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A promising birefringent crystal Ba2Na3(B3O6)2F
Optical Materials 38 (2014) 6–9
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
A promising birefringent crystal Ba2Na3(B3O6)2F Xing Wang a,b, Mingjun Xia a, R.K. Li a,⇑ a 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, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 17 September 2014 Accepted 18 September 2014 Available online 18 October 2014 Keywords: Ba2Na3(B3O6)2F Birefringent crystal Refractive indices Glan-Taylor polarizer
a b s t r a c t Bulk crystals of Ba2Na3(B3O6)2F (BNBF) have been successfully grown by top-seeded solution growth (TSSG) technique. Its transmittance spectra show a wide transparency range from 186 nm to 3000 nm. The refractive indices in 13 wavelengths were measured with high accuracy and the Sellmeier equations were obtained, which demonstrated that the title crystal displayed a birefringence (Dn = 0.1030 at 588 nm) comparable to that of the commercial birefringent crystal a-BBO (the high temperature form of BaB2O4). A prototype Glan-Taylor polarizer made of BNBF prisms was fabricated, which showed high transparency and large optical extinction ratio similar to the commercial polarizer made of a-BBO. In addition, BNBF crystal is less moisture sensitive than that of a-BBO, thus BNBF can be a potential new birefringent crystal. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As key materials in the optical communication and laser industry, birefringent crystals are traditionally utilized to fabricate polarizers, optical isolators, circulators and beam displacers [1,2]. More recently, they have been proposed as time dispersion compensator to compensate group velocity mismatch in harmonic generations [3,4] of ultrafast lasers or to generate flat top peak shape pulses for cathode laser driver [5]. Commercial birefringent materials show excellent performance in the visible range, such as calcite [6] and YVO4 [7], but some drawbacks hinder their application in the UV, especially the deep UV range, which is important for spectroscopy [8] as well as for controlling polarization of illuminating lasers in immersion lithography [9]. For example, mineral calcite crystals are mainly restricted by their low transmittance at the UV side due to poor crystal quality and various geological impurities, while YVO4 is simply not transparent at the wavelength below 400 nm. Over the past decades, more attentions have been paid to borate crystals for their excellent properties as nonlinear optical materials, e.g. low temperature phase BaB2O4 (b-BBO) [10] and LiB3O5 (LBO) [11]. Because of their high transmission in the UV range and large anisotropic polarizabilities of the planar BO3 or B3O6 groups, borates can also be adopted as birefringent crystals. For example, the high temperature phase BaB2O4 (a-BBO) exhibits ⇑ Corresponding author. Tel.: +86 010 82543711. E-mail address: [email protected] (R.K. Li). http://dx.doi.org/10.1016/j.optmat.2014.09.014 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
wide transmittance range (189–3500 nm) and large birefringence [12–15]. However, it is difficult to obtain perfect pure a-BBO crystal since it tends to crack during growth due to a phase transition at 925 °C [16]. Usually, doping of Sr into the Ba sites is necessary to stabilize the high temperature phase and in this case variation of the Sr contents is unavoidable in doped a-BBO crystal during growth and from batch to batch. Ca3(BO3)2 crystal is proposed as a new birefringent material with a short UV transmittance cutoff at 180 nm, but its birefringence is relatively small [17]. Some other borates, including YBa3(B3O6)3 [18], Ba2M(B3O6)2 (M = Mg, Ca) [19] have been recently proposed as new birefringent crystals, however, due to growth difficulties or low transmittance in the UV range, they are still not commercially available. We are aiming to find new birefringent crystals with high transmission in the UV range and other properties especially birefringence comparable to the commercial a-BBO crystal (e.g. with Dn > 0.10 at 589 nm, transmission cutoff at UV side kcutoff < 190 nm, transmission T > 90% at 200 nm–2 lm). Ba2Na3(B3O6)2F (BNBF) was discovered as a new compound during the growth of well-known BBO nonlinear optical crystal by Bekker et al. [20–23]. It belongs to the space group P63/m with cell parameters of a = 7.346(1) Å, c = 12.636(2) Å. The fundamental building units in the BNBF structure are also the isolated planar (B3O6)3 anion groups, which are distributed perfectly parallel similar to that in a-BBO and Ba2M(B3O6)2 structures. Based on our understanding of the structure–property relations of the borate crystals [24], BNBF can be envisaged as a new birefringent crystal due to its parallel arrangement of the B3O6 groups. In addition, it
7
X. Wang et al. / Optical Materials 38 (2014) 6–9
has been shown that the crystal could be easily grown into sufficient size and its UV transmittance is comparable to that of aBBO [23]. In this paper, bulk BNBF crystals have been grown and characterized by optical measurements. To assess the device performance, a Glan type polarizer made of two identical BNBF prisms has been designed and manufactured which showed high transmittance ranging from 300 nm to 2500 nm and large optical extinction ratio of at least 2.8 104:1.
