Text-chap8

_______________________________________________8 ELECTROOPTIC DEVICES

The electrooptic effect, which is the change in refractive index with an external applied electric field, is manifested in certain materials which show tremendous promise of applications such as light valves, beam deflectors, and optical displays for optical communications, in conjunction with solid state laser chips and optical fibers. Compared with liquid crystal devices, the ceramic electrooptic components, in general, possess advantages in terms of their response speed (µsec), particularly in falling time, contrast ratio (10 2) and gray scale (16 scales), and their ability to withstand high intensity illumination. On the other hand, the present ceramic components require relatively high drive voltages (1 kV) and production cost ($100). Therefore, the development of a simple mass-production process, electrode configurations with a narrow gap, and improvement of material properties, will be key factors in the actual commercialization of ceramic optical components.

8.1 ELECTROOPTIC EFFECT - REVIEW Let us initially review the operation of a light shutter based on the second-order electrooptic effect (Kerr effect). Birefringence ∆n is induced in a crystal of the appropriate type when an electric field E is applied, according to: ∆n = - (1/2) R n 3 E2,

(8.1)

where R is the quadratic electrooptic coefficient and n is the original refractive index of the crystal. When this sample is placed between crossed polarizers arranged at the 45o direction with respect to the E direction, and light is transmitted through the system as shown in Fig. 8.1, the output light intensity is represented by I = I 0 A sin 2 [(π R n 3 L / 2 λ) E2 ],

(8.2)

where I0 is the incident light intensity, A is an equipment constant, L is the path length (i.e., the sample thickness), and λ is the wavelength of the light. The voltage required for the first intensity maximum is an essential device parameter called the half wave voltage. 221

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Chapter 8 Analyzer

Polarizer

- 45 o

45 o L PLZT

Electric field

Fig. 8.1 Fundamental construction of an electrooptic light shutter.

8.2 TRANSPARENT ELECTROOPTIC CERAMICS Since the 1960s the non-linear polarizability of ferroelectrics has been investigated rather extensively, and various electrooptic and optical parametric devices have been developed. However, problems still remain in preparing high grade opticallyhomogeneous single crystals and, hence, manufacturing costs are generally high. The polycrystalline microstructure of a ferroelectric ceramic can also exhibit the electrooptic effect if it is sintered to a pore-free state to make it transparent. Relaxor ferroelectrics are of special interest for non-linear optic applications because an extraordinarily large apparent electrooptic Kerr effect can be observed even when the material is in its so-called paraelectric state. This section describes the fundamental electrooptic properties of perovskite-type polycrystalline and single crystal ferroelectrics. (1)

(Pb,La)(Zr,Ti)O3

The most useful ferroelectric electrooptic materials have traditionally come from the (Pb,La)(Zr,Ti)O3 system; they generally have good transparency in a wavelength range extending from the visible to infrared, and exhibit optical anisotropy with an applied electric voltage. Figure 8.2 shows the phase diagram of the (Pb 1-xLa x)(Zr 1y Tiy )1-x/4O 3 system, on which is indicated the electrooptic effects manifested for various phase regions. Notice that the valence of lanthanum ion (3+) in the a-site (2+) generates the vacancy of the b-site. The PLZT solid solution exhibits both the Pockels (primary) and Kerr (secondary) electrooptic effects, depending on the composition. Some examples of typical ∆n vs. E curves are shown in Fig. 8.3. The electrooptic coefficients of the PLZT system are much larger than the values in conventional crystals such as LiNbO3 and (Sr,Br)Nb 2 O6 (SBN) (see Table 8.1), which means that the voltage required for the electrooptic shutter is much less for the PLZT.

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Fig. 8.2 Relation between PLZT compostion and structure and electrooptic application.

Fig. 8.3 Polarization P and birefringence ∆n as a function of electric field E for some PLZT ceramics.

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Chapter 8

Table 8.1 materials.

