Electro-optic Mach-Zehnder modulators with polycrystalline BaTiO_3 thin films on MgO

摘要

The optical and electro-optical properties of epitaxially grown thin films of ferroelectric BaTiO_3 on MgO substrates have been established and high quality Mach-Zehnder waveguide modulators have been demonstrated. As a next step towards the integration of ferroelectric thin films on different substrates, we have modified the growth conditions by lowering the growth temperature to study polycrystalline, but still highly transparent BaTiO_3 (BTO) films. Polycrystalline BTO on MgO substrates has been grown by pulsed laser deposition (PLD). The growth temperature was reduced from 800 ℃ to 400 ℃ at an oxygen pressure of 2 x 10~(-3) mbar. Although polycrystalline, the BTO is still birefringent with n_o = 2.32 and n_e = 2.30. Ridge waveguides have been formed by ion beam etching. The estimated waveguide propagation loss is 4 dB/cm at 633 nm. Electro-optic Mach-Zehnder modulators have been realized. Using 3 mm long electrodes with a spacing of 10 μm, a V_π voltage of 14 V was obtained at 633 nm wavelength. This is half of the observed effective electro-optic coefficient measured at epitaxial BTO films. At 1.5 μm wavelength similar results were observed.