Then the linear polarized output light entered into the polarizer of BNBF Glan-Taylor or BBO device. As the device rotating, the intensity of output light was recorded by a Newport 918UV power detector. For the extinction ratio measurement, the input laser power was raised to its maximum and the maximum and minimum output powers of 5.589 mW and 200 nW were recorded when the polarization directions of the BNBF polarizer were positioned parallel or perpendicular to that of the calcite polarizer, respectively.
2. Experimental section 3. Results and discussion 2.1. Crystal growth BNBF crystals were grown by the typical top-seeded solution growth (TSSG) method. Analytical pure starting materials of BaCO3, Na2CO3, H3BO3, NaF were thoroughly mixed in the approximate ratios equivalent to BNBF:NaF = 1:x (x = 0.05–0.2). After homogenized at 900 °C in a platinum crucible with stirring for over 24 h, the melt was then cooled down to the saturation temperature around 820 °C, which was confirmed by seed crystal testing. And then a c-oriented seed crystal (2 2 10 mm3) with a rotating speed of 15–25 r/min was carefully dipped into the melt and the furnace temperature was decreased at a rate of 0.2 °C/day. Transparent BNBF crystals with hexagonal prism shape (Fig. 1) and sizes up to 22 22 30 mm3 (37 g) were obtained in a growth period of 5–7 days. 2.2. Crystal characterization
The BNBF crystal grown from the melt with slightly excess NaF (x = 0.05) is presented in Fig. 1, which shows a hexagonal prism morphology in agreement with its structural characteristics with space group P63/m. Since a near stoichiometric solution (5% in excess of NaF) is employed, the growth rate is much faster than that in Refs. [21,23] where high contents of BaF2 (BNBF: BaF2 = 1: 0.4 or 1: 1) flux were utilized for the growths. From the transmission spectrum of BNBF crystal (Fig. 2), the UV transmittance cut-off edge of present grown BNBF crystal is located at about kcutoff = 186 nm, which is slightly shorter than that of a-BBO (189 nm). High transmittance (T > 90%) was observed in the range of 350–2150 nm, but there is still an absorption loss of about 15% at 266 nm after correcting the reflection loss, which may be caused by the absorptions of impurity ions or by scattering centers in the grown crystal. It is well known that the planar B3O6 group has largest anisotropic polarizabilities among the borate groups [24]. If all
The room temperature transmission spectra of a polished cplate of BNBF crystal with thickness of 1.5 mm were recorded on a Lambda 900 UV/Vis/NIR (Perkin–Elmer) spectrophotometer in the range of 185–3000 nm in air and a McPherson Vuvas2000 spectrophotometer in the range of 120–220 nm in vacuum, respectively. The refractive indices of a BNBF prism with apex angle of 29.909(3)° were measured by the minimum deviation method at 13 spectral lines of Hg or other discharge lamps from 253.7 nm to 2.325 lm on a Trioptics SpectroMaster HR UV–VIS–IR spectrometer. 2.3. Polarizer design To evaluate its potential device applications as a birefringent crystal, a BNBF Glan-Taylor type polarizer was designed and fabricated for comparison tests with a commercial a-BBO Glan-Taylor polarizer (from Ultra Photonics, Inc. in Fuzhou, China). For polarizing test, a He–Ne laser (k = 633 nm) (Melles Griot 25-LHP-151-230) firstly passed through a linear polarizer made of calcite crystal.
Fig. 2. Transmission spectrum of BNBF crystal.
Table 1 Measured refractive indices for the BNBF crystal.