Pockels (1st) and Kerr (2nd) electrooptic coefficients for various

_______________________________________________________________ Material r (x10-10 m/V) _______________________________________________________________ LiNbO3 0.17 Ba2(K0.9Na0.1)Nb5O15 0.52 Primary electrooptic KH2PO4 0.52 coefficient (Sr 0.5Ba0.5)Nb2O6 2.10 PLZT 8/65/35 (GS=10µm) 5.23 PLZT 8/65/35 (GS=3µm) 6.12 _______________________________________________________________ R (x10-16 m2 /V2) _______________________________________________________________ KTa0.65Nb0.35O3 5.30 Secondary electrooptic PLZT 9/65/35 (GS=2µm) 9.12 coefficient PLZT 10/65/35 (GS=2µm) 1.07 _______________________________________________________________

Fig. 8.4 Grain size dependence of the electrooptic coefficients, R and g, for PLZT 9/65/35. Among the various PLZT compositions, 9/65/35 near the triple point between the tetragonal, rhombohedral and cubic phases is of particular interest because it exhibits a large electooptic effect (R = 9.1 x 10-16 m2 V-2 ) and is thus applicable for light shutters and optical displays. However, care must be taken to control grain size. Figure 8.4 shows the grain size dependence of the electrooptic coefficients, R and g (defined as ∆n = - (1/2)g n3 P2 ),

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for PLZT 9/65/35.1) The samples were prepared by hot-press sintering starting from coprecipitated PLZT powders. The electrooptic response is drastically decreased for samples with grain sizes below 2 µm, which corresponds approximately to the critical grain size below which the sample loses ferroelectric properties (maybe antiferroelectric state).2) Therefore, relatively large grain size is necessary to produce a reasonable electrooptic effect. On the other hand, a significant decrease in fracture toughness or durability occurs for particularly large grain size samples, probably due to the B-site vacancies in the crystal structure. A normally sintered transparent PLZT ceramic, with an average grain size of more than 6 µm, has a fracture toughness (mode I for tensile stress) of KIC =0.9 MNm-3/2, which corresponds roughly to durability for 108 cycles of repeated operation.2) This translates to about only 2 months when the PLZT is used for an image display (TV) driven at 30Hz. Example Problem 8.1_________________________________________________ PLZT 10/65/35, with a cubic symmetry shows an electrooptic coefficient (R11 R12 ) of 1.1 x 10 -16 [m2/ V2] and n0 = 2.49. Calculate the half wave electric field for a sample with L = 1 mm, when λ = 633 nm light is transmitted perpendicular to the electric field. Refer to Fig. 8.1. Hint The half wave voltage is calculated from Γy = (p/λ) n 0 3 E3 2 (R 11 - R12 ) L = π.

(P8.1.1)

Solution E3 = (λ / n 0 3 (R 11 - R12) L) 1/2 = (633 x 10-9 / 2.493 x 1.1 x 10 -16 x 1 x 10-3) 1/2 = 6.1 x 10 5 [V/m] . (P8.1.2) ___________________________________________________________________ (2)

Pb(Zn1/3 Nb2/3 )O3

Pb(Zn1/3Nb 2/3)O3 is a relaxor ferroelectric which can be used in single crystal form. Figure 8.5 shows the birefringence ∆n versus electric field E relation for a Pb(Zn1/3Nb 2/3)O3 single crystal in the paraelectric phase.3) The single crystal sample was made by a flux method using excess PbO. The parabolic curve in the low field region becomes a straight line in the high field region.

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Fig. 8.5 Birefringence Pb(Zn1/3Nb 2/3)O3 .

vs.

electric

field

response

of

paraelectric

A possible phenomenological analysis of this peculiar phenomenon is based on the model that the crystal is composed of coexisting ferroelectric and paraelectric phases.3) Suppose that the volume fraction of the paraelectric phase x(T) is given by an accumulated Gaussian distribution with respect to temperature, the birefringence ∆n is estimated by the summation of the linear and quadratic electrooptic effects:4) ∆n = [1 - x(T)] n 3 (r 33 - r 13 ) E/2 + x(T) n 3 R44 E2/2,

(8.3)

where n is the refractive index, and r and R represent the electrooptic Pockels and Kerr coefficients, respectively. Even if the experimental data can be represented phenomenologically, the actual situation may not be so simple as this model predicts as x(T) is also a function of the applied electric field E. Another more realistic description is found in terms of a microscopic domain reversal mechanism. Pb(Zn1/3Nb 2/3)O 3 has very small spindle-like domains (5 µm) with ambiguous boundaries arranged perpendicular to the external electric field. When a field greater than 0.5 kV/mm is applied, the domain walls within a certain region of the sample moves togeter, such that the micro-domains respond to the applied field in a cooperative manner (See Fig. 8.6). 5) It is noteworthy that the stripe period of the dark and bright domains (corresponding to up and down polarizations) will not be changed by domain reversal, and that each domain area

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changes under an AC external field with zero net polarization at zero field. The relaxor crystal can be electrically poled easily when an electric field is applied around the transition temperature, and depoled completely without any remanent polarization. This is the basis of the large "apparent" secondary non-linear effects such as electrostrictive and electrooptic Kerr phenomena, which occur without any hysteresis.

