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PMMA
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Optics & Laser Technology 35 (2003) 291 – 294 www.elsevier.com/locate/optlastec
Preparation and transmission loss of the nano-crystal and polymer composite &lm BTO/PMMA Hongliang Yanga;∗ , Quan Rena , Shiyi Guob , Guanghui Zhangb b State
a Department of Optics, Shandong University, Jinan 250100, People’s Republic of China Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China
Received 9 September 2002; accepted 14 January 2003
Abstract Bismuth titanate, Bi4 Ti3 O12 (BTO), is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices. Its nano-crystals were compounded by the chemical solution decomposition technique. The structure and size of BTO were analyzed by X-ray di:raction and transmissive electron microscopy. Two sorts of composite &lms BTO/polymethylmethacrylate with di:erent weight ratio of BTO were prepared by spin-coating method at certain conditions. In this article, the scattering loss in thin &lms was obtained using an imaging technique. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Nano-crystal; Transmission loss; Imaging technique
1. Introduction In recent years, the optical communication and optical information processing have made great progress. The photo-electronic devices are developing in the direction of miniaturization and integration. Polymer composite thin &lms satisfy these requirements because they have some advantages such as quick response, wide transparence, simple manufacture, low cost and ease for integration. Especially with the development of wavelength-division multiplexing all-optical communication network, polymer composite thin &lms have become a major focus of scienti&c research [1–3]. Bismuth titanate, Bi4 Ti3 O12 (BTO) is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices [4] because of its high dielectric constant, high Curie temperature and high breakdown strength. Its refractive index of the 0.5-m-thick &lm is about 2.33 at 520 nm and the dielectric constant at 1 kHz is about 200 [5]. Its Curie temperature is as high as 676◦ C [6]. The &lm deposited on MgO (1 1 0) shows large electro-optic characteristics with an e:ect electro-optic coeEcient of about 3:8 × 10−15 m2 =V2 [7]. ∗
Corresponding author. Tel.: +86-531-8364565; fax: +86-5318565403. E-mail addresses: [email protected] (H. Yang), [email protected] (Q. Ren).
In our research, BTO nano-crystals were selected as the nonlinear chromophore and the transparent polymethylmethacrylate (PMMA) was used as polymer host. We compounded BTO nano-crystals, the structure and size of which were analyzed by X-ray di:raction (XRD) and transmissive Electron microscopy (TEM). Two sorts of composite thin &lms of nano-crystal BTO and polymer PMMA, in which the weight concentrations of BTO were 10% and 20%, were prepared by spin-coating method. Polymers have attracted a lot of attention due to possible applications to optical interconnects and integrated devices in both optical &ber communication and computer network systems. However, polymers have high optical loss. In this work the transmission losses due to scattering in BTO/PMMA composite &lms were measured by using an imaging technique [8] set up by ourselves.
2. Experiment Nanocrystalline BTO has been synthesized by the CSD technique. First bismuth nitrate penta-hydrate [Bi(NO3 )3 · 5H2 O] was dissolved in 2-methoxyethanol. Then titanate butoxide [Ti(OC4 H9 )4 ] was added into the solution to form BTO precursor solution under stirring. The solution was baked to vaporize the solvent. So BTO ultra&ne precursor
0030-3992/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(03)00021-5
292
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
Fig. 1. The setup for measuring transmission loss.
