Growth and Nonlinear Optical Properties of GdAl3(BO3)4 in a Flux

Article pubs.acs.org/crystal

Growth and Nonlinear Optical Properties of GdAl3(BO3)4 in a Flux without Molybdate Yinchao Yue,† Yangyang Zhu,†,‡ Ying Zhao,† Heng Tu,† and Zhanggui Hu*,† †

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, P. R. China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Single crystals of the nonlinear optical (NLO) material GdAl3(BO3)4 (GdAB) were successfully grown by top-seeded solution growth method in the new flux Al2O3−B2O3−Li2O−NaF. The ultraviolet (UV) cutoff wavelength of GdAB grown from this new flux is approximately 175 nm, which is shorter than 266 nm. Measurement of second-harmonic generation powder suggested that the effect of the as-grown GdAB crystal is 3.5 times greater than that of KDP (KH2PO4), which was unchanged as the crystal grew from a molybdate-based flux. Therefore, the GdAB crystal obtained from the new flux is a promising candidate for NLO materials, which can be used to achieve fourthharmonic generation in the UV region.

the previous flux for growth, the size of the as-grown crystal is approximately 20 mm × 20 mm × 15 mm, and the NLO coefficient of the GdAB crystal is 3.5 times as much as that of KDP; however, GdAB crystals exhibit a near-UV cutoff edge, which limits their potential application in short wavelength because of the incorporation of Mo in the crystals.16 In our previous research, YAB crystal grown from a new flux overcame the drawback of Mo contamination.17,18 On the basis of the previous work, we developed a new flux, namely, Al2O3−B2O3− Li2O−NaF, to address the same issue. The results indicated that GdAB crystals can readily be grown from the Al2O3− B2O3−Li2O−NaF flux. Consequently, UV transmittance spectrum, second-harmonic generation (SHG) powder, and refractive indices were measured on the basis of the grown crystals.

1. INTRODUCTION Lasers in the ultraviolet (UV) region are important for various applications such as photolithography, high-resolution photoelectron spectroscopy, micromachining, and laser prototyping. UV light can be generated via frequency conversion of solidstate IR laser by using a series of nonlinear optical (NLO) crystals. To date, few NLO crystals can realize the fourthharmonic generation (FoHG) of an all-solid-state Nd:YAG laser.1−4 Each of these crystals presents a unique disadvantage. For instance, CLBO crystals demonstrate high hygroscopicity, and KABO crystals display a low NLO coefficient. Hence, an NLO crystal that can realize FoHG should be discovered. Recently, rare-earth aluminum borate crystals (RAl3(BO3)4; R = Y, La−Lu), which were discovered in the 1960s and belong to the carbonate huntite structure in the space group R32, have attracted considerable attention because of their excellent optical properties.5−8 As a member of the RAl3(BO3)4 family, YAl3(BO3)4 (YAB) crystal with UV cutoff wavelength of 165 nm presents a large NLO coefficient, which is four times as much as that of KDP.9 YAB crystals had been used as NLO crystals for FoHG, and high-powered UV laser at 266 nm with conversion efficiency of 12.3% was obtained using this method.10,11 However, YAB crystals with large sizes are difficult to obtain. Thus, a crystal that can realize FoHG and can be grown easily should be discovered. To realize FoHG, the NLO crystal must display a large NLO coefficient and short cutoff wavelength. Similar to Y3+ cation in crystals, Gd3+ can transmit light with wavelengths of 200 nm or even less.12 GdAl3(BO3)4 (GdAB) has been mainly studied as a host for laser applications doped with Nd3+ and Yb3+ ions in the past several years.13 Studies that focused on pure GdAB crystals indicated that the flux system was generally K2Mo3O10-B2O3.14,15 On the basis of © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Flux Selection. The polycrystalline of GdAB was prepared using solid-state reaction techniques. The mixture of the starting materials in different ratios based on the polycrystallinity of GdAB and the composition of the flux system were first ground in an agate mortar and then packed into a platinum crucible. The crucible with the starting materials was located in the center of a single crystal furnace and then heated to 1150 °C. This temperature was held for 3 to 10 h until the starting materials were molten to be transparent and clear. Afterward, the melt was allowed to cool at a rate of 0.1 to 2 °C per day. After the crystals that appeared on the surface of the solution became large enough to be removed from the flux, a platinum spoon was used to remove these crystals from the solution. Each crystal was identified Received: September 9, 2015 Revised: October 29, 2015