E // <111>

Fig. 8.6 Domain reversal mechanism in Pb(Zn 1/3Nb 2/3)O 3.

228 (3)

Chapter 8 Pb(Mg 1/3 Nb2/3 )O3 -PbTiO3

The development of new ceramic electrooptic materials with higher fracture toughness and larger electrooptic coefficients suitable for image display applications is necessary. The following conditions should be considered: 1) ceramic transparency requires almost zero birefringence in the zero-field state (that is, a pseudo-cubic structure) to suppress light scattering, 2) large fracture toughness may be obtained in a sufficiently dense structure (that is, ion vacancies are not suitable), 3) a large electrooptic effect is manifested by relaxor ferroelectrics. The Pb(Mg1/3Nb 2/3)O3 -PbTiO 3 system, which is known as a superior electrostrictive (secondary effect) material with very high fracture toughness (KIC = 1.7 MNm-3/2 ) is a good candidate for electrooptic applications.6) Samples of the (1 - x) Pb (Mg1/3Nb 2/3)O3 - x PbTiO3 system were prepared by hot-press sintering of oxide mixtures. The Curie temperature increases gradually with PbTiO 3 content, passing room temperature around x = 0.12, and the crystal structure is pseudo-cubic in the region below x = 0.4. Figure 8.7 shows the composition x dependence of optical transmittance (λ = 633 nm) of a 0.5 mm thick sample from the (1 - x) Pb(Mg1/3Nb 2/3)O3 - x PbTiO3 system. The transmittance is reduced drastically above x = 0.14, probably due to scattering caused by the spontaneous birefringence. The best transmittance 49% is still smaller than the 62% observed for PLZT. This suggests that a more sophisticated powder preparation technique will be required for fabricating adequately transparent PMN-PT ceramics.

Fig. 8.7 Transmittance of a 0.5 mm thick sample of (1 - x) Pb(Mg1/3Nb 2/3)O 3- x PbTiO3 (λ = 633 nm).

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The refractive index n (λ = 633 nm) is plotted as a function of composition x in Fig. 8.8, and shows a small maximum around x = 0.12. The values are slightly larger than the n = 2.49 of PLZT 10/65/35. The most interesting data come from electrooptic measurements. Figures 8.9(a) and 8.9(b) show the electrooptic R coefficient and its corresponding hysteresis for λ = 633 nm, respectively, plotted as a function of composition x. The maximum electrooptic R coefficient of 22 x 10-16 m2 V-2 for x = 0.12 is more than twice that of PLZT 9/65/35 (9.1 x 10-16 m2 V-2 ). The hysteresis, defined as an equivalent coercive electric field obtained from the experimental ∆n curve, increases drastically above x = 0.16 and, hence, samples in this region cannot be used practically.

Fig. 8.8 Refractive index as a function of composition x for (1 - x) Pb(Mg1/3 Nb 2/3)O 3 - xPbTiO3.

Changes in the electrooptic coefficient R (a) and the corresponding Fig. 8.9 hysteresis (b) in (1 - x)Pb(Mg 1/3Nb 2/3)O 3 - xPbTiO3.

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Chapter 8

The data indicate that the 0.88Pb(Mg1/3Nb 2/3)O 3-0.12PbTiO3 has the potential to be a better electrooptic ceramic than PLZT with high mechanical toughness. Higher optical transmittance must be achieved, however, by optimizing the fabrication process.

