Light transmittance memory effect of lead lanthanum zirconate titanate ...

APPLIED PHYSICS LETTERS 93, 192102 共2008兲

Light transmittance memory effect of lead lanthanum zirconate titanate induced by the electrical imprint field Toshinori Ohashi,a兲 Hiroshi Hosaka, and Takeshi Moritab兲 Department of Human Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan

共Received 11 August 2008; accepted 10 October 2008; published online 10 November 2008兲 The possibility of inducing a light transmittance memory effect in a ferroelectric material was investigated. Lead lanthanum zirconate titanate was examined for a light transmittance memory effect, and it was confirmed that the light transmittance had two stable values in the absence of an electrical field after control of the imprint electrical field. This memory effect was demonstrated as an optical shutter under pulsed voltage operation, resulting in decreased energy consumption and simple operation. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3021068兴 Lead lanthanum zirconate titanate 共PLZT兲 is a transparent ferroelectric material that has various nonlinear electrooptic effects, such as variable birefringence and variable light scattering,1–4 and has been applied for optical scanners, optical switches, and optical shutters.5–8 Conventional operation of these devices requires a continuous external electrical field, so as to maintain the certain optical value, and the polarization direction of the ferroelectric material is not reversed. These optical devices consume a significant amount of electrical power and require complicated drivers for continuous application of the electrical field. Recently, our group has focused on memory effects by utilizing an imprint electrical field.9–11 An imprint electrical field is observed mainly in ferroelectric thin films and most investigations have focused on removal of the imprint electrical field.12 We have indicated that the various properties of ferroelectric materials, such as strain, permittivity, and light transmittance, can obtain memory effects in the presence of the imprint electrical field.9–11 The imprint electrical field can be intentionally induced by applying a high voltage electrical field to the ferroelectric material at a high temperature over a long period. A shape memory piezoelectric actuator and refractive index memory optical switch had been demonstrated using this technique.9–11 In this study, a light transmittance memory effect is proposed using PLZT as an optical shutter or optical switch. The optical memory can be realized by pulsed operation and enables the on or off states to be maintained in the absence of an external electrical field; thus, the energy consumption decreases, and simple operation becomes possible. The light transmittance PLZT changes by application of an electrical field due to its variable light scattering effect. The principle of this effect is based on discontinuities of the refractive index at domain and grain boundaries. Variations of ferroelectric domain size, the density of strain relief at the 71° and 109° domains, and the proportion of rhombohedral to tetragonal grains and domains as a function of polarization induce the variable light scattering effect.1 The relationship between the light transmittance and the intensity of the electrical field is described by a butterfly-shaped curve, as shown

in Fig. 1. The light transmittance of PLZT becomes a maximum when the electrical field matches the coercive electrical field. At the initial state, the butterfly-shape of the light transmittance curve is symmetric against the electrical field; thus, there is no memory effect, even with polarization reversal. However, in the presence of an imprint electrical field, the relationship between light transmittance and the applied electrical field shifts to the horizontal axis, as shown in Fig. 1, and then two stable light transmittances can be maintained depending on the direction of polarization. The imprint electrical field is sometimes observed in research on the ferroelectric thin films for ferroelectric random access memory 共FeRAM兲 devices.13 The residual strain or the lattice defects at the boundary surface must be related to this imprint electrical field; however, the conclusive mechanism of this imprint electrical field is still unclear. The imprint electrical field is a serious problem in FeRAM research; however, it is the principle phenomenon exploited in this investigation. An optical shutter was fabricated using PLZT, and in the presence of an imprint electrical field, it has two stable light transmittances in the absence of an external electrical field. Each light transmittance value can be selected by controlling the polarization direction via adjustment of the pulsed voltages. Pulsed voltages can be easily generated from low voltage using a transformer so that a large amplifier is not required.

a兲

Electronic mail: [email protected]. Author to whom correspondence should be addressed. Electronic mail: [email protected].

b兲

0003-6951/2008/93共19兲/192102/3/$23.00

FIG. 1. Principle of the memory effect induced by control of the imprint electrical field.

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© 2008 American Institute of Physics

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Appl. Phys. Lett. 93, 192102 共2008兲

FIG. 2. Photograph of the fabricated optical shutter.

