K2Al2B2O7, (K0.8Na0.2)2Al2B2O7, (K0.6Na0.4)2Al2B2O7 and (K0.4Na0.6)2Al2B2O7 samples were synthesised by solid state reaction using a mixture of analytically pure K2CO3, Na2CO3, Al2O3, and H3BO3 in the appropriate ratio as starting materials. According to our powder diffraction patterns (as shown in Fig. 2), we can found that as the content of Na+ increases, X-ray diffraction peaks shift towards higher 2h values from pure KABO gradually, which indicated the (K1 xNax)2Al2B2O7 crystal adopted the same structure type of KABO. In their paper [7], they reported that the XRD patterns of (K1 xNax)2Al2B2O7 system revealed that single phase samples with the KABO structure can be prepared up to x = 0.7 and the lattice constants obtained from the refinement of the XRD peak positions monotonically drop till x = 0.8 for the KABO structure, leading to about 10% volume contraction. The patterns of the as-grown (K1 xNax)2Al2B2O7 crystal are very close to that of (K0.6Na0.4)2Al2B2O7, therefore, we could deduce that the amounts of Na+ in the as-grown (K1 xNax)2Al2B2O7 crystal were similar to (K0.6Na0.4)2Al2B2O7. The contents of Na in the as-grown (K1 xNax)2Al2B2O7 crystal were determined by ICP-AES (Varian 710-ES, US). The results were shown as follows: (K1 xNax)2Al2B2O7 (mol): K: Na: Al: B = 0.62:0.38:0.98:0.98 The molecular formula of the crystal could be described as (K0.62Na0.38)2Al2B2O7, which was very close to (K0.6Na0.4)2Al2B2O7, and it was in accord with the conclusion that we drew from Xray powder diffraction. The effective segregation coefficient for Na, which was defined as the ratio of Na content in the crystal to the Na content in the reactant, was measured to be 0.38/ 0.5 = 0.76. Because the growth solution contained Li+, the amounts of Li in the crystal was also measured, and the results showed that only 0.2 mol% of Li+ was doped into the as-grown (K0.62Na0.38)2Al2B2O7 crystal. The contents of Li+ were so little that we ignored the effect of such impurity on the properties of (K0.62Na0.38)2Al2B2O7 crystal. No iron could be detected at ppm level in the crystal by ICP-AES. The transmission spectrum of the as-grown (K0.62Na0.38)2Al2B2O7 crystal was recorded on a Lambda 900UV–VIS–NIR (Perkin– Elmer) spectrophotometer at room temperature with the range of 185–800 nm (as shown in Fig. 3b), and the sample shown in

b 60

40

a 20

0 185200

300

400

500

600

700

800

Wavelength (nm) Fig. 3. Transmittance spectra of the KABO crystal (a) grown from NaF flux and the as-grown (K0.62Na0.38)2Al2B2O7 crystal (b).

Fig. 1b was cut from the as-grown crystal with a thickness of 2.0 mm. In comparison with our previous work [9], the KABO crystal (a) was grown from NaF flux and its molecular formula was (K0.89Na0.11)2Al2B2O7 determined by ICP, it can be seen that the transmittance of (K0.62Na0.38)2Al2B2O7 (b) crystal was improved from 300 nm to 200 nm and the transmittance of the absorption peak at 230 nm was also up to 54.75%. It was clear that Na+ can easily enter into the KABO crystal lattice substituting for K+, and the transmittance of the slice increased especially below 250 nm with large amounts of Na+ doped into the crystal. The absorption of KABO below 300 nm was caused by the Fe3+ impurity occupying tetrahedral sites in the crystal lattice, the higher the content of Fe3+ in the crystal, the more the absorption, and there was no absorption in 200–300 nm of KABO crystal with no Fe3+ impurity [6]. Large amounts of Na+ entering into the KABO crystal lattice leaded to volume contraction [7] and the radius of Fe3+ is larger than that of Al3+, thus the contents of Fe3+ substituting Al3+ in the crystal lattice would decreased. Large amounts of Na+ doped into KABO can decrease the crystal growth temperature more than 50 °C, this may also reduce the contents of Fe3+ substituting Al3+ in the crystal lattice. As a result, the transmittance of (K0.62Na0.38)2Al2B2O7 (b) crystal was higher than that of (K0.89Na0.11)2Al2B2O7 (a) crystal in the range of 200–300 nm. The synergetic effect between Na+ and K+ may be another reason why (K0.62Na0.38)2Al2B2O7 crystal had the higher UV transparency. Therefore, the KABO crystal grown with large amounts of Na+ substitution could reduced the UV absorption, and could promote its applications in generating UV laser output below 300 nm, e.g. through 4HG to 266 nm or SFG to 193 nm. A picosecond mode-locked Nd:YAG laser (PL2140 from Ekspla, Lithuania) was employed in measurements of the NLO properties of the as-grown (K0.62Na0.38)2Al2B2O7 crystal. The laser was operated at a repetition rate of 10 Hz, using LBO for SHG output, with a 25 ps duration and a beam diameter of 1.5 mm. Optically polished crystal specimen with dimension of 3.5  3.2  6.74 mm3 was cut from the as-grown (K0.62Na0.38)2Al2B2O7 crystal (as shown in Fig. 1c), which yielded type-I phase matching with h = 58.1° and u = 0° for optimum fourth harmony generation (FOHG). During the course of the experiment, scattering phenomena from inhomogeneous media was not observed in the inner part of the crystal and the angles of the crystal cut at for this experiment was 2.25° deviation from phase matching angle. Fig. 4 showed the dependency of the efficiency of the 266 nm radiation on the 532 nm input peak power density. When the 532 nm input peak power density was 1.91 GW/cm2, we could obtain the highest conversion efficiency 20.1%, which added