powder was obtained. Finally the dried powder was annealed by rapid thermal processing (RTA) at 520◦ C for 10 min and transformed into nanocrystalline BTO. The crystal structure of the microcrystals was identi&ed by XRD with CuK 1 (= 0:15406 nm) radiation operated at 40 kV and 20 mA. The size of BTO microcrystals was evaluated by TEM (H-800 Vitachi). Two sorts of BTO/PMMA composite thin &lms were prepared by spin-coating method. The weight ratio of BTO microcrystals to the polymer PMMA is 10% and 20%, respectively. In the following, we use B10 to represent the &lm in which the ratio of BTO is 10% and B20 the &lm in which the ratio of BTO is 20%. After PMMA was dissolved into monochlorobenzene, BTO microcrystals were put into the solution and the mixture was blended quite well under the condition of supersonic. After BTO microcrystals were dispersed after 20 min, the solution was directly deposited onto a indium-tin-oxide (ITO) glass substrate by spin coating at 2000 r=min for 40 s. So BTO/PMMA composite thin &lm samples were prepared. The transmission loss of the composite thin &lm is one of the very important property parameters. The precise evaluation of the propagation characteristics of the &lm optical waveguides will provide reliable basis for the design of electro-optic devices. At present, several techniques have been proposed to measure the transmission loss of planar waveguides. The optical-&ber probe, reported &rstly by Goell and Stanley [9], exhibits very good spatial resolution due to the small cross section of the probe. However, since it must be moved along the light streak while maintaining a constant distance to the waveguide surface, the system becomes diEcult to align. Wang [10] proposed to use a Coblentz mirror which o:ers stable, accurate measurements with a relatively high signal-to-noise ratio by collecting light in a complete hemisphere. The outstanding problems are the fabrication of the Coblentz mirror, the positioning of the waveguide under test and the detector at the center of the hemisphere, and the construction of a mechanism to translate the sliding mask. In our research, a new experimental apparatus (see Fig. 1) for measuring scattering loss in thin &lms was set up using an imaging technique. It involves
Fig. 2. The XRD pattern of BTO nano-crystal.
a digital camera, a corresponding data-processing software and a prism-coupling system. A He–Ne laser (633 nm) was used as the light source. A beam of it was focused by a lens and coupled into the waveguide &lm by a right-angle glass (ZLaF1) coupling prism. The &lm was kept in close contact with the prism base. The prism was mounted on a rotation stage, which was used to orient the focused laser beam with respect to the coupling area in order to excite a particular propagation mode. A digital camera was set to observe the light streak scattered out of the thin &lm. The camera was hooked up with the serial port of a microcomputer by the special transmission line, and the light streak images were obtained by running the software for downloading image. 3. Results and discussion The crystal structure of BTO microcrystals was identi&ed by XRD. Fig. 2 shows the XRD pattern of BTO powder. The peaks (1 7 1), (2 0 0), (1 7 3), (1 1 1), etc. have appeared. It conforms to the standard XRD pattern of BTO. The size of BTO microcrystals was evaluated by TEM. Fig. 3 is the TEM photograph of BTO microcrystals. It is known from the picture that most of the microcrystals are spheric and their distribution is uniform. The size di:erence of BTO microcrystals is small and most of them have an diameter of about 20 nm. From the above analysis, it is known that the samples synthesized by the CSD technique are good quality BTO nano-crystals. The transmission loss in thin &lm waveguides is de&ned as the attenuation of the output optical intensity to the input one. In the composite thin &lm waveguides the principal transmission loss is due to scattering, whose intensity is directly related to the waveguide loss. As light transmits in composite thin &lm, its energy decreases exponentially with increasing transmission distance, so we have P0 1 Px = P0 exp(−x); = ln (cm)−1 ; (1) x Px
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
293
Fig. 5. The three-dimensional pro&le of the scattered intensity. Fig. 3. The TEM photograph of BTO nano-crystal.
4.5
log (Px) (a. u.)
4.0
3.5
3.0
2.5
Fig. 4. The streak image.
L=−
10 Px (dB=cm): lg x P0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Propagation Length (cm)
(2)
Eq. (2) can be written as lg Px − lg P0 L = −10 × (dB=cm); (3) x−0 where x is the distance between probing point and input coupling point, P0 is the light intensity at the input coupling point, Px is the light intensity at the probing point, is the attenuation coeEcient and L is the scattering loss. Fig. 4 shows the streak image taken by a digital camera. From the &gure it can be seen that the light intensity decreases with the propagation of the light in &lm, which can be described in the form of exponential attenuation. So it conforms to the universal Equation (1). At the same time, the light brightness oscillates during the light propagation, which is an experimental phenomenon and has been observed in other several samples. To explain the phenomenon satisfactorily, much more discussion and work need to be done. After the images are downloaded by the microcomputer, they are &rst processed by Photoshop. Then using Borland C++, the optical intensity values of each point on
Fig. 6. The scattering loss curve for BTO/PMMA &lm.