Fig. 1. The as-grown BNBF crystal (a) and its morphology (b).
k (lm)
no
ne
Dn
0.2537 0.3650 0.4047 0.4358 0.4800 0.5461 0.5876 0.6438 0.7065 0.8521 1.0140 1.5296 2.3254
1.713237(7) 1.6518320(7) 1.6428486(4) 1.6375874(3) 1.6319364(3) 1.6259556(4) 1.6231987(2) 1.6202321(3) 1.6176589(4) 1.6134025(5) 1.60997(5) 1.60305(2) 1.58896(3)
1.582676(8) 1.5402356(7) 1.5339346(2) 1.5302376(6) 1.5262696(4) 1.5220936(3) 1.5201902(6) 1.5181695(8) 1.5164578(4) 1.513781(1) 1.51185(3) 1.50960(7) 1.50583(5)
0.1306 0.1116 0.1089 0.1073 0.1057 0.1039 0.1030 0.1021 0.1012 0.0996 0.0981 0.0935 0.0831
8
X. Wang et al. / Optical Materials 38 (2014) 6–9
as proposed in Refs. [24–26], gave the refractive indices as, no = 1.5876 and ne = 1.4834 with Dn = 0.1042 at 589 nm. The refractive indices of the BNBF crystal were determined experimentally by the minimum deviation method. The extraordinary (ne) and ordinary refractive indices (no) at the measured wavelengths are summarized in Table 1. Sellmeier equations obtained by the least-square fitting of all the measured data points (Fig. 3) are as follows:
n2o ¼ 2:5895ð7Þ þ
n2e ¼ 2:2776ð5Þ þ
Fig. 3. The measured refractive indices and the fitted curve by Sellmeier equations (inset: BNBF prism used for the measurement).
Fig. 4. Transmission spectrum of BNBF Glan type polarizer.
the planar B3O6 groups are arranged parallel to each other in a structure as in BNBF crystal, the material will display large birefringence. Our theoretical calculation based on the same procedure
0:0163ð2Þ 2
k 0:0173ð5Þ 0:0113ð1Þ k2 0:0149ð6Þ
0:0124ð2Þk2
ð1Þ
0:0022ð2Þk2
ð2Þ
where k represents the wavelength in unit of lm. The results show that BNBF is a negative uniaxial optical crystal and exhibits large birefringence (Dn = 0.0831–0.1306) in the measured wavelength range (2325.4–253.7 nm). In order to compare the BNBF crystal to commercial BBO as a polarizer, a BNBF Glan type polarizer was fabricated with two identical prisms which were cut at apex angles of 38°550 towards the crystal optic c axis. The two prisms were parallel placed in an aluminum alloy holder with a small air space. At this specific angle, the extraordinary wave would pass through the Glan type polarizer, while the ordinary one would be totally reflected in the wavelength range of 250–2500 nm according to the obtained Sellmeier equations. The transmittance spectrum of the polarizer for epolarized light (Fig. 4) shows high transmittance (T 80%) between 300 and 3000 nm, especially account for the reflection losses due to the 4 crystal/air interfaces of the uncoated prisms. The slight disagreement of the wavelength range may come from the misalignments of the prisms in the holder or the optical pass in the spectrometer. Fig. 5 shows the relationships between output light intensities versus rotating angles for both the Glan type polarizers constructed from BNBF and commercial a-BBO. The extinction ratios of 2.8 104:1 and 3.9 104:1 were recorded for BNBF polarizer and a-BBO polarizer, respectively. 4. Conclusion BNBF crystals have been successfully grown by the TSSG method. In the BNBF crystal structure, the perfectly parallel B3O6 planar groups generate the major anisotropic polarizabilities,
Fig. 5. The performances of the Glan-Taylor polarizers constructed from BNBF crystal (a) and BBO (b).
X. Wang et al. / Optical Materials 38 (2014) 6–9
thereby producing large birefringence (Dn = 0.087–0.196) in the range from 2000 nm to 186 nm, which is as large as that of wellknown a-BBO birefringent crystal. The optical tests indicate that the Glan type polarizer made from BNBF has similar performance and same level of extinction ratio (104) as that made of a-BBO. In addition, BNBF crystal is less moisture sensitive, possesses shorter UV transmittance cutoff than a-BBO and is relatively easy to grow. Therefore, BNBF crystal can be a potential substitute for the well-known a-BBO as birefringent crystal in the visible to the IR range (350–2150 nm) and it may be superior to a-BBO in the UV wavelength region if the absorption around 200–300 nm range can be overcome. Acknowledgments We thank Mr. Lei Bai for the refractive index measurements. We also appreciate the financial support by the National Science Foundation of China (Nos. 90922036 and 51032004/E0201).