8.3 BULK ELECTROOPTIC DEVICES (1)

Ferpic

One of the earliest applications is Ferpic (Ferroelectric Picture Memory Device). Figure 8.10 shows the principle of the Ferpic.7) Initally, a PLZT 7/65/35 ceramic plate is uniformly DC-poled laterally [see Fig. 8.10(a)]. Then, storage is achieved by switching domains at points corresponding to the image's high-intensity regions. To switch domains, a high-contrast transparency is placed in front of the Ferpic and illuminated [Fig. 8.10 (b)], creating low-impedance regions in the photoconductive film. The writing voltage supply will then cause switching in these regions only. Viewing/reading the memorized image is accomplished by passing polarized light through the Ferpic and an analyzer as shown in Fig. 8.10(c). When the polarizer and analyzer are parallel, the regions with remnant polarization normal to the plate produce a bright image, and the other regions produce a dark image. (2)

Eye Protection Application

Sandia National Laboratories designed PLZT goggles for the U.S. Air Force to provide thermal and flashblindness protection for aircraft personnel.8) The goggle is basically a transverse-mode shutter using an interdigital surface electrode configuration similar to that shown in Fig. 8.11. (3)

Stereo TV Application

PLZT eye glasses for stereo TV (see Fig. 8.11) have been fabricated using the light shutter principle. 9) The lenses consist of a pair of optically isotropic PLZT (9/65/35) discs sandwiched between two crossed polarizers. When zero voltage is present between the electrodes, light will not be transmitted. The transmitted light intensity increases with increasing applied voltage, and reaches a maximum when a phase difference (retardation) of 180o is induced in the PLZT disc (at the half-wave voltage). Stereo TV images of an object are taken by two video cameras corresponding to the two eyes and the signal from each camera is mixed alternately to make a frame for the right and left eyes. When viewing, the right and left PLZT shutters are triggered synchronously to each image frame, resulting in a stereo image.

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Fig. 8.10 Principle of Ferpic: (a) initial DC poling, (b) writing process using a photoconductive film, (c) reading process using a pair of parallel polarizers.

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TV cameras left

right

left image

right image

PLZT glasses

Fig. 8.11 A stereo TV system using a pair of PLZT glasses.

(4)

Two-Dimensional Displays

The current requirements for high definition TV are stringent, and several systems have been proposed. One of the promising devices is a projection type TV utilizing one-dimensional10) or two-dimensional PLZT displays.11) The operating principle for a projection TV utilizing two-dimensional PLZT displays is introduced in this section. The development of a simple mass-production process and the design of electrode configurations with a narrow gap are the key factors in the production of the PLZT displays. A design shown in Fig. 8.12 produces a very bright image with small crosstalk-related problems and is easy to manufacture. Fabrication Process of the 2-D Display The fabrication process for the two-dimensional PLZT light valve array is outlined in Fig. 8.13. 11) Coprecipitated PLZT 9/65/35 powders were mixed with organic solvent and binder and formed into a green sheet. Platinum internal electrodes were printed on the green sheets. The electroded sheets were then laminated, with the electrodes alternating by 90o between sheets, under a pressure of 3000 psi. The laminated body was sintered in an oxygen-controlled atmosphere, and the bulk was cut and polished. Finally the external electrodes were applied for vertical and horizontal addressing for individual pixels on the display.

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Fig. 8.12 A two-dimensional PLZT optical display: (a) a matrix segment and (b) detail of an activated portion of the display.

Figure 8.14(a) shows the electrode configuration of a (10x10) matrix light valve. The shaded portion of the device in the figure represents one image unit (pixel). The separated internal electrodes are connected by external electrodes printed on the surface of the device. The continuous (plate-through) electrodes are embedded 100 µm below the optical surface to avoid shorting with the surface electrodes connecting the separated internal electrodes. Figure 8.14(b) shows a picture of an actual display. Note that the layer thickness is about 0.35 mm. Characteristics of the light valve array The optical transmittance of the PLZT device fabricated by the tape casting technique was 62% at λ = 633 nm, which is comparable with 63% transmittance for the ideal bulk sample prepared by hot-pressing. The brightness for red, green and blue light was measured as a function of applied voltage (Fig. 8.15), where the electrode gap was 0.45 mm. 11) The contrast ratio, defined as the ratio of the brightness on a screen with the application of the half-wave voltage to the brightness with zero volt applied (220 cd/m2/2.8 cd/m2 ), of about 80, is superior to the values for conventional cathode ray tubes (CRT) or liquid crystal displays (LCD). The response time (both rising and falling) of a single pixel on the display is less than 10 µsec, which is rapid enough to drive this shutter array at a raster frequency comparable to the conventional CRT.