the image can be obtained. The pro&le of the scattered intensity versus the image cell can be plotted. Fig. 5 shows the three-dimensional pro&le of the scattered intensity. Here we selected x-axis as the transmission direction of light in the &lm, y-axis as the direction in the &lm perpendicular to the transmission direction of light and z-axis as the scattered power correspondingly. By integrating the scattered intensity over y-axis, the two-dimensional scattered intensity pro&le along x-axis was obtained. According to the scale put before the glass substrate when taking pictures, the image cells can be converted into the real length. Then the pro&le of the scattering intensity versus the propagation length can be obtained. If the pro&le of the common logarithm of scattered intensity versus propagation length is line &tted according to the formula y = A + Bx in Origin, the value of B can be obtained. From Eq. (3), the value of minus ten times B is the scattering loss. Fig. 6 shows the scattered loss curve of BTO/PMMA thin &lm, obtained by using the linear &tting. By doing so, the scattering losses of &lms
294
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
are measured. The scattering loss of B10 &lm at 633 nm is 0:7198 dB=cm and the loss of B20 &lm is 1:8203 dB=cm. 4. Conclusion Now polymers have drawn increasing attention for their potential as cost e:ective waveguide material for integrated optic devices. In this paper, the polymer PMMA was selected as host owing to its well known optical properties. The BTO nano-crystals were compounded using the CSD technique. The structure and size of BTO nano-crystals were analyzed by XRD and TEM. The polymer composite &lms BTO/PMMA were prepared by spin-coating method. A new experimental apparatus for measuring scattering loss in thin &lms was set up using an imaging technique. The scattering loss in thin &lms was obtained by processing the images taken by CCD camera. The scattering loss of B10 &lm at 633 nm is 0:7198 dB=cm and the loss of B20 &lm is 1:8203 dB=cm. The transmission loss increases when more BTO microcrystals are added into the polymer PMMA. This phenomenon is consistent with the theoretical analysis. The imaging technique has no mechanical operation in the system. So it is a nondestructive measurement. It better excludes the inRuences of the surroundings and the instability of lasers than the two methods mentioned above. But there are also several factors which have e:ect on the measurement accuracy. The scale should be parallel to the propagation light in the &lm. Otherwise it will greatly inRuence the measurement accuracy. And we can take several photos once and average them to get more reliable result. On the whole, the imaging technique is a convenient and accurate method to measure the light transmission loss in &lms. Of course some need to be improved and we will do more work to make the technique perfect.
Acknowledgements The authors acknowledge the &nancial support of the Chinese State Key Laboratory of Crystal Materials, the National Natural Science Foundation (Grant Nos. 60077016 and 50272037) and the 863 National Plan (Grant No. 2002AA313070) of China.
References [1] Service RF. Nonlinear competition heats up. Science 1995; 267(5206):1918–21. [2] Roberts CC, Yang GR, et al. High electro-optic side-chain polymer by vapor deposition polymerization. Appl Phys Lett 1996;68(15): 2067–9. [3] Schildkraut JS. Photoconducting electro-optic polymer &lms. Appl Phys Lett 1991;58(4):340–2. [4] Jiang AQ, Li GH, Zhang LD. Dielectric study in nanocrystalline Bi4 Ti3 O12 prepared by chemical coprecipitation. J Appl Phys 1988;83(9):4878–83. [5] Joshi PC, Mansingh A, et al. Structural and optical properties of ferroelectric Bi4 Ti3 O12 thin &lms by sol–gel method. Appl Phys Lett 1991;59(19):2389–90. [6] Fouscova A, Cross LE. Electrical and optical properties of ferroelectric Bi4 Ti3 O12 single crystals. J Appl Phys 1968;39(5): 2268–74. [7] Jo W, Cho J, et al. Studies on structural and electro-optic properties of ferroelectric bismuth titanate thin &lms. Ferroelectrics 1994;152: 139–44. [8] Okamura Y, Yoshinaka S, et al. Measuring mode propagation losses of integrated optical waveguides: a simple method. Appl Opt 1983;22(23):3892–4. [9] Gleine W, MSuller J. Low-pressure chemical vapor deposition silicon-oxynitride &lms for integrated optics. Appl Opt 1992;31(12):2036–40. [10] Wang H. Measurement of optical waveguide scattering loss: an improved method by the use of a Coblentz mirror. Appl Opt 1994;33(9):1707–14.
