Electro-Optical Properties of Na 0.5 K 0.5 NbO 3 Films on Si by Free-Space Coupling Technique
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Electro-Optical Properties of Na0.5K0.5NbO3 Films on Si by Free-Space Coupling Technique Alexander M. Grishin and Sergey I. Khartsev, Department of Condensed Matter Physics, Royal Institute of Technology, SE-164 40 Stockholm-Kista, SWEDEN
ABSTRACT We report electro-optic performance of highly polar axis oriented Na0.5K0.5NbO3 (NKN) films grown directly on Pt(100nm)/Ti(10nm)/SiO2/Si(001) substrates by rf-magnetron sputtering. Semitransparent gold electrodes (diameter ∅ = 2 mm) were deposited ontop the NKN films by a thermal evaporation through the contact mask. Processing parameters have been specially optimized to obtain “electrosoft” NKN films with a non-linear fatigue-free P-E characteristics: low remnant Pr = 3.6 µC/cm2 and high induced polarization P = 26 µC/cm2 @ 522 kV/cm, and the coercive field Ec = 39 kV/cm. Electro-optical characterization of NKN/Pt/Si films has been performed using waveguide refractometry: a free-space coupling of a light beam into the thinfilm waveguide modes. Intensity of TM- and TE-polarized light of 670 nm laser diode reflected from the free surface of NKN film and Au-cladding NKN/Pt/Si waveguide was recorded at zero and 30 V (100 kV/cm) bias electric field. Extraordinary and ordinary refractive indices as well as electro-optic coefficient have been determined by fitting these experimental data to the Fresnel formulas. Applying 160 V (530 kV/cm) across the parallel plate NKN capacitor (∅ = 2 mm, thickness 3 µm), modulation of the reflected light as high as 40% was achieved. INTRODUCTION Since the first fabrication of Ti indiffused waveguides in LiNbO3 in 1974 [1], this ferroelectric become the leader in the materials arsenal of lightwave systems. Numerous optical devices have been extensively studied which exploit electro-optic, acousto-optic, and nonlinear properties of this material. Integrated optics in LiNbO3 has already reached a stage of maturity. [2] However, such devices suffer from unavailability of low-cost fiber assembly. Retarded development of pigtailing structures and packaging of LiNbO3 chips is one of the reasons for the scarce commercial availability of LiNbO3 integrated optics components. Therefore, an engineering of new materials which enable “true integration” of lasing crystal, waveguides, optical processing circuits and, perhaps, memory all on the same chip remains the challenge in photonics. Ferroelectric alkaline niobate NaxK1-xNbO3 (NKN) solid solutions occupy the niche between “electrohard” LiNbO3 and “electrosoft” Pb(Zr,Ti)O3 ceramics. They possess unique combination of functional properties: remnant polization Pr as high as 18 µC/cm2, piezoelectric constant d33 ~ 160 pC/N, dielectric permittivity ε ~ 400 which promise various applications in the wide frequency range from rf- to the millimeter wave band. Also, clinical tests proved NKN ceramics to be a useful biocompatible implant material. Recently, NKN ceramics attracts revived attention
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since high performance NKN films have been grown by rf-magnetron sputtering [3-5] and pulsed laser d
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