Growth and optical properties of a new nonlinear ... - OSA Publishing

Growth and optical properties of a new nonlinear Na3La9O3(BO3)8 crystal Jianxiu Zhang, Guiling Wang, Zuoliang Liu, Lirong Wang, Guochun Zhang, Xin Zhang, Yang Wu, Peizhen Fu, and Yicheng Wu* Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China *[email protected]

Abstract: New nonlinear crystals Na3La9O3(BO3)8 (abbreviated as NLBO) with desired morphologies, high quality and weight exceeding 40g have been grown along different directions, such as [001], [110], and [100], by top-seeded solution growth(TSSG) method. The refractive indices were accurately measured over the full transmission range, and the second-order nonlinear optical coefficients were determined by the Maker fringe technique. The optimal phase-matching (PM) conditions and the corresponding effective nonlinear coefficient were calculated for second harmonic generation (SHG) at different wavelengths. In order to confirm the correctness of our calculation, we also performed the SHG experiments under 1064 and 800 nm pumping, respectively. In addition, we directly compared the SHG performance of NLBO with that of LBO under the same experimental conditions with the 1064 nm pumping source. As the results, a conversion efficiency of 58.3% at 532 nm was obtained for NLBO, and whereas only 21.5% was obtained for LBO, indicating that NLBO is a highly attractive nonlinear material for frequency conversion of pulses into the visible and ultraviolet. ©2009 Optical Society of America OCIS codes: (160.4430) Nonlinear optical crystals; (190.2620) Frequency conversion.

References and links 1. 2. 3. 4. 5. 6. 7. 8.

D. Cyranoski, “Materials science: China’s crystal cache,” Nature 457(7232), 953–955 (2009). C. Chen, Y. Wang, B. Wu, K. Wu, W. Zeng, and L. Yu, “Design and synthesis of an ultraviolet-transparent nonlinear optical crystal Sr2Be2B2O7,” Nature 373(6512), 322–324 (1995). Y. C. Wu, G. C. Zhang, P. Z. Fu, C. T. Chen, Chinese Patent, Application No., 01134393.1, November 2, 2001, Publication No. CN052I010563. P. Gravereau, J. P. Chaminade, S. Pechev, V. Nikolov, D. Ivanova, and P. Peshev, “Na3La9O3(BO3)8, a new oxyborate in the ternary system Na2O_La2O3_B2O3: preparation and crystal structure,” Solid State Sci. 4(7), 993–998 (2002). G. Zhang, Y. Wu, Y. Li, F. Chang, S. Pan, P. Fu, and C. Chen, “Flux growth and characterization of a new oxyborate crystal Na3La9O3(BO3)8,” J. Cryst. Growth 275(1-2), e1997–e2001 (2005). Y. Li, Y. Wu, G. Zhang, P. Fu, and X. Bai, “Flux growth and optical properties of Na3La9O3(BO3)8 crystals,” J. Cryst. Growth 292(2), 468–471 (2006). C. Cascales, R. Balda, V. Jubera, J. P. Chaminade, and J. Fernández, “Optical spectroscopic study of Eu3+ crystal field sites in Na3La9O3(BO3)8 crystal,” Opt. Express 16(4), 2653–2662 (2008). A. H. Reshak, S. Auluck, and I. V. Kityk, “X-ray photoelectron spectroscopy and full potential studies of the electronic density of state of ternary oxyborate Na3La9O3(BO3)8,” J. Alloy. Comp. 472(1-2), 30–34 (2009).

1. Introduction Borate-based nonlinear optical (NLO) crystals have been used widely for frequency conversion in NLO devices and modern laser systems due to their excellent properties such as high laser damage threshold, high optical quality, high ultraviolet (UV) transparency and good chemical stability. In 1993, the anionic group theory and the corresponding molecular design system were developed by C. Chen, and which have greatly promoted the discovery of new borate NLO crystals, such as KBe2BO3F2 (KBBF), Sr2Be2B2O7 (SBBO) etc [1,2], By using these crystals, the range of laser wavelengths has been successfully expanded from the #119350 - $15.00 USD

(C) 2010 OSA

Received 4 Nov 2009; revised 8 Dec 2009; accepted 8 Dec 2009; published 23 Dec 2009