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
References [1] E. Hecht, Optics, Addison-Wesley, 1998. [2] G. Chartier, Introduction to Optics, Springer Science + Business Media Inc, 1995. [3] Y.C. Chen, X.J. Fang, J. Li, X.Y. Liang, H.L. Zhang, B.H. Feng, X.L. Zhang, L.A. Wu, Z.Y. Xu, Appl. Opt. 40 (2001) 2579–2582. [4] J.J. Huang, Y.Q. Chang, T. Shen, Y.Q. Yang, Opt. Commun. 281 (2008) 5244–5248.
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I. Will, G. Klemz, Opt. Express 16 (2008) 14922–14937. G. Ghosh, Opt. Commun. 163 (1999) 95–102. L.G. DeShazer, Proc. SPIE 10 (2002) 4481. L.R. McCunn, D.I.G. Bennett, L.J. Butler, H. Fan, F. Aguirre, S.T. Pratt, J. Phys. Chem. A 110 (2006) 843–850. H. Nomura, Y. Furutono, Microelectron. Eng. 85 (2008) 1671–1675. C.T. Chen, B.C. Wu, A.D. Jiang, G.M. You, Sci. Sin. Ser. B 28 (1985) 235. C.T. Chen, Y.C. Wu, A.D. Jiang, B.C. Wu, G.M. You, R.K. Li, S.J. Lin, J. Opt. Soc. Am. B 6 (1989) 616. V.P. Solntsev, E.G. Tsvetkov, V.A. Gets, V.D. Antsygin, J. Cryst. Growth 236 (2002) 290–296. G.Q. Zhou, J. Xu, X.D. Chen, H.Y. Zhong, S.T. Wang, K. Xu, P.Z. Deng, F.X. Gan, J. Cryst. Growth 191 (1998) 517–519. S.F. Wu, G.F. Wang, J.L. Xie, X.Q. Wu, Y.F. Zhang, X. Lin, J. Cryst. Growth 245 (2002) 84–86. R. Appel, C.D. Dyer, J.N. Lockwood, Appl. Opt. 41 (2002) 2470–2480. K.H. Hubner, Neues Jahrb. Mineral. Monatsh. (1969) 335–342. S.Y. Zhang, X. Wu, Y.T. Song, D.Q. Ni, B.Q. Hu, T. Zhou, J. Cryst. Growth 252 (2003) 246–250. M. He, X.L. Chen, Y.P. Sun, J. Liu, J.T. Zhao, C.J. Duan, Cryst. Growth Des. 7 (2007) 199–201. R.K. Li, Y.Y. Ma, Cryst. Eng. Commun. 14 (2012) 5421–5424. A.E. Kokh, N.G. Kononova, T.B. Bekker, P.P. Fedorov, E.A. Nigmatulina, A.G. Ivanova, Crystallogr. Rep. 54 (2009) 146–151. T.B. Bekker, N.G. Kononova, A.E. Kokh, S.V. Kuznetsov, P.P. Fedorov, Crystallogr. Rep. 55 (2010) 877–881. T.B. Bekker, P.P. Fedorov, A.E. Kokh, Cryst. Growth Des. 12 (2012) 129–134. T.B. Bekker, A.E. Kokh, N.G. Kononova, P.P. Fedorov, S.V. Kuznetsov, Cryst. Growth Des. 9 (2009) 4060–4063. F.L. Qin, R.K. Li, J. Cryst. Growth 318 (2011) 642–644. W.L. Bragg, Proc. R. Soc. London Ser. A 105 (1924) 370–386. R.K. Li, Z. Kristallogr. 228 (2013) 526–531.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
A promising birefringent crystal Ba2Na3(B3O6)2F Xing Wang a,b, Mingjun Xia a, R.K. Li a,⇑ a 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, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 11 August 2014 Received in revised form 17 September 2014 Accepted 18 September 2014 Available online 18 October 2014 Keywords: Ba2Na3(B3O6)2F Birefringent crystal Refractive indices Glan-Taylor polarizer
a b s t r a c t Bulk crystals of Ba2Na3(B3O6)2F (BNBF) have been successfully grown by top-seeded solution growth (TSSG) technique. Its transmittance spectra show a wide transparency range from 186 nm to 3000 nm. The refractive indices in 13 wavelengths were measured with high accuracy and the Sellmeier equations were obtained, which demonstrated that the title crystal displayed a birefringence (Dn = 0.1030 at 588 nm) comparable to that of the commercial birefringent crystal a-BBO (the high temperature form of BaB2O4). A prototype Glan-Taylor polarizer made of BNBF prisms was fabricated, which showed high transparency and large optical extinction ratio similar to the commercial polarizer made of a-BBO. In addition, BNBF crystal is less moisture sensitive than that of a-BBO, thus BNBF can be a potential new birefringent crystal. