Integrated (Pb,La)(Zr,Ti)O 3 Heterostructure Waveguide Devices Fabricated by Solid-Phase Epitaxy
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ABSTRACT Heterostructures of a Pb(Zr,Ti)0 3 (PZT) waveguide/(Pb,La)(Zr,Ti)0 3 (PLZT) system buffer layer were grown on a Nb-doped SrTiO 3 (Nb:ST) substrate by solid-phase epitaxy. The propagation loss in the PLZT heterostructure waveguides was on the order of I dB/cm. An electro-optic beam deflection device with an ITO prism electrode on the surface of the PLZT heterostructure waveguide presented efficient deflection of the coupled laser beam by applying a voltage between the electrode and the substrate. A beam deflection greater than 10 mrad at 5 V and frequency response as fast as 13 MHz were observed. An apparent electro-optic coefficient as large as 39 pmJV was estimated from the deflection characteristics for the TE mode and TM mode suggesting the polarization independent nature of the PZT waveguide. For integrating the electrooptic PLZT heterostructure waveguides, channel waveguides were fabricated in the PZT waveguides using a simple wet-etching process. Based on a low-voltage drive structure, lowloss waveguide process, and fine patterning process, a fabricated digital matrix switch showed a 10 dB cross-talk at a voltage as low as 7.5 V. INTRODUCTION It is very attractive to fabricate an electrode/ferroelectric waveguide/semiconductor structure since low-voltage drive devices will be realized by the reduced electrode gap. For such a structure, it is of great interest to use (Pb,La)(Zr,Ti)O3 (PLZT) ferroelectric materials which have
excellent electrooptic properties [1] and Nb-doped SrTiO 3 (Nb:ST) semiconductor substrates. However, it is not easy to prepare a low-loss PLZT waveguide on a Nb:ST substrate in terms of crystal growth and absorption by the Nb:ST substrates. Actually, practical PLZT single crystal wafers are not available because of difficulty of growing PLZT single crystals. Therefore. epitaxial growth is essential for fabricating PLZT optical waveguide devices. However, it has been difficult to grow epitaxial PLZT waveguides with a propagation loss of less than a few dB/cm by vapor-phase processes. We did demonstrate heteroepitaxial growth of high quality LiNbO 3 waveguides [2] and PZT waveguides [3,4] by solid-phase epitaxy. The solid-phase epitaxy is a simple and useful technique regarding stoichiometric composition control, uniform doping, uniform large-area fabrication, low-loss capability, and waveguide patterning. The substrates were spin-coated with methoxyethoxide precursor solutions and preannealed to form amorphous thin films followed by the solid-phase epitaxial crystallization of the thin films. To avoid the absorption loss created by the substrate, we fabricated a heterostructure with a PZT waveguide/PLZT buffer layer on a Nb:ST substrate by the solid-phase epitaxy [5]. For integrated optical waveguide devices for switching, modulation, or deflection, it is important to fabricate micro-optics including channels in addition to the preparation of low-loss waveguides. To make directional coupler switches, Mach-Zehnder switches, or digital switches based on LiNbO 3 waveguides, Ti diffu
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