Keiichi Nashimoto

Preparation and Characterization of Sol-Gel Derived Epitaxial and Oriented Pb(Zr0.52Ti0.48)O3 Thin Films

Epitaxial and oriented Pb(Zr0.52Ti0.48)O3 (PZT) thin films were prepared from nonhydrolyzed polymeric precursors. PZT thin films with a single (001) orientation were prepared on SrTiO3 (100) and MgO (100) substrates by solid-phase epitaxial growth. Crystallization of sol-gel PZT with a (111) preferred orientation was also observed on sapphire (0001) substrates. The guided wave modes were excited by a prism coupling method for PZT thin films crystallized on MgO (100) at 700° C, and the verified refractive index was as high as 2.561. Dielectric constant and remnant polarization of epitaxial PZT thin films, crystallized on conductive Nb-SrTiO3 (100) substrates at 650° C, were 448 at 10 kHz and 15 µ C/cm2, respectively.

Control of crystallinity in sol-gel derived epitaxial LiNbO3 thin films on sapphire

Control of crystallinity in solid phase epitaxial growth of LiNbO 3 thin films derived from methoxyethoxide solution on sapphire substrates has been attempted by investigating growth variables. Crystallization at 700°C for longer than 30min was necessary for full crystallization of LiNbO 3 thin films, although the orientations of the LiNbO 3 crystal planes parallel to the substrates were almost independent of crystallization temperature and crystallization time. Layer-by-layer crystallization and preparation of an ultrathin initial layer have been found to be important in the growth of high-quality epitaxial LiNbO 3 thin films. X-ray diffraction analysis revealed that perfectly single-plane-oriented epitaxial LiNbO 3 thin films, with rocking curve full width at half-maximum of less than 0.07°, were produced on sapphire (001) substrates. Refractive index of 2.24 and optical propagation loss of 3.0 dB/cm were achieved.

Fabrication of electro-optic Pb(Zr, Ti)O3 heterostructure waveguides on Nb-doped SrTiO3 by solid-phase epitaxy

Pb(Zr, Ti)O3 (PZT) heterostructure optical waveguides were grown on low resistivity Nb-doped SrTiO3(100) substrates by solid-phase epitaxy. The propagation loss was reduced to 1.7 dB/cm at the wavelength of 1.3 μm by introducing an epitaxial buffer layer between the PZT waveguide and the Nb-doped SrTiO3 substrate. An electro-optic beam deflector with an indium–tin–oxide prism electrode on the surface of the PZT waveguide showed efficient laser beam deflection as great as 3.3° (58 mrad) by applying 20 V between the prism electrode and the substrate. An index change higher than 0.001 at 5 V and an average apparent electro-optic coefficient larger than 46 pm/V were estimated from the deflection characteristic.

Electrooptical Properties of Heterostructure (Pb, La)(Zr, Ti)O3 Waveguides on Nb–SrTiO3

The electrooptic behavior of (Pb, La)(Zr, Ti)O3 (PLZT) heterostructure waveguides, including PLZT/Pb(Zr0.95Ti0.05)O3 [PZT (95/5)] and Pb(Zr0.52Ti0.48)O3 [PZT (52/48)]/Pb(Zr0.85Ti0.15)O3 [PZT (85/15)] on Nb–ST substrates was examined. Electrooptic behavior was characterized by fabricating beam deflectors using these heterostructures. The heterostructure waveguides exhibited hysteresis-free electrooptic behavior after the application of initial voltage. The electrooptic behavior of PLZT/PZT (95/5) and PZT (52/48)/PZT (85/15) heterostructure waveguides presented a nonlinear dependence and a linear dependence, respectively. The apparent electrooptic coefficient of the PLZT/PZT (95/5) heterostructure obtained from the linear part of the V–θ characteristic above +7.5 V was 53 pm/V, and that of the PZT (52/48)/PZT (85/15) heterostructure was 37 pm/V. The electrooptic coefficient of the PZT (52/48) waveguide layer using an effective voltage was estimated to be 53 pm/V.