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(K0.62Na0.38 ) 2Al 2B2O7: 3.5*3.2*6.74mm 3 532nm

24

Al2B2O7 crystal has a very good radiation resistance to UV light. We will carry out a higher average power laser experiment using the sample with a cooling device in the next work so as to obtain an even higher output power. The (K1 xNax)2Al2B2O7 crystal with large amounts of Na+ could be easily grown at a lower temperature and the properties of the (K1 xNax)2Al2B2O7 crystal could be improved with the change of the growth environments, therefore, we believed that much higher conversion efficiency could be obtained. This efforts are in progress, and it will promote the application of (K1 xNax)2Al2B2O7 crystal for the FOHG of Nd:YAG lasers.

266nm

Conversion Efficiency (%)

20.1%

20 16 12 8

4. Conclusions

25ps , 10Hz

4 0 0.0

0.5

1.0

1.5

2.0

2.5

532nm Peak-power density (GW/cm2) Fig. 4. Conversion efficiency of 266 nm radiation generation realized in (K0.62Na0.38)2Al2B2O7 crystal as a function of 532 nm input peak power density.

4HG with (K 0.62 Na0.38 ) 2 Al2 B2 O7 (3.5*3.2*6.74mm3 )

400

353mw

266nm Power (mW)

350 300 250 200

Single crystalline (K1 xNax)2Al2B2O7 with the reactants K2CO3 and Na2CO3 in a ratio of 1:1 had been successfully grown using NaF + LiF flux system by the top-seeded solution growth technique. The best composition of the fluxes was NaF and LiF in the molar ratio of 1:1 and we obtained a large bulk of (K1 xNax)2Al2B2O7 crystal with the dimensions of 70  30  12 mm3. The ICP-AES results showed the molecular formula of the as-grown crystal was (K0.62Na0.38)2Al2B2O7, which was in accord with the deduction of X-ray powder diffraction. The transmittance increased significantly below 300 nm with a large amount of Na+ entering into the KABO crystal lattice. FOHG conversion efficiency from 532 to 266 nm reached 20.1% on a specimen 6.74 mm in length for a picosecond mode-locked Nd:YAG laser. Coherent light with an output power of 353 mW at 266 nm was generated using an nanosecond Qswitched Nd:YAG laser system with 10 kHz, 10 ns pulse on the specimen. Expecting to achieve an even higher output power, now we are working hard to use this specimen to carry out a high average power laser experiment with a cooling device.

150

References 100 50 0 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28

532nm Power (W) Fig. 5. Output power at 266 nm from (K0.62Na0.38)2Al2B2O7 crystal versus input power at 532 nm.

the uncoated loss up to 22.1%. This result was improved significantly compared with that were reported by Zhang et al. [17] and Lv et al. [22], since the transmittance of (K0.62Na0.38)2Al2B2O7 (b) crystal increased in the range of 200–300 nm. However, it was lower than that of the KABO crystal which was grown from non-oxygen atmosphere using NaF as flux reported by Liu et al. [23] recently, for the (K0.62Na0.38)2Al2B2O7 crystal just reduced the contents of Fe3+ but could not eliminate iron from the crystal completely. No sign of damage induced by the photorefractive effect was observed during the whole measurement. Then a high average power laser experiment was carried out on the same sample with a Q-switched Nd:YAG laser system at a repetition rate of 10 kHz and 10 ns. The highest output power at 266 nm reached 353 mW when the 532 nm input power was up to 26.8 W (shown in Fig. 5). This result was a highest output for KABO crystal at present. However, the obtained conversion efficiency of 1.3% was rather low, which would be caused by the abnormal UV absorption of the specimen at 266 nm. After this experiment, there was no sign of damage observed by eyes. This means that the (K0.62Na0.38)2

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