4 January 2010 / Vol. 18, No. 1 / OPTICS EXPRESS 237

near infrared (IR) to the deep-UV spectral region. Recently, a new promising borate NLO crystal Na3La9O3(BO3)8 (NLBO) was discovered in our group [3], and the parallel arrangements of their BO3 anionic groups are, according to the anionic group theory, favorable for producing large macroscopic second harmonic generation (SHG) coefficients. Subsequently P. Gravereau [4] prepared the single crystals by spontaneous crystallization and solved the structure by using X-ray data. NLBO crystallizes in the hexagonal system with space group P62m. It has excellent physical chemical features such as good mechanical properties, chemically very stable and free from moisture etc. which make it an attractive candidate for a wide range of frequency conversion applications in the visible and UV spectral regions. Since the discovery of this material, a number of experiments and theoretical calculations have been performed in it. These include: investigation of new fluxes for crystal growth [5,6], spectroscopic properties of rare-earth doped NLBO [7], and ab initio studies of its electronic structure etc [8], Despite the encouraging results, high quality crystals with large sizes are difficult to obtain, which therefore limit the optical applications of NLBO. In this paper, we report that high quality and large bulk NLBO crystals with the desired morphologies have been successfully grown based on the top-seeded solution growth (TSSG) method. The refractive indices were measured accurately over the full transmission range, and the second-order nonlinear optical coefficients were determined by the Maker fringe technique. We also present the numerical calculations of the optimal phase-matching(PM) conditions and the corresponding effective nonlinear coefficient. SHG experiments were also performed for the first time by a Ti:Sapphire laser with the central wavelength of 800 nm and a mode-locked Nd:YAG laser with working wavelength 1064 nm, respectively. A conversion efficiency of 58.3% at 532 nm was obtained for NLBO, and whereas only 21.5% was obtained for LBO under the same experimental conditions. Our results confirm that NLBO is a new competitive candidate for SHG conversion applications. 2. Crystal Growth and Optical Homogeneity Previous attempts to grow NLBO crystals were mentioned in Refs.5 and 6. However, at that time the crystals obtained were comparatively small sizes and poor quality, so they are difficult to meet the requirement of further characterization and optical applications. Recently, we optimized technological parameters of the equipments to improve the growth conditions. As a result, a series of even larger good quality bulk NLBO crystals with weight exceeding 40g have been successfully grown along different crystallographic directions, such as [001], [110], and [100] etc., respectively by TSSG method. As an example, the as-grown NLBO crystal with seed orientations along [210] directions is given in Fig. 1(a). From this figure, we can see that the crystal has the good quality, and furthermore the morphologies were much more suitable for optical applications than those grown ever before. The optical homogeneity of a NLBO sample with dimensions of 8 × 6 × 1.45 mm3 was measured by a Veeco interferometer Wyko RTI 4100. The optical source in the instrument was a He–Ne laser of wavelength 633 nm and the incident beam laser was parallel to the crystal optical axis. The optical homogeneity characterized by the root-mean-squared of the gradient of refractive index was measured to be about 4.15 × 10−6 cm−1, indicating that the optical quality of this crystal was very good. 3. Measurements of the refractive indices The refractive indices were measured by, up to now, the most accurate refractive index measurement system HR SpectroMaster UV-VIS-IR (Trioptics, Germany) at 12 different wavelengths over the full transmission range of NLBO. The sample was cut as right-angle prism with apex angle about 30° and kept at 21°C during the measurement. The experimental values with a high accuracy of 1 × 10−5 and calculated values of refractive indices are compared in Table 1.

#119350 - $15.00 USD

(C) 2010 OSA



Table 1. Comparison of the refractive indices between the experimental and calculated values for NLBO λ(µm)

ne Exp

Cal

0.3630

1.8333412

0.4047

1.8194391

0.4358 0.4800

no ∆

Exp

Cal

∆

1.833463

−0.0001

1.9336767

1.933862

−0.0002

1.819214

0.00022

1.9164917

1.916182

0.00031

1.8114123

1.811383

2.9E-05

1.9065969

1.906528

6.9E-05

1.8029553

1.802946

9.5E-06

1.8962102

1.896167

4.4E-05

0.5461

1.7939887

1.79413

−0.0001

1.8851935

1.885356

−0.0002

0.5875

1.7900643

1.790111

−5E-05

1.8803420

1.880417

−7E-05

0.6438

1.7858227

1.785861

−4E-05

1.8750837

1.875168

−8E-05

0.7065

1.7822428

1.782274

−3E-05

1.8706116

1.870694

−8E-05

0.8521

1.7767532

1.776709

4.4E-05

1.8635602

1.863574

−1E-05

1.0140

1.7729684

1.772926

4.3E-05

1.8584565

1.858431

2.5E-05

1.5300

1.7665892

1.766532

5.8E-05

1.8482910

1.848043

0.00025

2.3250

1.7599801

1.760007

−3E-05

1.8336265

1.833717

−9E-05

Figure 1(b) shows the fitted dispersion curves of the NLBO prism over the full transmission range, and the Sellmeier's equations fitted by the least squares fitting method were given as following:

