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Optical birefringence of β-SrB4O7 crystals with strontium substitutions
|Alexandre I. Zaitsev 1,2, Maxim S. Molokeev 1, Aleksandre V. Cherepakhin 1,2, Nikita V. Radionov 1, Anatolii V. Zamkov 1|
1. L.V.Kirensky Institute of Physics Siberian Branch of the Russian Academy of Science (IPH SB RAS), Academgorodok, 50, Krasnoyarsk 660036, Russian Federation
Borate compounds with the noncentrosymmetric crystal structure are attractive materials for nonlinear optical (NLO) applications, especially in the UV region . Orthorhombic α-SrB4O7 (α-SBO) crystals have been established to possess high NLO coefficients and a very short absorption edge but they have not sufficient birefringence for the existence of the second harmonic generation (SHG) phase matching conditions [2, 3, 4]. Recently, strontium tetraborate crystals have been obtained with another structure modification and their structure has been established. The structure of this β-SrB4O7 (β-SBO) with the trigonal symmetry belongs to the noncentrosymmetric P3 space group, the unit cell parameters being: a = 17.145 (1) Å, c = 4.2527 (5) Å . The dispersion dependence of the refraction indexes β-SBO has been investigated and SHG phase matching conditions have been calculated . According to  β-SBO crystals can be used in Type I SHG process down to 672 nm of the fundamental wavelength. Though the NLO coefficients of β-SBO haven't been measured yet, some attempts to extend investigations on the solid solution β-SBO:Pb have already been made .
Small single crystals of the solid solutions β-SBO:Ba and β-SBO:Pb have been obtained and main optical birefringence of these crystals has been measured.
β-SBO itself is a metastable crystal modification of this compound , so β-SBO crystals cannot be obtained with a temperature higher than ~ 780°C (the stable α-SBO melting temperature is 1000°C). Solid solution crystals β-SBO:Ba have been grown from a supercooled melt of the compositions (1-x)SrB4O7 × xBaB4O7 with x = 0.05; 0.1; 0.3 at about 700 – 720°C. β-SBO:Pb has been grown at 680°C from the melt with 15 mol % of PbB4O7.
Single crystal X-ray diffraction data on the obtained solid solution crystals were collected using diffractometer SMART APEX II (Bruker) with CCD detector.
The birefringence of the crystals has been measured by the compensation method with a Karl ZEISS AxioScop 40 polarized light microscope and a Berek compensator. The birefringence temperature dependence has been measured with a Lincam LTS 350 temperature controlled stage in the range from – 150 to +150°C.
The results are presented in Table 1.
One can see the solid solution birefringence increase with the increase of barium concentration. The effect of lead in the solid solution on the β-SBO birefringence is much higher, so the β-SBO:Pb birefringence is slightly higher than that for β-SBO:Ba with the dopant concentration being twice higher.
The birefringence temperature dependence has a sufficiently weak linear character for all the crystals with the temperature coefficient in range 1.5-3×10-6 grad-1.
Barium substitutes strontium in the β-SBO structure only in three from five nonequivalent positions, though lead is distributed between all the positions.
This study is supported by SB RAS Project 103, by Grant of the President of the Russian Federation for the support of leading scientific schools SS-4828.2012.2 and by RFBR Grant 11-02-00596-а.
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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Alexandre I. Zaitsev
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-04-15 16:02 Revised: 2013-04-15 20:59