Optics & Laser Technology 35 (2003) 291 – 294 www.elsevier.com/locate/optlastec
Preparation and transmission loss of the nano-crystal and polymer composite &lm BTO/PMMA Hongliang Yanga;∗ , Quan Rena , Shiyi Guob , Guanghui Zhangb b State
a Department of Optics, Shandong University, Jinan 250100, People’s Republic of China Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China
Received 9 September 2002; accepted 14 January 2003
Abstract Bismuth titanate, Bi4 Ti3 O12 (BTO), is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices. Its nano-crystals were compounded by the chemical solution decomposition technique. The structure and size of BTO were analyzed by X-ray di:raction and transmissive electron microscopy. Two sorts of composite &lms BTO/polymethylmethacrylate with di:erent weight ratio of BTO were prepared by spin-coating method at certain conditions. In this article, the scattering loss in thin &lms was obtained using an imaging technique. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Nano-crystal; Transmission loss; Imaging technique
1. Introduction In recent years, the optical communication and optical information processing have made great progress. The photo-electronic devices are developing in the direction of miniaturization and integration. Polymer composite thin &lms satisfy these requirements because they have some advantages such as quick response, wide transparence, simple manufacture, low cost and ease for integration. Especially with the development of wavelength-division multiplexing all-optical communication network, polymer composite thin &lms have become a major focus of scienti&c research [1–3]. Bismuth titanate, Bi4 Ti3 O12 (BTO) is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices [4] because of its high dielectric constant, high Curie temperature and high breakdown strength. Its refractive index of the 0.5-m-thick &lm is about 2.33 at 520 nm and the dielectric constant at 1 kHz is about 200 [5]. Its Curie temperature is as high as 676◦ C [6]. The &lm deposited on MgO (1 1 0) shows large electro-optic characteristics with an e:ect electro-optic coeEcient of about 3:8 × 10−15 m2 =V2 [7]. ∗
Corresponding author. Tel.: +86-531-8364565; fax: +86-5318565403. E-mail addresses: [email protected] (H. Yang), [email protected] (Q. Ren).
In our research, BTO nano-crystals were selected as the nonlinear chromophore and the transparent polymethylmethacrylate (PMMA) was used as polymer host. We compounded BTO nano-crystals, the structure and size of which were analyzed by X-ray di:raction (XRD) and transmissive Electron microscopy (TEM). Two sorts of composite thin &lms of nano-crystal BTO and polymer PMMA, in which the weight concentrations of BTO were 10% and 20%, were prepared by spin-coating method. Polymers have attracted a lot of attention due to possible applications to optical interconnects and integrated devices in both optical &ber communication and computer network systems. However, polymers have high optical loss. In this work the transmission losses due to scattering in BTO/PMMA composite &lms were measured by using an imaging technique [8] set up by ourselves.
2. Experiment Nanocrystalline BTO has been synthesized by the CSD technique. First bismuth nitrate penta-hydrate [Bi(NO3 )3 · 5H2 O] was dissolved in 2-methoxyethanol. Then titanate butoxide [Ti(OC4 H9 )4 ] was added into the solution to form BTO precursor solution under stirring. The solution was baked to vaporize the solvent. So BTO ultra&ne precursor
0030-3992/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(03)00021-5
292
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
Fig. 1. The setup for measuring transmission loss.
