Monolithically integrated optical modulator based on polycrystalline Ba_0.7 Sr_0.3 Ti O₃ thin films

Good-quality polycrystalline Ba0.7 Sr0.3 Ti O3 (BST0.7) thin films were deposited on fused silica substrates and Si substrates having a thick amorphous Si O2 layer at a relatively low temperature of 550 °C by spin-coating metal organic solutions. The thin films were highly transparent to light in the ultraviolet to near-infrared wavelength regions. The optical propagation loss for a 5-μm -wide and 300-nm -thick polycrystalline waveguide based on BST0.7 was about 17 dBcm at a wavelength of 632.8 nm. An electro-optic Mach-Zehnder interferometer modulator based on the polycrystalline BST0.7 thin film was monolithically integrated on a Si substrate with standard lithography and wet etching. Optical modulation was successfully demonstrated. The estimated electro-optic coefficient (6.7 pmV) is the highest reported so far for this kind of film deposited on a fused silica substrate or a Si substrate with an amorphous Si O2 layer.