Stefan Abel

A strong electro-optically active lead-free ferroelectric integrated on silicon

The development of silicon photonics could greatly benefit from the linear electro-optical properties, absent in bulk silicon, of ferroelectric oxides, as a novel way to seamlessly connect the electrical and optical domain. Of all oxides, barium titanate exhibits one of the largest linear electro-optical coefficients, which has however not yet been explored for thin films on silicon. Here we report on the electro-optical properties of thin barium titanate films epitaxially grown on silicon substrates. We extract a large effective Pockels coefficient of r(eff) = 148 pm V(-1), which is five times larger than in the current standard material for electro-optical devices, lithium niobate. We also reveal the tensor nature of the electro-optical properties, as necessary for properly designing future devices, and furthermore unambiguously demonstrate the presence of ferroelectricity. The integration of electro-optical active films on silicon could pave the way towards power-efficient, ultra-compact integrated devices, such as modulators, tuning elements and bistable switches.

Controlling tetragonality and crystalline orientation in BaTiO3 nano-layers grown on Si

A hybrid growth process was developed in order to epitaxially integrate nano-layers of the multi-functional perovskite BaTiO₃ onto Si(001) substrates. In particular, we combined molecular beam epitaxy (MBE) with radio-frequency sputtering. Due to its strong influence on the functional properties, the crystalline structure of the layers was thoroughly investigated throughout our study. MBE-grown seed layers are tetragonal and c-axis oriented up to a thickness of 20 nm. A transition into a-axis films is visible for thicker layers. When the seed layer thickness exceeds 6 nm, subsequently sputtered BaTiO₃ films are epitaxial. However, their crystalline structure, their orientation with respect to the substrate, and their morphology are strongly dependent on the deposition and post-deposition thermal budget. Consistently with their crystalline symmetry, thin MBE BaTiO₃ films are piezo- and ferroelectric with a spontaneous polarization perpendicular to the surface. Also for thick films, the functional response, as determined via piezo-force microscopy, is in good agreement with the structural properties.

A Hybrid Barium Titanate–Silicon Photonics Platform for Ultraefficient Electro-Optic Tuning

Ultrafast and highly efficient optical modulators that are based on the Pockels effect are key components of todays optical communication networks. For the next generation of photonic links, silicon photonic technology is used to establish a new wave of densely integrated optic components. However, this new technology cannot exploit the advantages of using the Pockels effect for optical switching for two reasons: First, silicon does not exhibit any Pockels effect, and second, attempts to combine nonlinear materials with silicon photonics have been cumbersome. Here, we demonstrate a path to integrate barium titanate thin films with strong Pockels coefficients into silicon photonic structures. We highlight various design options, discuss the actual fabrication process, and present experimental results of functional passive and active structures. Examples include couplers and interferometers, as well as active, electrically driven nonvolatilely tunable ring resonators with a tunability of 4 μW/nm. Our results represent a major advancement in the field of ultralow-power silicon photonic switches based on nonlinear oxides, and demonstrate the potential of novel applications based on the hybrid barium titanate-silicon photonic platform.

Monolithically Integrated Microelectromechanical Systems for On-Chip Strain Engineering of Quantum Dots

Elastic strain fields based on single crystal piezoelectric elements represent an effective way for engineering the quantum dot (QD) emission with unrivaled precision and technological relevance. However, pioneering researches in this direction were mainly based on bulk piezoelectric substrates, which prevent the development of chip-scale devices. Here, we present a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN-PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. After transferring the QD-containing nanomembranes onto these MEMS, the nonclassical emissions (e.g., single photons) from single QDs can be engineered by the strain fields. We envision that the strain tunable QD sources on the individually addressable and monolithically integrated MEMS pave the way toward complex quantum photonic applications on chip.

Electro-Optical Active Barium Titanate Thin Films in Silicon Photonics Devices

We demonstrate the integration of single-crystalline barium titanate thin films on silicon substrates, with Pockels coefficients of rEff ~ 148 pm/V. We further show the implementation of such layers into silicon photonic devices such as ring resonators.

Barium titanate (BaTiO 3 ) RF characterization for application in electro-optic modulators

Barium titanate (BaTiO3 or BTO) is currently one of the most promising ferroelectric materials for enabling Pockels modulation that is compatible with silicon photonic circuits. The relative permittivity of BTO has been characterized in thin films deposited on a silicon-on-insulator (SOI) substrate. High values between 800 and 1600 have been estimated at 20 GHz. Furthermore, no substantial difference has been obtained by using BTO grown by molecular beam epitaxy and sputtering. The obtained permittivity has been used to properly design the RF electrodes for high-speed modulation in hybrid BTO/Si devices. Electrodes have been fabricated and the possibility of achieving modulation bandwidths up to 40 GHz has been demonstrated. The bandwidth is limited by the microwave propagation losses and, in this case, different losses have been measured depending on the BTO growth process.

Silicon CMOS compatible transition metal dioxide technology for boosting highly integrated photonic devices with disruptive performance

In this work we will present the objectives and last results of the FP7-ICT-2013-11-619456 SITOGA project. The SITOGA project will address the integration of transition metal dioxides (TMO) materials in silicon photonics and CMOS electronics. TMOs have unique electro-optical properties that will offer unprecedented and novel capabilities to the silicon platform. SITOGA will focus on two disruptive TMO materials, barium titanate (BaTiO3) and vanadium didioxide (VO2), for developing advanced photonic integrated devices for a wide range of applications. Innovative integration processes with silicon photonics circuits and CMOS electronics will be developed. The whole technology chain will be validated by two functional demonstrators: a 40 Gbit/s DPSK transceiver and an 8×8 switching matrix with 100 Gbit/s throughput.

Microstructure and ferroelectricity of barium titanate thin films on Si for integrated photonics

Significant progress has been made in integrating novel materials into silicon photonic structures to extend the functionality of photonic circuits. One of these promising optical materials, BaTiO3 (BTO), exhibits a large Pockels coefficient as required for high-speed light modulators. Here, we employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.

Integration of functional oxides on SOI for agile silicon photonics

Photonic devices enabling light modulation and switching are of major importance for modern telecommunication. Silicon-based electro-optic modulators are intensively investigated because of their direct compatibility to CMOS fabrication processes. However, the performances of Si based modulators are intrinsically limited, in terms of data transmission rate and energy consumption. To overcome this limitation, ferroelectric oxides with naturally strong electro-optical coefficients combined to small footprint could be ideal candidates for high-speed modulators. We have designed an integration scheme to implement these materials on SOI photonic platform, for 1.55 μm Datacom systems. Monolithic integration of ferroelectric oxides on SOI via SrTiO3 templates is presented. The devices are based on a horizontal (crystalline Si / ferroelectric oxide / amorphous Si) slot waveguide configuration.

Barium-titanate integrated with silicon photonics for ultra-efficient electro-optical performance

We demonstrate a technology to integrate optical nonlinear barium titanate thin films into silicon photonic structures. We designed and successfully fabricated passive structures, such as couplers and interferometers, and active, electrically non-volatilely tuneable ring resonators with a tunability of 4μW/nm.