powder was obtained. Finally the dried powder was annealed by rapid thermal processing (RTA) at 520◦ C for 10 min and transformed into nanocrystalline BTO. The crystal structure of the microcrystals was identi&ed by XRD with CuK 1 (= 0:15406 nm) radiation operated at 40 kV and 20 mA. The size of BTO microcrystals was evaluated by TEM (H-800 Vitachi). Two sorts of BTO/PMMA composite thin &lms were prepared by spin-coating method. The weight ratio of BTO microcrystals to the polymer PMMA is 10% and 20%, respectively. In the following, we use B10 to represent the &lm in which the ratio of BTO is 10% and B20 the &lm in which the ratio of BTO is 20%. After PMMA was dissolved into monochlorobenzene, BTO microcrystals were put into the solution and the mixture was blended quite well under the condition of supersonic. After BTO microcrystals were dispersed after 20 min, the solution was directly deposited onto a indium-tin-oxide (ITO) glass substrate by spin coating at 2000 r=min for 40 s. So BTO/PMMA composite thin &lm samples were prepared. The transmission loss of the composite thin &lm is one of the very important property parameters. The precise evaluation of the propagation characteristics of the &lm optical waveguides will provide reliable basis for the design of electro-optic devices. At present, several techniques have been proposed to measure the transmission loss of planar waveguides. The optical-&ber probe, reported &rstly by Goell and Stanley [9], exhibits very good spatial resolution due to the small cross section of the probe. However, since it must be moved along the light streak while maintaining a constant distance to the waveguide surface, the system becomes diEcult to align. Wang [10] proposed to use a Coblentz mirror which o:ers stable, accurate measurements with a relatively high signal-to-noise ratio by collecting light in a complete hemisphere. The outstanding problems are the fabrication of the Coblentz mirror, the positioning of the waveguide under test and the detector at the center of the hemisphere, and the construction of a mechanism to translate the sliding mask. In our research, a new experimental apparatus (see Fig. 1) for measuring scattering loss in thin &lms was set up using an imaging technique. It involves
Fig. 2. The XRD pattern of BTO nano-crystal.
a digital camera, a corresponding data-processing software and a prism-coupling system. A He–Ne laser (633 nm) was used as the light source. A beam of it was focused by a lens and coupled into the waveguide &lm by a right-angle glass (ZLaF1) coupling prism. The &lm was kept in close contact with the prism base. The prism was mounted on a rotation stage, which was used to orient the focused laser beam with respect to the coupling area in order to excite a particular propagation mode. A digital camera was set to observe the light streak scattered out of the thin &lm. The camera was hooked up with the serial port of a microcomputer by the special transmission line, and the light streak images were obtained by running the software for downloading image. 3. Results and discussion The crystal structure of BTO microcrystals was identi&ed by XRD. Fig. 2 shows the XRD pattern of BTO powder. The peaks (1 7 1), (2 0 0), (1 7 3), (1 1 1), etc. have appeared. It conforms to the standard XRD pattern of BTO. The size of BTO microcrystals was evaluated by TEM. Fig. 3 is the TEM photograph of BTO microcrystals. It is known from the picture that most of the microcrystals are spheric and their distribution is uniform. The size di:erence of BTO microcrystals is small and most of them have an diameter of about 20 nm. From the above analysis, it is known that the samples synthesized by the CSD technique are good quality BTO nano-crystals. The transmission loss in thin &lm waveguides is de&ned as the attenuation of the output optical intensity to the input one. In the composite thin &lm waveguides the principal transmission loss is due to scattering, whose intensity is directly related to the waveguide loss. As light transmits in composite thin &lm, its energy decreases exponentially with increasing transmission distance, so we have P0 1 Px = P0 exp(−x); = ln (cm)−1 ; (1) x Px
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
293
Fig. 5. The three-dimensional pro&le of the scattered intensity. Fig. 3. The TEM photograph of BTO nano-crystal.
4.5
log (Px) (a. u.)
4.0
3.5
3.0
2.5
Fig. 4. The streak image.
L=−
10 Px (dB=cm): lg x P0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Propagation Length (cm)
(2)
Eq. (2) can be written as lg Px − lg P0 L = −10 × (dB=cm); (3) x−0 where x is the distance between probing point and input coupling point, P0 is the light intensity at the input coupling point, Px is the light intensity at the probing point, is the attenuation coeEcient and L is the scattering loss. Fig. 4 shows the streak image taken by a digital camera. From the &gure it can be seen that the light intensity decreases with the propagation of the light in &lm, which can be described in the form of exponential attenuation. So it conforms to the universal Equation (1). At the same time, the light brightness oscillates during the light propagation, which is an experimental phenomenon and has been observed in other several samples. To explain the phenomenon satisfactorily, much more discussion and work need to be done. After the images are downloaded by the microcomputer, they are &rst processed by Photoshop. Then using Borland C++, the optical intensity values of each point on
Fig. 6. The scattering loss curve for BTO/PMMA &lm.
