Active silicon integrated nanophotonics: ferroelectric BaTiO3 devices (1401.4184v2)

Abstract: The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 plus minus 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.

• The paper demonstrates nanoscale silicon photonic circuits using epitaxially grown BaTiO3, achieving an effective Pockels coefficient of 213 ± 49 pm/V.
• It employs a horizontal slot waveguide design that enhances the overlap of optical and RF fields, enabling a modulation bandwidth of 4.9 GHz.
• Crystalline quality is validated via XRD and TEM, paving the way for integrating ferroelectric materials in advanced photonic systems.

Active Silicon Integrated Nanophotonics: Ferroelectric BaTiO Devices

This paper presents a comprehensive exploration of integrating ferroelectric barium titanate (BaTiO$_3$, or BTO) on silicon-on-insulator (SOI) substrates as a mechanism for advancing capabilities in silicon photonic circuits. The research addresses the critical need for innovative material systems in integrated photonics, aiming to exploit the distinct electro-optic properties of BTO for high-performance optical modulation.

Key Contributions and Results

A major contribution of the paper is the successful demonstration of nanophotonic circuits using BTO, achieving high levels of electro-optic performance on an SOI platform. The authors employ reactive molecular beam epitaxy (MBE) to grow single-crystalline BTO films directly on the SOI substrates. This epitaxial growth is facilitated by an intermediate strontium titanate (STO) buffer layer, which aids in overcoming lattice mismatch challenges between the BTO and the silicon substrate.

The most notable numerical result is the effective Pockels coefficient of 213 ± 49 pm/V, measured in the engineered BTO thin films. This coefficient exceeds the conventional values offered by lithium niobate-based optical modulators by more than sixfold. The demonstrated BTO-based optical modulators also offer a modulation bandwidth reaching up to 4.9 GHz, illustrating substantial potential for broadband applications.

Technical Innovations

To address the refractive index mismatch, where BTO (n ≈ 2.38) is lower than silicon (n ≈ 3.47), the authors design and fabricate a horizontal slot waveguide structure. This geometry leverages a confined light path within a thin BTO layer flanked by silicon layers, enhancing the overlap between optical and RF electric fields, thus maximizing the electro-optic interaction.

Various photonic components, such as microring resonators and Mach-Zehnder interferometers (MZI), are realized on the BTO-on-silicon substrate. High extinction ratios and quality factors are consistently observed, with a specific 7,000 quality factor for the microring resonator, indicating lower optical losses and efficient light management within the waveguides.

Material Science Considerations

The research details the crystalline structure and interface quality using X-ray diffraction (XRD) and transmission electron microscopy (TEM), which confirm the high-quality fabrication of epitaxial BTO layers. The paper highlights the persistent material challenges, such as optical losses due to oxygen vacancies, suggesting potential improvements through post-growth annealing techniques and the implementation of hydrogen barrier layers.

Implications and Future Work

The integration of ferroelectric oxides like BTO in silicon photonics opens new avenues for the development of optical components with enhanced functionalities. Specifically, the high effective Pockels coefficient and potential rapid modulation capabilities make BTO an attractive candidate for next-generation photonic circuits.

This paper paves the way for the implementation of ferroelectric materials in nonlinear optical applications, including wavelength conversion and generation of non-classical light sources. Future work may focus on optimizing material growth processes to further reduce defects and enhance device performance, including higher optical Q-factors and lower propagation losses.

Overall, the presented research validates the concept of embedding ferroelectric functionalities in silicon photonics, positioning BTO as a promising material for a range of photonic applications, and fostering advancements in integrated optics technology.