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As key materials in the optical communication and laser industry, birefringent crystals are traditionally utilized to fabricate polarizers, optical isolators, circulators and beam displacers [1,2]. More recently, they have been proposed as time dispersion compensator to compensate group velocity mismatch in harmonic generations [3,4] of ultrafast lasers or to generate flat top peak shape pulses for cathode laser driver [5]. Commercial birefringent materials show excellent performance in the visible range, such as calcite [6] and YVO4 [7], but some drawbacks hinder their application in the UV, especially the deep UV range, which is important for spectroscopy [8] as well as for controlling polarization of illuminating lasers in immersion lithography [9]. For example, mineral calcite crystals are mainly restricted by their low transmittance at the UV side due to poor crystal quality and various geological impurities, while YVO4 is simply not transparent at the wavelength below 400 nm. Over the past decades, more attentions have been paid to borate crystals for their excellent properties as nonlinear optical materials, e.g. low temperature phase BaB2O4 (b-BBO) [10] and LiB3O5 (LBO) [11]. Because of their high transmission in the UV range and large anisotropic polarizabilities of the planar BO3 or B3O6 groups, borates can also be adopted as birefringent crystals. For example, the high temperature phase BaB2O4 (a-BBO) exhibits ⇑ Corresponding author. Tel.: +86 010 82543711. E-mail address: [email protected] (R.K. Li). http://dx.doi.org/10.1016/j.optmat.2014.09.014 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
wide transmittance range (189–3500 nm) and large birefringence [12–15]. However, it is difficult to obtain perfect pure a-BBO crystal since it tends to crack during growth due to a phase transition at 925 °C [16]. Usually, doping of Sr into the Ba sites is necessary to stabilize the high temperature phase and in this case variation of the Sr contents is unavoidable in doped a-BBO crystal during growth and from batch to batch. Ca3(BO3)2 crystal is proposed as a new birefringent material with a short UV transmittance cutoff at 180 nm, but its birefringence is relatively small [17]. Some other borates, including YBa3(B3O6)3 [18], Ba2M(B3O6)2 (M = Mg, Ca) [19] have been recently proposed as new birefringent crystals, however, due to growth difficulties or low transmittance in the UV range, they are still not commercially available. We are aiming to find new birefringent crystals with high transmission in the UV range and other properties especially birefringence comparable to the commercial a-BBO crystal (e.g. with Dn > 0.10 at 589 nm, transmission cutoff at UV side kcutoff < 190 nm, transmission T > 90% at 200 nm–2 lm). Ba2Na3(B3O6)2F (BNBF) was discovered as a new compound during the growth of well-known BBO nonlinear optical crystal by Bekker et al. [20–23]. It belongs to the space group P63/m with cell parameters of a = 7.346(1) Å, c = 12.636(2) Å. The fundamental building units in the BNBF structure are also the isolated planar (B3O6)3 anion groups, which are distributed perfectly parallel similar to that in a-BBO and Ba2M(B3O6)2 structures. Based on our understanding of the structure–property relations of the borate crystals [24], BNBF can be envisaged as a new birefringent crystal due to its parallel arrangement of the B3O6 groups. In addition, it
7
X. Wang et al. / Optical Materials 38 (2014) 6–9
has been shown that the crystal could be easily grown into sufficient size and its UV transmittance is comparable to that of aBBO [23]. In this paper, bulk BNBF crystals have been grown and characterized by optical measurements. To assess the device performance, a Glan type polarizer made of two identical BNBF prisms has been designed and manufactured which showed high transmittance ranging from 300 nm to 2500 nm and large optical extinction ratio of at least 2.8 104:1.
