Ferroelectric BaTiO3 thin-film optical waveguide modulators

High-quality ${\mathrm{BaTiO}}_3$ epitaxial thin films on MgO substrates have been grown by pulsed-laser deposition. Both, $c$ -axis and $a$ -axis ${\mathrm{BaTiO}}_3$ orientations were studied. Mach–Zehnder optical waveguide modulators with a fork angle of $\text{1.7°}$ have been fabricated by ion-beam etching. The waveguides are of the ridge type, the ${\mathrm{BaTiO}}_3$ thickness is 1 μm, the ridge step 50 nm, and the width 2 μm. Light was coupled into the waveguides from optical fibers. The ${\mathrm{BaTiO}}_3$ waveguide propagation losses are 2–3 dB/cm. Electrodes of 3 mm length were deposited besides the waveguides. Electro-optic modulation has been demonstrated with ${\mathrm{V}}_{\pi }\text{=6.3\hspace{0.17em}}\mathrm{V}$ at 632 nm wavelength and ${\mathrm{V}}_{\pi }\text{=9.5\hspace{0.17em}}\mathrm{V}$ at 1550 nm wavelength for the $a$ -axis samples, and with ${\mathrm{V}}_{\pi }\text{=8\hspace{0.17em}}\mathrm{V}$ at 632 nm wavelength and ${\mathrm{V}}_{\pi }\text{=15\hspace{0.17em}}\mathrm{V}$ at 1550 nm for the $c$ -axis samples. Theoretical modelling of the Mach–Zehnder modulators for both crystalline orientations of the ${\mathrm{BaTiO}}_3$ films gave the Pockels coefficients $r_{51}\text{=80\hspace{0.17em}}\mathrm{pm/V}$ for the c-axis film and an effective Pockels coefficient $r_{\mathrm{eff}}\text{=734\hspace{0.17em}}\mathrm{pm/V}$ for the $a$ -axis films at 632 nm wavelength.