Ferroelectric Phase Shifters in Silicon Photonics for Novel Types of Optical Computing

Abstract

Integrated photonic circuits efficiently combine multiple optical functions on a single physical platform, such as guiding, modulating, splitting, or detection of light. The integration of these functions removes barriers for designing and realizing large optical circuits, as present when using discrete components like bulky lenses or mirrors in tabletop optical networks. With the rise of silicon photonics, integrated photonic circuits (PICs) are becoming increasingly large and highly functional, eventually allowing unprecedented concepts of photonic computing. Examples are integrated photonic quantum processors, microwave photonic filters and processors, and optical accelerators to train and execute neural networks. An important building block in integrated optical circuits is an efficient link between the optical and electrical domain. Wellknown examples of such links are integrated high-speed modulators to convert electrical signals into optical signals at very high-speed, and low-power tuning elements to compensate for variations in the device operation temperature and for deviceto-device variations during fabrication. To enable such electro-optic links, the two most widely used physical effects are the plasma-dispersion effect and Joule heating. Although these effects are attractive to use due to their compatibility with standard photonic fabrication processes, their performance in integrated devices is intrinsically limited by high insertion losses and high-power dissipation. Over the past decade, we established an alternative electro-optic switching technology by embedding a Pockels material into silicon-based photonic devices [1]. We reached this goal by developing a process to fabricate ferroelectric barium-titanate (BTO) thin films on silicon substrates using advanced epitaxial deposition techniques and by developing a BTO process technology [2]. We correlated the electro-optical properties of the thin films with their structural properties such as porosity and crystalline symmetry to show guidelines for improving the functional properties. By realizing integrated hybrid BTO/silicon devices, we demonstrated record-high, in-device Pockels coefficients of >900 pm/V. The Pockels effect in BTObased photonic devices indeed enables extremely fast data modulation at rates beyond >40 Gbps and ultra-low-power electrooptic tuning of silicon and silicon-nitride waveguides. We also show ways of how to integrate and use BTO in plasmonic slot waveguide structures for very compact optical devices. With the development of a wafer-level integration scheme of singlecrystalline BTO layers to a 200 mm process [3], we could demonstrate a viable path to combine the BTO-technology with existing fabrication routes. With major breakthroughs in the past years, BTO has emerged as a strong candidate for a novel generation of electro-optic devices. Major achievements of the BTO technology will be covered in the presentation, ranging from important materials aspects, device development, integration concepts, and novel applications in the area of quantum computing, high-speed communication, and neuromorphic optical computing. [1] S. Abel et al., “Large Pockels effect in micro- and nano- structured barium titanate integrated on silicon,” Nat. Mater., vol. 18, no. 1, pp. 42–47, 2019. [2] S. Abel et al., “A strong electro-optically active lead-free ferroelectric integrated on silicon,” Nat. Commun., vol. 4, p. 1671, 2013. [3] F. Eltes et al., “A BaTiO3-Based Electro-Optic Pockels Modulator Monolithica