Then the linear polarized output light entered into the polarizer of BNBF Glan-Taylor or BBO device. As the device rotating, the intensity of output light was recorded by a Newport 918UV power detector. For the extinction ratio measurement, the input laser power was raised to its maximum and the maximum and minimum output powers of 5.589 mW and 200 nW were recorded when the polarization directions of the BNBF polarizer were positioned parallel or perpendicular to that of the calcite polarizer, respectively.
2. Experimental section 3. Results and discussion 2.1. Crystal growth BNBF crystals were grown by the typical top-seeded solution growth (TSSG) method. Analytical pure starting materials of BaCO3, Na2CO3, H3BO3, NaF were thoroughly mixed in the approximate ratios equivalent to BNBF:NaF = 1:x (x = 0.05–0.2). After homogenized at 900 °C in a platinum crucible with stirring for over 24 h, the melt was then cooled down to the saturation temperature around 820 °C, which was confirmed by seed crystal testing. And then a c-oriented seed crystal (2 2 10 mm3) with a rotating speed of 15–25 r/min was carefully dipped into the melt and the furnace temperature was decreased at a rate of 0.2 °C/day. Transparent BNBF crystals with hexagonal prism shape (Fig. 1) and sizes up to 22 22 30 mm3 (37 g) were obtained in a growth period of 5–7 days. 2.2. Crystal characterization
The BNBF crystal grown from the melt with slightly excess NaF (x = 0.05) is presented in Fig. 1, which shows a hexagonal prism morphology in agreement with its structural characteristics with space group P63/m. Since a near stoichiometric solution (5% in excess of NaF) is employed, the growth rate is much faster than that in Refs. [21,23] where high contents of BaF2 (BNBF: BaF2 = 1: 0.4 or 1: 1) flux were utilized for the growths. From the transmission spectrum of BNBF crystal (Fig. 2), the UV transmittance cut-off edge of present grown BNBF crystal is located at about kcutoff = 186 nm, which is slightly shorter than that of a-BBO (189 nm). High transmittance (T > 90%) was observed in the range of 350–2150 nm, but there is still an absorption loss of about 15% at 266 nm after correcting the reflection loss, which may be caused by the absorptions of impurity ions or by scattering centers in the grown crystal. It is well known that the planar B3O6 group has largest anisotropic polarizabilities among the borate groups [24]. If all
The room temperature transmission spectra of a polished cplate of BNBF crystal with thickness of 1.5 mm were recorded on a Lambda 900 UV/Vis/NIR (Perkin–Elmer) spectrophotometer in the range of 185–3000 nm in air and a McPherson Vuvas2000 spectrophotometer in the range of 120–220 nm in vacuum, respectively. The refractive indices of a BNBF prism with apex angle of 29.909(3)° were measured by the minimum deviation method at 13 spectral lines of Hg or other discharge lamps from 253.7 nm to 2.325 lm on a Trioptics SpectroMaster HR UV–VIS–IR spectrometer. 2.3. Polarizer design To evaluate its potential device applications as a birefringent crystal, a BNBF Glan-Taylor type polarizer was designed and fabricated for comparison tests with a commercial a-BBO Glan-Taylor polarizer (from Ultra Photonics, Inc. in Fuzhou, China). For polarizing test, a He–Ne laser (k = 633 nm) (Melles Griot 25-LHP-151-230) firstly passed through a linear polarizer made of calcite crystal.
Fig. 2. Transmission spectrum of BNBF crystal.
Table 1 Measured refractive indices for the BNBF crystal.