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DOI: 10.1021/acs.cgd.5b01304 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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by powder X-ray diffraction (XRD). On the basis of the results, the ratio of GdAB−Al2O3−B2O3−Li2O−NaF = 1:3:7−10:3−5:1−3 was selected to represent the grown flux. 2.2. Growth of Single Crystal. A large single crystal was grown by top-seeded solution growth (TSSG) method in an electric resistance furnace. The mixture of the starting materials of Gd2O3, Al2O3, B2O3, Li2CO3, and NaF was heated in an 80 mm × 70 mm platinum crucible at 1150 °C. The melt was stirred using a platinum blade for 1 day until the melt became transparent. The lid was then fitted on the crucible. The attempted seed was used to determine the exact saturation temperature. Before the 1 mm × 1 mm [0 0 1] direction seed was introduced into the melt, the melt was heated until the temperature was 1 to 3 °C higher than the saturation temperature. Afterward, the melt was allowed to cool at a rate of 0.1 to 2 °C per day. When the growth was completed, the crystal was hung over the surface of the solution. Subsequently, the crystal was brought down to room temperature at a rate of 10 °C per hour. During growth, the seed crystal was rotated at 10 to 40 rpm. 2.3. Measurement of as-Grown Crystal. To verify the crystal structure, X-ray powder diffraction (XRD) was conducted with a Bruker D8 focus diffractometer equipped with Cu Kα radiation (λ = 1.54 Å). Data were collected in the 2θ range of 10° to 90° at room temperature. A 3-mm-thick slab of GAB crystal polished to the optical gradient on both sides was used to measure the UV transmittance spectra by employing a Lambda 900 UV/vis/NIR (Perkin−Elmer) spectrophotometer with a wavelength range of 160 to 220 nm at room temperature. The SHG of the GAB crystal was measured via Kurtz method with microcrystalline KDP powder as reference. The fundamental wavelength was 1064 nm, as generated by a Q-switched Nd:YAG laser. A rectangular prism with a corner angle of 26.9° was cut to measure the refractive index (n) of the GAB crystal by using SpectroMaster UV/vis/IR (Trioptics, Germany) at room temperature.

Figure 2. XRD pattern of GdAB crystal.

measured again in the new flux. The transmittance curve in the UV region is shown in Figure 3, in which the UV cutoff edge of

3. RESULTS AND DISCUSSION 3.1. Crystal Growth. On the basis of the selected system, the GdAB crystal was obtained via TSSG method. The grown large GdAB single crystal was colorless, transparent, and displaying a few defects; this crystal was successfully grown with dimensions of 18 mm × 16 mm × 15 mm from the solution by using Al2O3−B2O3−Li2O−NaF as flux. The grown crystal is shown in Figure 1.

Figure 3. Transmittance spectrum of GdAB crystal.

GdAB is around 175 nm, and the wavelength at which the transmittance drops is approximately 205 nm. A weak absorption band located at around 202 nm is also detected. This phenomenon was not only observed in the GAB crystal but also in another borate NLO crystal, namely, KABO (K2Al2B2O7).18 The presence of the absorption band may be attributed to residual Fe3+ impurities in the growth process. At present, we continue our efforts in solving the incorporation problem of Fe3+. Thus, without Mo6+ ions in the new flux, the cutoff wavelength of the GdAB crystal decreases. As such, this crystal is good enough to work as NLO crystal in the UV region or even deeper. 3.3. Powder Second-Harmonic Generation. The relationship between SHG intensity and particle size in the range of 60 to 175 μm is shown in Figure 4. The SHG intensity initially increased as the particle size of GdAB increased, reaching the maximum when the particle size reached approximately 125 μm, and then became independent of the particle size. In the experimental particle size range, the SHG effect is approximately 2.5 times larger than that of KDP. Therefore, regardless of which flux the crystal is grown, the high SHG efficiency of GdAB is unchanged.13 3.4. Refractive Indices. As a function of wavelength λ, the refractive indices n(λ) of GdAB were measured using the minimum deviation method. The refractive indices n(λ) in the

