Materials Letters 60 (2006) 2617 – 2619 www.elsevier.com/locate/matlet
Optical properties of Ca3TaGa3Si2O14 single crystal Baolin Wang a,⁎, Aijian Wei a,b , Xuzhong Shi b , Duorong Yuan b , Haifeng Qi a a
Department of Information Science and Engineering, Shandong University, Jinan 250100, China b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 14 September 2005; accepted 18 January 2006 Available online 24 February 2006
Abstract The interference patterns of polarized light are measured by polarizing microscopy. The specific rotation ρ of Ca3TaGa3Si2O14 (CTGS) piezoelectric single crystal is determined from 200 to 850nm by measuring the optical transmission between parallel polarizers in b001N direction. It is shown that Ca3TaGa3Si2O14 has quite large a value of ρ which is a little smaller than that of Ca3NbGa3Si2O14 (CNGS). © 2006 Elsevier B.V. All rights reserved. Keywords: Optical properties; Optical activity
1. Introduction
2. Experimental
Recently, a series of new ordered piezoelectric single crystals with calcium gallium germinate CGG-type [1] structure which belong to point group 32 and space group P321 such as Sr3NbGa3Si2O14 (SNGS) , Sr3TaGa3Si2O14 (STGS) , CNGS and CTGS have been of considerable interest due to their promising piezoelectric properties and high frequency stability. Their piezoelectric properties have been actively investigated [2–9]. They are in the same family as La3Ga5SiO14 (LGS), La3Ga5.5 Ta0.5O14 (LGT) and La3Ga5.5Nb0.5O14 (LGN) but have a lower density, differing coupling coefficients, higher velocity and differing temperature coefficients. The preparation of CTGS polycrystal was firstly mentioned in Ref. [2]. In Refs. [8,9], its growth, structure, differential thermal analysis, thermal gravity analysis, specific heat and thermal expansion coefficients were investigated. But its optical properties have not been systematically studied yet. In the current paper we study optical properties including polarized light interference and optical activity of CTGS single crystal.
The CTGS single crystal grown along b010N direction as shown in Fig. 1 can be successfully grown by using the Czochralsky technique. Its melting point is 1368°C and the lattice parameters are a = 0.8098 nm, c = 0.4977nm.
⁎ Corresponding author. Tel.: +86 531 88361208. E-mail address:
[email protected] (B. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.01.110
2.1. Polarized light interference The interference patterns of convergent polarized light propagating along b001N direction which are investigated by polarizing microscopy are shown in Fig. 2, where θ is the angle between the fitted polarizer and the rotating analyzer. Fig. 2 shows that the CTGS crystal has large optical activity. With the angle θ increasing, the interference patterns expand around the center, then we can get that the CTGS crystal is right-handed,
Fig. 1. The picture of as-grown CTGS crystal.
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B. Wang et al. / Materials Letters 60 (2006) 2617–2619
Fig. 2. Interference patterns of convergent polarized light.
which is the same as the result determined by the method described in Ref. [3]. 2.2. Optical activity Firstly the transmittance spectra of CTGS crystal along b001N direction shown in Fig. 3 were measured from 200 to 850 nm by using TU-1901 UV–VIS (ultraviolet–visible) spec-
trophotometer. From Fig. 3 we can get that the cutoff wavelength is 259nm. As a comparison, the transmittance spectra of CNGS are also shown in Fig. 3. It can be seen that CTGS single crystal has slightly better transparency from visible light up to infrared light wavelength region. Since for CTGS crystal there is no typical absorption in visible light region, the sample we measured is transparent with no color. In order to determine the optical activity of CTGS crystal, we constructed one measuring system based on the method introduced by Ref. [3]. According to our own condition, we added two parallel polarizers in the light path, whose position could easily determined via observing the intensity of transmitted light displayed in screen. And the transmittance spectra of CTGS crystal with dimension of 8 × 6 × 5 mm (X × Y × Z) between the parallel polarizers along b001N direction were recorded by TU1901 UV–VIS spectrophotometer, as shown in Fig. 4. When a beam of linearly polarized monochromatic light passes through the optically active material along b001N direction, the vibration plane will rotate a certain angle — φ which is proportional to the thickness d of the sample: φ = ρ · d with the wavelength-dependent specific rotation ρ(λ) whose unit is degrees per millimeter. According to Fresnel's circular birefringence theory [10], the rotation dispersion of ρ(λ) is: qðkÞ ¼ pðnl −nr Þ=kvac
Fig. 3. Transmittance spectra of CTGS crystal in b001N direction.
ð1Þ
with the refractive indices nl and nr of, respectively, a lefthanded and a right-handed circularly polarized wave, and the wavelength λvac in vacuum. With the measuring wavelengths decreasing from 850 to 250 nm the angle of rotation increases consequently. Hence, the transmission reaches a minimum at φ = 90° when the vibration plane of the light is perpendicular to that of the analyzer. And the transmission achieves a maximum at φ = 180°, when the vibration plane of the light is parallel to that of the analyzer, and so on. In general, minima will appear at uðkÞ ¼ qðkÞd ¼ md180-−90-ðm ¼ 1; 2; 3 N Þ;
ð2Þ
Table 1 Measured angle of rotation and corresponding specific rotation with wavelength dependence
Fig. 4. Optical transmission of CTGS crystal between parallel polarizers.
λ (nm) φ (°) ρ (°/mm)
835 90 18
595 180 36
487 270 54
430 360 72
390 450 90
363 540 108
340 630 126
325 720 144
310 810 162
301 900 180
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shown in Fig. 5. And the corresponding Boltzmann's coefficients A1 = 10.579° nm, A2 = 0.4926 * 10− 6° nm3. The specific rotation in the b001N direction of the CTGS crystal was determined by a TU-1901 spectrophotometer via measuring the optical transmission in dependence on the wavelength between crossed polarizers. From Fig. 5 we can see that the specific rotation of CTGS is relatively smaller than that of CNGS, but still larger than that of quartz whose optical activity is already known large. High specific rotation can make the CTGS crystals useful for electric–optical switches and optical insulating instruments. References
Fig. 5. Fitted curve of specific rotation dispersion of CTGS crystal.
while maxima at uðkÞ ¼ qðkÞd ¼ md180-ðm ¼ 1; 2; 3 N Þ:
ð3Þ
From Fig. 3 we can see that there are intense absorptions below 300 nm, therefore, we did not take the extrema under 300 nm into account. The minimum or maximum, the degree of rotation φ and the corresponding specific rotation ρ with the wavelength dependence are shown in Table 1. According to Ref. [3], the specific rotation dispersion ρ(λ) can be fitted to Boltzmann's equation: qðkÞ ¼ A1 =k2 þ A2 =k4 :
ð4Þ
The fitted curve of ρ(λ) is shown in Fig. 3. For comparison, the specific rotation dispersions of CNGS and SiO2 are also
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