Fig. 1. The as-grown BNBF crystal (a) and its morphology (b).
k (lm)
no
ne
Dn
0.2537 0.3650 0.4047 0.4358 0.4800 0.5461 0.5876 0.6438 0.7065 0.8521 1.0140 1.5296 2.3254
1.713237(7) 1.6518320(7) 1.6428486(4) 1.6375874(3) 1.6319364(3) 1.6259556(4) 1.6231987(2) 1.6202321(3) 1.6176589(4) 1.6134025(5) 1.60997(5) 1.60305(2) 1.58896(3)
1.582676(8) 1.5402356(7) 1.5339346(2) 1.5302376(6) 1.5262696(4) 1.5220936(3) 1.5201902(6) 1.5181695(8) 1.5164578(4) 1.513781(1) 1.51185(3) 1.50960(7) 1.50583(5)
0.1306 0.1116 0.1089 0.1073 0.1057 0.1039 0.1030 0.1021 0.1012 0.0996 0.0981 0.0935 0.0831
8
X. Wang et al. / Optical Materials 38 (2014) 6–9
as proposed in Refs. [24–26], gave the refractive indices as, no = 1.5876 and ne = 1.4834 with Dn = 0.1042 at 589 nm. The refractive indices of the BNBF crystal were determined experimentally by the minimum deviation method. The extraordinary (ne) and ordinary refractive indices (no) at the measured wavelengths are summarized in Table 1. Sellmeier equations obtained by the least-square fitting of all the measured data points (Fig. 3) are as follows:
n2o ¼ 2:5895ð7Þ þ
n2e ¼ 2:2776ð5Þ þ
Fig. 3. The measured refractive indices and the fitted curve by Sellmeier equations (inset: BNBF prism used for the measurement).
Fig. 4. Transmission spectrum of BNBF Glan type polarizer.
the planar B3O6 groups are arranged parallel to each other in a structure as in BNBF crystal, the material will display large birefringence. Our theoretical calculation based on the same procedure
0:0163ð2Þ 2
k 0:0173ð5Þ 0:0113ð1Þ k2 0:0149ð6Þ
0:0124ð2Þk2
ð1Þ
0:0022ð2Þk2
ð2Þ
where k represents the wavelength in unit of lm. The results show that BNBF is a negative uniaxial optical crystal and exhibits large birefringence (Dn = 0.0831–0.1306) in the measured wavelength range (2325.4–253.7 nm). In order to compare the BNBF crystal to commercial BBO as a polarizer, a BNBF Glan type polarizer was fabricated with two identical prisms which were cut at apex angles of 38°550 towards the crystal optic c axis. The two prisms were parallel placed in an aluminum alloy holder with a small air space. At this specific angle, the extraordinary wave would pass through the Glan type polarizer, while the ordinary one would be totally reflected in the wavelength range of 250–2500 nm according to the obtained Sellmeier equations. The transmittance spectrum of the polarizer for epolarized light (Fig. 4) shows high transmittance (T 80%) between 300 and 3000 nm, especially account for the reflection losses due to the 4 crystal/air interfaces of the uncoated prisms. The slight disagreement of the wavelength range may come from the misalignments of the prisms in the holder or the optical pass in the spectrometer. Fig. 5 shows the relationships between output light intensities versus rotating angles for both the Glan type polarizers constructed from BNBF and commercial a-BBO. The extinction ratios of 2.8 104:1 and 3.9 104:1 were recorded for BNBF polarizer and a-BBO polarizer, respectively. 4. Conclusion BNBF crystals have been successfully grown by the TSSG method. In the BNBF crystal structure, the perfectly parallel B3O6 planar groups generate the major anisotropic polarizabilities,
Fig. 5. The performances of the Glan-Taylor polarizers constructed from BNBF crystal (a) and BBO (b).
X. Wang et al. / Optical Materials 38 (2014) 6–9
thereby producing large birefringence (Dn = 0.087–0.196) in the range from 2000 nm to 186 nm, which is as large as that of wellknown a-BBO birefringent crystal. The optical tests indicate that the Glan type polarizer made from BNBF has similar performance and same level of extinction ratio (104) as that made of a-BBO. In addition, BNBF crystal is less moisture sensitive, possesses shorter UV transmittance cutoff than a-BBO and is relatively easy to grow. Therefore, BNBF crystal can be a potential substitute for the well-known a-BBO as birefringent crystal in the visible to the IR range (350–2150 nm) and it may be superior to a-BBO in the UV wavelength region if the absorption around 200–300 nm range can be overcome. Acknowledgments We thank Mr. Lei Bai for the refractive index measurements. We also appreciate the financial support by the National Science Foundation of China (Nos. 90922036 and 51032004/E0201).
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