the image can be obtained. The pro&le of the scattered intensity versus the image cell can be plotted. Fig. 5 shows the three-dimensional pro&le of the scattered intensity. Here we selected x-axis as the transmission direction of light in the &lm, y-axis as the direction in the &lm perpendicular to the transmission direction of light and z-axis as the scattered power correspondingly. By integrating the scattered intensity over y-axis, the two-dimensional scattered intensity pro&le along x-axis was obtained. According to the scale put before the glass substrate when taking pictures, the image cells can be converted into the real length. Then the pro&le of the scattering intensity versus the propagation length can be obtained. If the pro&le of the common logarithm of scattered intensity versus propagation length is line &tted according to the formula y = A + Bx in Origin, the value of B can be obtained. From Eq. (3), the value of minus ten times B is the scattering loss. Fig. 6 shows the scattered loss curve of BTO/PMMA thin &lm, obtained by using the linear &tting. By doing so, the scattering losses of &lms
294
H. Yang et al. / Optics & Laser Technology 35 (2003) 291 – 294
are measured. The scattering loss of B10 &lm at 633 nm is 0:7198 dB=cm and the loss of B20 &lm is 1:8203 dB=cm. 4. Conclusion Now polymers have drawn increasing attention for their potential as cost e:ective waveguide material for integrated optic devices. In this paper, the polymer PMMA was selected as host owing to its well known optical properties. The BTO nano-crystals were compounded using the CSD technique. The structure and size of BTO nano-crystals were analyzed by XRD and TEM. The polymer composite &lms BTO/PMMA were prepared by spin-coating method. A new experimental apparatus for measuring scattering loss in thin &lms was set up using an imaging technique. The scattering loss in thin &lms was obtained by processing the images taken by CCD camera. The scattering loss of B10 &lm at 633 nm is 0:7198 dB=cm and the loss of B20 &lm is 1:8203 dB=cm. The transmission loss increases when more BTO microcrystals are added into the polymer PMMA. This phenomenon is consistent with the theoretical analysis. The imaging technique has no mechanical operation in the system. So it is a nondestructive measurement. It better excludes the inRuences of the surroundings and the instability of lasers than the two methods mentioned above. But there are also several factors which have e:ect on the measurement accuracy. The scale should be parallel to the propagation light in the &lm. Otherwise it will greatly inRuence the measurement accuracy. And we can take several photos once and average them to get more reliable result. On the whole, the imaging technique is a convenient and accurate method to measure the light transmission loss in &lms. Of course some need to be improved and we will do more work to make the technique perfect.
Acknowledgements The authors acknowledge the &nancial support of the Chinese State Key Laboratory of Crystal Materials, the National Natural Science Foundation (Grant Nos. 60077016 and 50272037) and the 863 National Plan (Grant No. 2002AA313070) of China.
References [1] Service RF. Nonlinear competition heats up. Science 1995; 267(5206):1918–21. [2] Roberts CC, Yang GR, et al. High electro-optic side-chain polymer by vapor deposition polymerization. Appl Phys Lett 1996;68(15): 2067–9. [3] Schildkraut JS. Photoconducting electro-optic polymer &lms. Appl Phys Lett 1991;58(4):340–2. [4] Jiang AQ, Li GH, Zhang LD. Dielectric study in nanocrystalline Bi4 Ti3 O12 prepared by chemical coprecipitation. J Appl Phys 1988;83(9):4878–83. [5] Joshi PC, Mansingh A, et al. Structural and optical properties of ferroelectric Bi4 Ti3 O12 thin &lms by sol–gel method. Appl Phys Lett 1991;59(19):2389–90. [6] Fouscova A, Cross LE. Electrical and optical properties of ferroelectric Bi4 Ti3 O12 single crystals. J Appl Phys 1968;39(5): 2268–74. [7] Jo W, Cho J, et al. Studies on structural and electro-optic properties of ferroelectric bismuth titanate thin &lms. Ferroelectrics 1994;152: 139–44. [8] Okamura Y, Yoshinaka S, et al. Measuring mode propagation losses of integrated optical waveguides: a simple method. Appl Opt 1983;22(23):3892–4. [9] Gleine W, MSuller J. Low-pressure chemical vapor deposition silicon-oxynitride &lms for integrated optics. Appl Opt 1992;31(12):2036–40. [10] Wang H. Measurement of optical waveguide scattering loss: an improved method by the use of a Coblentz mirror. Appl Opt 1994;33(9):1707–14.