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Chapter 8

Fig. 8.13 Fabrication process for the two-dimensional PLZT optical display.

Construction of the Image Projector The driving circuit for the display is shown schematically in Fig. 8.16(a). When the terminals of the device are addressed as shown in Fig. 8.16(b), the image appearing in Fig. 8.16(c) (letter "F") is generated on the screen.11)

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Fig. 8.14 (a) Schematic electrode configuration of a (10x10) matrix PLZT light valve. (b) Top view photograph of a PLZT light valve array with external electrodes.

Fig. 8.15 Brightness on a screen vs. applied voltage for red, green or blue light. Note that the half-wave voltage differs for these three lights.

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Chapter 8

Fig. 8.16 (a) Driving circuit for the two-dimensional display. (b) The driving signal waveforms of the driving signal. (c) An image of "F" on a screen illuminated through the PLZT projector.

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Fig. 8.17 Crosstalk test system. The light through a slit focused on the screen is measured.

Fig. 8.18 Crosstalk patterns for three different input combinations: (a) vertical type, (b) oblique type and (c) complex type.

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Chapter 8

Crosstalk was monitored on the 2-D display using the setup shown in Fig. 8.17 with monochromatic light.6) The test was made by keeping one vertical terminal (separated electrode) on (i.e., Ground) and applying high voltage to multiple horizontal terminals (continuous plate-through electrodes) simultaneously. There are three different crosstalk patterns: vertical, horizontal and oblique types; that is, light leakage observed at vertically, horizontally and obliquely adjacent pixels. The results are shown in Fig. 8.18(a)-(c) for three different input combinations, where the top and bottom of figures in a pixel indicate the light intensity in µW on the screen for the ON and OFF state, respectively. The leakage light intensity associated with the vertical and horizontal crosstalk is 20 - 30% and 10 - 20% of the main-peak intensity, respectively, which does not affect the image contrast significantly. On the other hand, oblique type crosstalk causes non-negligible leakage, up to 50% depending on the applied voltage and the number of continuous electrodes addressed (horizontal address) (called combination type crosstalk). Modification of the internal electrode configurations is necessary to eliminate the crosstalk problem completely. Example Problem 8.2_________________________________________________ In Fig. 8.15, the first maximum in the light intensity is obtained at different voltages for red, green and blue light; 160 V for red, 150 V for green and 130 V for blue. (1) Explain the reason physically. (2) Supposing that the refractive index n (= 2.49) and the electrooptic coefficient (R 11 - R12) (3.6 x 10-16 [m2/ V2]) does not change significantly for each light, calculate the wavelength of these three lights. Hint The half wave voltage is calculated from Γy = (p/λ) n 0 3 E3 2 (R 11 - R12 ) L =π.

(P8.2.1)

Solution (1) Since the half wave voltage is provided by Eq. (P8.2.1), according to the illumination light wavelength, the required voltage differs: for shorter wavelengths, a smaller electric field is required. λ = n 0 3 E32 (R 11 - R12) L .

(P8.2.2)

(2) Taking into account the electrode gap of 0.45 mm, E3 = 3.55, 3.33 and 2.89 x 10 5 [V/m] for R, G and B, respectively, and a pathlength L given by (1.0 - 0.1) mm (note that the surface depth 0.1 mm is an inactive layer):

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λ = 2.493 x (3.55x105 )2 (3.6 x 10-16 ) (0.9 x 10-3) = 630 [nm] (for red), λ

= 555 [nm] (for green),

λ = 418 [nm] (for blue). (P8.2.3) ___________________________________________________________________

8.4 WAVEGUIDE MODULATORS Light waveguides can be fabricated by depositing a high-refractive index layer on a substrate. The principle of the waveguide is shown schematically in Fig. 8.19.12) Like an optical fiber, the light tends to bend toward high refractive-index side, so that the light should be confined in the narrow high refractive-index layer fabricated on the crystal. Nb-diffused LiNbO3 single crystals are commonly used. Figures 8.20(a) and 8.20(b) are typical planar and ridge type electrooptic waveguides.13) The fabrication of a planar type is easy, but the nonuniform distribution of the applied electric field is a problem. On the other hand, as you can imagine, the ridge type requires sophisticated manufacturing technology, but the device function is close to the ideal. The transmitted light intensity is easily modulated by applying a relatively low voltage. Phase modulation by 1 radian can be achieved by applying a voltage of 0.3 V with power consumption of several µW/MHz.