An optical shutter was fabricated using a PLZT plate 共Zr: Ti= 65: 35, La= 8.19%, 5 ⫻ 1.5⫻ 0.5 mm3兲共Fig. 2兲. PLZT exhibits the Pockels 共primary兲 or Kerr 共secondary兲 electro-optic effects, depending on the chemical composition. For the experiment, PLZT, which exhibits the Pockels effect, was used, because the light transmittance characteristics against external electrical field produce a butterflyshaped curve. On one side of the PLZT, a brass block 共5 ⫻ 1.5⫻ 30 mm3兲 was attached with a conductive adhesive 共Fujikura Kasei Dotite 705A兲 as a bottom electrode, and on the other side, the same adhesive was pasted as a rectangular top electrode. After fabricating both electrodes, their side surfaces 共5 ⫻ 0.5 mm2兲 were polished to a mirror finish using a precision polishing machine 共Musashino Denshi MA150兲 for penetration of the laser beam 共0.48 mm spot size兲. An imprint electrical field is sometimes induced during the forming process of ferroelectric thin films, especially epitaxial thin films. The imprint electrical field might be related to residual strain due to lattice mismatching. However, the bulk PLZT does not have an imprint electrical field in the initial condition. In order to induce an imprint electrical field, a 2.5 kV/mm electrical field was applied in the thickness direction for 10 h at 120 ° C in silicone oil. The electrical field was supplied from an amplifier 共NF HVA4321兲 and the silicone oil was heated by an electric hot plate 共IKA RH digital KT/C兲. Research on shape memory piezoelectric actuators has shown that the intensity of the imprint electrical field can be controlled, depending on parameters such as the temperature, the intensity of the electrical field, and the duration.10 The change in the light transmittance of PLZT was measured before and after the electrical imprint field treatment. A laser 共633 nm, 0.8 mW, 0.48 mm spot Edmund 61318-I兲 beam was passed through the PLZT, and the electrical voltage was supplied from a function generator 共NF WF1974兲 through a high-speed amplifier 共NF 4010, M-2601兲. A photodiode 共Edmund 54522-I兲 was set approximately 300 mm from the PLZT to detect the intensity of the penetrated laser. The relationship between light transmittance and the electrical field is shown in Fig. 3 for application of a triangular electrical field 共1.4 kVp.-p., 0.1 Hz兲共a兲 before the imprint electrical field treatment and 共b兲 after the imprint electrical field treatment. Prior to the electrical imprint field treatment, the relationship between the light transmittance and the applied voltage was symmetric. In the presence of the imprint electrical field, the PLZT obtained a memory effect and it

FIG. 3. Relationship between the light transmittance and applied electrical field of PLZT without an imprint electrical field 共a兲 and with an imprint electrical field 共b兲.

was confirmed that the transmittance characteristics displayed two stable light transmittances in the absence of an external electrical field due to the internal electrical imprint field. The light transmittance of the PLZT was then controlled using pulsed voltages, as shown in Fig. 4. The applied pulse voltage was ⫾1400 V, and the pulse width was 86 ms. The intensity of the pulsed voltage was the same as the triangular electrical field using for measurement of the butterfly-shaped curve. Without an external electrical field, the light transmittance maintained each stable value and was controlled by the direction of polarization, thereby successfully demonstrating the light transmittance memory effect. The amount of the

FIG. 4. Control of the light transmittance using pulsed voltages.


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Appl. Phys. Lett. 93, 192102 共2008兲

Ohashi, Hosaka, and Morita

memory effect, which is the difference between the minor light transmittance and the major light transmittance, corresponded to the amount presumed from the butterfly-shaped curve, as shown in Fig. 3. Up to the present experiment, the amount of the memory effect did not vary greatly with elapsed time and the number of the applied pulsed voltage. It was confirmed that the shape memory piezoelectric actuator maintained a certain amount of the memory effect after 104 pulsed operation at least.14 The shape memory effect is based on the imprint electrical field as well as the light transmittance memory effect; then the optical memory effect is expected to show the same trend. Detailed research for these stabilities is ongoing. In this study, the principle of a light transmittance memory effect was proposed by the control of the electrical imprint field of PLZT material, and this principle was successfully demonstrated. Furthermore, the light transmittance could be controlled by adjusting the pulsed voltage in the presence of an imprint electrical field. The transmittance characteristics maintained a certain value depending on the direction of polarization. A detailed mechanism for the imprint electrical field is currently under investigation. This research was partially supported by the Ministry of Education, Culture, Sports, Science, and Technology through

a Grant-in-Aid for Scientific Research on Priority Areas under Grant No. 438, “Next Generation Actuators Leading Breakthroughs,” and by the Murata Science Foundation, the Futaba Electrons Memorial Foundation, the Japan Chemical Innovation Institute, and the Support Center for Advanced Telecommunications Technology Research 共SCAT兲. C. E. Land, Ferroelectrics 7, 45 共1974兲. J. R. Maldonado and A. H. Meitzler, Proc. IEEE 59, 368 共1971兲. 3 W. D. Smith and C. E. Land, Appl. Phys. Lett. 20, 169 共1972兲. 4 C. E. Land and W. D. Smith, Appl. Phys. Lett. 23, 57 共1973兲. 5 T. Kosaka, D. Fukunaga, K. Uchiyama, and T. Shiosaki, Proceedings of the ISAF2007, 2007 共unpublished兲, Paper No. 29Ps-A-6. 6 K. Hikita and Y. Tanaka, Ferroelectrics 94, 73 共1989兲. 7 K. Nashimoto, S. Nakamura, T. Morikawa, H. Moriyama, M. Watanabe, and E. Osakabe, Jpn. J. Appl. Phys., Part 1 38, 5641 共1999兲. 8 P. E. Shames, P. C. Sun, and Y. Fainman, Appl. Opt. 37, 3717 共1998兲. 9 T. Morita, Y. Kadota, and H. Hosaka, Appl. Phys. Lett. 90, 082909 共2007兲. 10 Y. Kadota, H. Hosaka, and T. Morita, Jpn. J. Appl. Phys., Part 1 47, 217 共2008兲. 11 T. Ohashi, H. Hosaka, and T. Morita, Jpn. J. Appl. Phys. 47, 3985 共2008兲. 12 M. Grossmann, O. Lohse, D. Bolten, U. Boettger, and T. Schneller, J. Appl. Phys. 92, 2680 共2002兲. 13 T. Morita and Y. Cho, Jpn. J. Appl. Phys., Part 1 45, 4489 共2006兲. 14 Y. Kadota, H. Hosaka, and T. Morita, Proceedings of the ICMA2008, 2008 共unpublished兲, p. 165. 1 2
