As data grows, so does the energy consumption required to store and process it, which is why scientists around the world are turning to light (photonics) as a means of moving data.
The Pockels effect, an important phenomenon in certain materials, allows changing the optical properties of the material through an electrical stimulus. The effect is only present in special materials, such as lithium niobate, and applied in high-speed optical modulators and switches for optical communication systems. Since these discrete devices are bulky, their application is limited to complex optical networks such as data centers and telecom networks. For these applications, silicon photonics has emerged as a technology for integrating complex optical circuits on a small footprint with low manufacturing cost. While many components such as waveguides and detectors have been developed, a material with a strong Pockels effect has not been available on silicon platforms until now.
In a paper published today in the scientific journal Nature Materials, my team at IBM Research – Zurich demonstrated a record-high electro-optic (EO) response in silicon photonic devices by applying nanometer-thick, crystalline layers of a material exhibiting the Pockels effect. The results were obtained in collaboration with academic partners at ETH – Zurich (Switzerland), Nanophotonics Technology Center (Valencia) and the University of Texas at Austin (US). We developed a process to deposit a dedicated optical material of very high crystalline quality, barium titanate (BTO), on silicon substrates. As a bulk crystal, BTO has one of the largest Pockels coefficients among all known materials, exceeding that of the well-known lithium niobate by a factor of more than 50. In our paper we unambiguously confirm the presence of a very large Pockels effect also on such ultra-thin, integrated layers, both on the micro- and on the nanoscale. Making such material available on silicon allowed us to realize several examples of practical devices: a high-speed photonic and a high-speed plasmonic modulator. We were able to modulate these micro-meter sized devices with 50 billion operations per second (50 Gbit/s).
Our work to integrate these BTO crystals on silicon was motivated by some limitations of silicon photonics, which has become a platform for dense and low-cost integrated photonic circuits for a wide range of applications such as high-speed transceiver systems and optical sensors, all of which require fast, energy-efficient tuning elements, EO modulators and switches. Today, such components are realized by locally changing the concentration of charge carriers in silicon waveguides (plasma dispersion effect). However, since these devices can only be optimized for either high speed or power efficiency, compromises must be made in circuit design. For example, while modulators made from lithium niobate single crystals have been used for decades in discrete components in long-haul telecommunications, simply transferring that well-known technology to silicon is not possible since the integration of lithium niobate on silicon can only be performed on small wafer scale.
We developed an alternative platform to enhance silicon photonics based on perovskite BTO. It offers large Pockels coefficients, scalability to large substrates, good process stability, and compatibility with passive photonic structures. Our current study confirms also the superior properties in these structures when actively switching them – a feature whose presence has previously been under discussion. We discovered the largest Pockels effect ever observed in integrated photonic structures and demonstrated high-speed EO switching up to frequencies of up to 65 GHz. These features are central to a large number of fields in photonics particularly in the area of high-speed communication as they provide the function of encoding electrical data onto an optical carrier.
The ability to use the Pockels effect in integrated photonic devices provides an additional degree of freedom for designers of optical circuits, which can be used in a novel generation of high-speed devices in optical communication and beyond. Near- and mid-infrared integrated sensors can strongly benefit from devices operated at reduced speeds and reduced power consumption, neuromorphic optical circuits benefit from the non-volatile memory capability in BTO, and microwave-to-optical quantum converters from the record-high optical nonlinearities.