Fig. 8.19 Diagrams of (a) slab and (b) graded-index waveguides. The wavefunctions for the TE0 and TE1 modes are shown in the refractive-index profiles.12)

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Chapter 8

Fig. 8.20 Electrooptic waveguides: (a) planar-type and (b) ridge-type.12)

CHAPTER ESSENTIALS_________________________________ 1.

Relaxor ferroelectrics are widely applicable for electrooptic light valve/display applications. The superior characteristics of these materials are attributed primarily to the easy poling of the ferroelectric micro-domains.

2.

A new electrooptic ceramic 0.88Pb(Mg 1/3Nb 2/3)O 3 -0.12PbTiO3 with high mechanical toughness is one of the more promising new materials for longlifetime display applications.

3.

A new type of PLZT two-dimensional light valve, fabricated by a tape casting technique, is one excellent example of a design well-suited to mass-production at a low manufacturing cost.

4.

Light waveguides can be fabricated by depositing a high refractive index layer on a substrate such as LiNbO3 . ___________________________________________________________________

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CHAPTER PROBLEMS 8.1

Let us consider a PLZT thin film (1 µm in thickness) deposited on a glass plate with the following two electrode configurations: (a) surface electrode for lateral electric field and (b) surface electrode for normal electric field.

Light direction (a)

PLZT film

Light direction (b)

(1) Discuss the merits and demerits of the above two electrode configurations. (2) Suppose that the electrode in part (b) of the figure above is made of a transparent material such as SnO2. Do you think the device will work for light transmitted normal to the film? Hint Consider the birefringence and the shape of the optical indicatrix induced by the electric field. 8.2

Consider Kerr electrooptic effect for a crystal with m3m symmetry. (1) Derive a secondary electrooptic coefficient matrix for this symmetry. (2) Discuss the change in the optical ni dicatrix shape (refractive index ellipsoid), when an electric field is applied along the z axis. (3) Calculate the retardation, when the light is transmitted perpendicular to the electric field, that is, along the y axis. The pathlength is L.

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Hint The electrooptic coefficient matrix is given as

R 11 R 12 R 12 0 0 0 R 12 R 11 R 12 0 0 0 R 12 R 12 R 11 0 0 0 0

0

0 R

0 0

0 0

0 0

44 0

0

0 R 44 0 0 0 R 44

REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

K. Tokiwa and K. Uchino: Ferroelectrics 94,87 (1989). K. Uchino and T. Takasu: Inspec. 10, 29 (1986). F. Kojima, J. Kuwata and S. Nomura: Proc. 1st. Mtg. on Ferroelectric Mater. & Appl. (Kyoto) p.155 (1977). J. Kuwata, K. Uchino and S. Nomura: Ferroelectrics 22, 863 (1979). R. Ujiie and K. Uchino: Proc. IEEE Ultrasonic Sympl (Hawaii) p.725 (1990). K. Uchino: Ceramics International 21, 309 (1995). L. M. Levinson edit.: Electronic Ceramics, Marcel Dekker (New York), Chap.7, p.371 (1988). J. T. Cutchen: Proc. 49th Annual Sci. Mtg. Aerospace Medical Assoc., New Orleans, May (1978). A. Kumada, K. Kitta, K. Kato and T. Komata: Proc. Ferroelectric Mater. & Appl., 2, p.205 (1977). K. Murano: Ceramic Transactions 14 Electro-Optics and Nonlinear Optic Materials, p.283 (1990). K. Uchino, K. Tokiwa, J. Giniewicz, Y. Murai and K. Ohmura: Ceramic Transactions 14 Electro-Optics and Nonlinear Optic Materials, p.297 (1990). M. E. Lines and A. M. Glass: Principles and Applications of Ferroelectrics and Related Materials, p. 604, Clarendon Press, Oxford (1977). I. P. Kaminov: Trans. IEEE, M. T. T. 23, 57 (1975).