0.02825765 − 0.005254 × λ2 . λ − 0.0147568 0.0350044 no2 = 3.4339330 + 2 − 0.014413 × λ2 . λ − 0.0180403 ne2 = 3.1207853 +

2

Fig. 1. (a) NLBO crystal grown along [210] directions; (b) The fitted dispersion curves of the NLBO prism over the full transmission range

4. Measurement of the NLO Coefficients NLBO is a negative uniaxial optical crystal with space group P62m, so it has only one nonzero independent SHG coefficient, i.e. d22, assuming Kleinman symmetry relations. Here the magnitude of the coefficient d22 was determined by the Maker fringe technique. In this experiment the pulsed Q-switched Nd: YAG laser (Spectral Physics, Model, Pro 230) at 1064.2 nm with the pulse width 10ns and the repetition frequency 10Hz was used as the fundamental light source. The SH signal from the sample was selectively detected by the photomultiplier tube (Hamamatsu, Model R105), averaged by the fast gated integrators and boxcar averagers (Stanford Research Systems), and then recorded. The sample was uncoated and cut along c directions with sizes of 6 × 8 × 1.45 mm3. Figure 2(a) shows the orientation of #119350 - $15.00 USD

(C) 2010 OSA



the c-cut NLBO sample to measure the Maker fringes of d22, the Eω is the fundamental light and the E2ω is the SH light. A KDP crystal was cut along [110] directions as the calibrated sample. The type-I Maker fringes of d22 was shown in Fig. 2(b), where the solid and dashed curves represent the experimental and calculated values, respectively. The d22 coefficient of NLBO crystals relative to d36 (KDP) was then derived as d22(NLBO) = (5.925 ± 0.171)d36 (KDP) = (2.31 ± 0.07) pm/V, by the ratio of the central inserted values of the envelopes (dashed curves in the figures) between the crystals to be measured and the KDP crystal.

Fig. 2. (a) Orientation of the c-cut NLBO crystal to measure the Maker fringes of d22; the (E)ω is the fundamental light and the (E)2ω is the SH light. (b) (Color online) Experimental Maker fringe (type-I) of d22(solid curve); theoretical fringe and theoretical envelope(dashed curves).

5. Phase-matching By using the above measured Sellmeier equations and nonlinear optical coefficients, we calculated PM directions for SHG and the corresponding effective nonlinear coefficients. The PM curves for different wavelengths for type I (PM-I) and type II (PM-II) are given in Fig. 3(a). From this figure, we learned that NLBO is phase matchable in the region from 560~5000 nm for a PM-I and 790~4344 nm for a PM-II, respectively. Spatial walk-off angle is an important parameter which effectively reduces the gain length for SHG, and therefore effects the attainment of maximum SHG output power and efficiency. The variation curve of walk-off angle for PM-I shown in Fig. 3(b), indicating that the walk-off angle for PM-I varies between ~0 mrad and ~52 mrad for fundamental wavelengths between 0.56 and 1.5 µm. As can also be seen from this plot, over the fundamental range of 1.5 to 5 µm the walk-off angle varies from 0 mrad to ~44 mrad. Generally, several samples with different lengths could be used to find out the optimum conditions for obtaining maximum SHG conversion efficiency and output power. 5

Type I

Walk off angle (mrad)

50 Wavelength (µm)

4

Type I

Type II

3

PM-I (1064 nm) PM-I (800 nm)

2 1

a 0 20

30

40

50 60 70 θ (degrees)

80

90

40 30 20

b 10

1

2 3 Wavelength ( µ m)

4

5

Fig. 3. (a) Phase-matching curves for type I and type II; (b) Variation of walk-off angle for type I as a function of fundamental wavelength

Figure 4(a) shows the spatial configuration of the PM angles and the corresponding magnitude of the effective nonlinear coefficient for PM-I (red) and PM-II (blue) as a function of fundamental wavelength. The calculation results indicate that the variation of the effective nonlinear coefficient deff for PM-I is ranging from 0 to 2.06 pm/V across the tuning range #119350 - $15.00 USD

(C) 2010 OSA



560~5000 nm, and for PM-II, ranging from 0 to 1.44 pm/V across the tuning range 790~4344 nm. As can also be seen form Fig. 4(a), the effective nonlinear coefficient deff for PM-I and PM-II is periodic variation in the azimuthal angle range 0