Orientation of MgO Thin Films on Si(100) and GaAs(100) Prepared by Electron-Beam Evaporation

MgO thin films were prepared on Si(100) and GaAs(100) by electron-beam evaporation. MgO thin films with (100) orientation were obtained at 610°C with the deposition rate of 0.5 A/s, and those with (111) orientation were obtained below 440°C with deposition rate higher than 8 A/s, on Si substrates. (100) oriented MgO thin films, however, grew on Si at 440°C upon decreasing the deposition rate to 0.3 A/s. MgO thin films with (100) orientation having cube-on-cube epitaxy were obtained on GaAs substrates at the temperature as low as 280°C even at the deposition rate of 1.4 A/s.

Metal Alkoxide Solution-Derived Epitaxial Lead Titanate-Based Thin-Film Optical Waveguides

Lead titanate-based thin films were prepared for optical waveguide studies by nonhydrolyzed methoxyethoxide-solution-derived solid-phase epitaxy. PbTiO3 thin films crystallized with preferred (001) and (100) orientations on SrTiO3 (100) and MgO (100), respectively. PLT (28/100) thin films had obvious grain structure although they crystallized with a single (110) orientation on sapphire (012) substrates. Epitaxial PZT thin films with a single (100) orientation crystallized on MgO (100) also had obvious grain structure. Epitaxial PZT (52/48) thin films on SrTiO3 (100) had a single (001) orientation, rocking curve full width at half-maximum of around 0.1°, and smooth surface of rms roughness less than 2 nm. The epitaxial PZT thin films on SrTiO3 showed optical propagation loss as small as 8 dB/cm at 633 nm.

Electro-optic beam deflector using epitaxial Pb(Zr,Ti)O3 waveguides on Nb-doped SrTiO3

Pb(Zr,Ti)O3 (PZT) thin-film optical waveguides were grown on Nb-doped SrTiO3 substrates by solid-phase epitaxy to fabricate an electrode/waveguide/conductor structure. The propagation loss was relatively large for a PZT waveguide on a lightly Nb-doped SrTiO3 as compared with the propagation loss of 4 dB/cm for the structure using undoped SrTiO3. An electro-optic beam deflection device was fabricated by preparing a prism electrode on the surface of the PZT waveguide on a lightly Nb-doped SrTiO3 substrate. Efficient deflection of the coupled laser beam in the PZT waveguide as large as 11 mrad was observed by applying 35 V between the prism electrode and the substrate.

PLZT Waveguide Devices for High Speed Switching and Filtering

High-speed optical switches were developed in electro-optic (Pb,La)(Zr,Ti)O3 (PLZT) waveguides. A 1×2 optical path switch showed <;2.5 ns switching time. An 8×1 wavelength selective switch based on a tunable AWG showed 15 ns switching time.

Patterning of (Pb, La)(Zr, Ti)O3 waveguides for fabricating micro-optics using wet etching and solid-phase epitaxy

The patterning process of (Pb, La)(Zr, Ti)O3 heterostructure waveguides was examined for fabricating micro-optics, including channels and lenses. After the patterning of the amorphous thin films derived from spin-cast methoxyethoxide precursors by a wet etching, the patterned amorphous film was crystallized by solid-phase epitaxy. A 5-μm-wide ridge-type channel waveguide was fabricated in a Pb0.91La0.09(Zr0.65Ti0.35)O3/Pb(Zr0.52Ti0.48)O3 heterostructure on a SrTiO3 (100) substrate by the process. The optical confinement was successfully observed. A mode index lens was also fabricated in the Pb0.91La0.09(Zr0.65Ti0.35)O3/Pb(Zr0.30Ti0.70)O3 heterostructure on the SrTiO3 (100) substrate. A deflection of the collimated light by the lens was observed.

A 31-FDL Buffer Based on Trees of 1x8 PLZT Optical Switches

We have demonstrated a 31-fiber-delay-line buffer based on 1x8 PLZT optical switches in a tree structure. Error-free optical buffering with BERs of less than 10-10 has been achieved at 10 Gbit/s.