Figure 1. As-grown GdAB single crystal oriented in [0 0 1].

The powder XRD pattern of the as-grown GdAB crystal is shown in Figure 2. On the basis of the pattern of the GdAB crystal in the Inorganic Crystal Structure Database (number 100831), the as-grown crystal can be determined as a GAB crystal. 3.2. UV Transmittance. Given the incorporation of Mo into the crystals, the UV cutoff wavelength of GdAB grown with a molybdate-based flux is approximately 310 nm.15 This phenomenon hinders the application of GdAB crystals in the UV region. The transmittance spectrum without molybdate was B

DOI: 10.1021/acs.cgd.5b01304 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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agreement with the experimental data (Figure 5). From the Sellmeier equation, the phase-matched angle, as a function of wavelength λ, can be calculated with CASIX computation program and the phase-matched factor in double-frequency output. The results showed that the shortest SHG wavelength is 248 nm, indicating that a laser output of 266 nm can be achieved by the double-frequency phase-matchable NLO material.

4. CONCLUSION A GdAB single crystal was grown for the first time from a flux without molybdate. The transmittance spectrum indicated that the cutoff edge of the as-grown GdAB in UV region is shifted to 175 nm, which is good enough to be a NLO crystal in the UV region. Moreover, the SHG effect is approximately 2.5 times larger than that of KDP in the particle size range of 60 to 175 μm, which is unchanged as the crystal grew from a molybdatebased flux. The refractive indices showed that the GdAB crystal yielded a large value of birefringence. The calculated results indicated that the GAB crystal is phase-matchable; moreover, the shortest SHG wavelength is 248 nm, suggesting that the GdAB crystal can realize a laser output of 266 nm. Overall, the measured properties support the potential application of this crystal as an NLO material in the UV region.

Figure 4. SHG efficiency of GdAB in different sizes.

wavelength range of 175 to 310 nm cannot be obtained from the previous crystals grown in the molybdate-based flux. Given that GdAB belongs to the hexagonal space group R32, all the refractive indices can be obtained by measuring two refractive indices, namely, ne and no. The refractive indices n(λ) of GdAB are displayed in Figure 5. Considering that the value of ne is

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82543721. Fax: +86-10-82543709. E-mail: hu@ mail.ipc.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant Nos. 51202259, 51132005, and 91122023).

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Figure 5. Black circles indicate the experimental data of ne, and black curve indicates the calculated data of ne; red circles indicate the experimental data of no, and red curve indicates the calculated data of no.

smaller than that of no, the GdAB crystal is an optically negative biaxial crystal. In addition, the differential Δn between ne and no is approximately 0.12, which is larger than that in other NLO crystals used to realize FoHG. The dispersion parameters of the refractive index ni were fitted by least-squares method in accordance with Sellmeier equation as follows: ne 2(λ) = 2.88482 +

0.016617 − 0.010365λ 2 λ − 0.012645

no 2(λ) = 3.14309 +

0.020116 − 0.022736λ 2 λ 2 − 0.014776

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where λ is the wavelength in micrometers, and the constants are the Sellmeier parameters. To verify the Sellmeier equation, the differences between the measured data and the calculated values were calculated. The calculated results are in good C

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DOI: 10.1021/acs.cgd.5b01304 Cryst. Growth Des. XXXX, XXX, XXX−XXX