Towards Integrated Barium Titanate Electro-Optic Modulators Through Epitaxial Film Growth

Electro-optic modulators are a backbone of the efficient data transfer necessary for modern optical communications. These modulators directly connect a driving electric field with optical properties of a material, and allow for transforming an electrical signal to an optical signal. Modern data centers and high speed internet, as well as sensing systems, heavily rely on these devices. The electro-optic modulators are often bulk crystals with proton-exchanged or titanium-indiffused waveguides, limiting their size, speed, and efficiency. Modern developments in integrated lithium niobate on insulator enables photonic device integration akin to the integrated circuit. In this work, we explore expanding the materials palette for integrated photonic devices with thin film materials such as barium titanate.<br/>The current industry standard for integrated electro-optic modulators are typically Silicon or indium phosphide. Here, carrier injection and dynamics of the modulation mechanism limit performance through optical absorption, operating speed, or half-wave voltage. Bulk lithium niobate devices are also used (electro-optic coefficient r33 ~ 30 pm/V), but cannot be scaled in the same manner.<br/>We use oxide molecular beam epitaxy to grow high quality, single crystal thin film of non-centrosymmetric materials that have superior electro-optic properties. For example, barium titanate (BaTiO3) exhibits an extremely high electro-optic coefficient (r42 ~ 900 pm/V), a reasonably high band gap (&gt; 3eV), and relatively high refractive index (n = 2.4). Additionally, previous work has shown that the ferroelectric properties of BaTiO3 vary significantly in epitaxially strained thin films. Since ferroelectric and electro-optic properties are both rooted in the crystal structure of these materials, we expect that the electro-optic properties will be equally affected by epitaxial strain.<br/>Here we grow BaTiO3 on strontium titanate substrates, as well as on a variety of scandate substrates, to understand and characterize the impact of epitaxial stran on the electro-optic tensor of BaTiO3. Furthermore, in order to facilitate scalability and creation of devices with BaTiO3, we show our progress in transferring our thin films onto silicon substrates. This is accomplished through growth of a dissolvable interlayer between the growth substrate and the BaTiO3 that can be used to easily separate the two.