Photonic Devices



In addition to the deposition and functional characterization of electro-optical materials, we have developed and realized concepts of bringing such layers into Si- photonic structures. The strong Pockels coefficient of barium titanate thin films can ultimately result in a new generation of compact, integrated devices with superior switching and optical memory properties.

Our research is focused on what are known as slot waveguide structures to achieve enhanced optical confinement in the BaTiO3 layer and to reduce the optical bending losses. Based on this waveguide geometry, our work focuses on manufacturing passive and active devices such as grating couplers, Mach–Zehnder interferometers, and ring resonators.

(Left) Schematics of a hybrid BaTiO3/Si slot waveguide structure with tungsten electrodes on the side of the waveguide. (Center) Transmission electron microscopy image of the waveguide core. (Right) Scanning electron microscopy image of several ring resonator structures.

A major challenge for this type of waveguide structures are the typically large propagation losses of more than 50 dB/cm. We identified the thin strontium titanate interfacial layer commonly used to enable the epitaxial growth of BaTiO3 on silicon as a major source of absorption. By adjusting the waveguide fabrication process we developed low-loss BaTiO3/Si waveguides required for large scale integration.

Active switches are obtained by applying an electrical field to the active BaTiO3 layer in the waveguide region. Indeed, the waveguide mode index changes as a function of the applied voltage, as for example visible in the shifts of the resonance wavelength in BaTiO3/Si ring resonator structures. We demonstrated that such devices can be used for high-speed modulation and ultralow-power optical tuning, and that the Pockels effect is the main physical origin of the electro-optical switching in such devices. In addition to investigating individual devices on dedicated research wafers, we showed how to integrate BaTiO3 photonic components on photonic integrated circuits (PICs) in a CMOS-compatible fabrication route.

(Left) Comparison of previously reported propagation losses in BaTiO3-based waveguides on MgO and Si substrates. With our fabrication process we are able to achieve BaTiO3 waveguides in silicon photonic structures with low propagation losses. (Right) Transmission spectra of an active racetrack resonator. Applying a voltage results in a spectral shift of the resonances, with a tunability of 18 pm/V.

Plasmonic switches

In addition to photonic devices, we are also working on integration of BaTiO3 in high-speed plasmonic switches. We have demonstrated very compact high-speed plasmonic modulators with low power consumption by utilizing the large Pockels effect as well the stability and durability of BaTiO3 thin films. These devices feature strongly improved stability compared to alternative plasmonic switches based on other active materials such as nonlinear polymers.

Integrated optical beamforming

The SKA telescope and the DOME project

The Square Kilometre Array (SKA) project is an international effort to build the world’s largest radio telescope, with eventually over a square kilometre (one million square metres) of collecting area.

The ASTRON & IBM Center for Exascale Technology is a research center located in Dwingeloo, Drenthe, on the Campus of ASTRON, The Netherlands Institute for Radio Astronomy. At this center ASTRON and IBM jointly carry out fundamental research into technologies needed to develop the SKA radio telescope in the latter half of this decade and the first half of the next decade. The collaboration is funded through the DOME project, supported by grants from the Dutch EL&I Ministry and the Province of Drenthe.

IBM Research in Zurich and Astron are collaborating on the realization of an optical beamformer to evaluate its application in the SKA system.

Optical technologies for the SKA telescope

A radio telescope of the size of SKA cannot be built as a traditional single dish, but must be composed from arrays of thousands to millions of antennas. The signals from these individual antennas are then synchronized and combined such that the whole array, with all antennas operating in unison, acts as a single instrument of unequaled performance.

A large part of the infrastructure budget and operating power of the telescope is expected to be consumed by the optical communication network that transports the signals from the single antennas to central locations for processing. A possible path to reduce this cost is to perform an early step in the data processing chain, “beamforming”, close to the antennas. In this beamforming step, signals from a group of antennas are preprocessed such that the whole group “looks into the same direction”. The signals from all antennas in the group can then be added and sent through a single optical communication link. This significantly reduces the number of long distance communication channels and corresponding infrastructure cost and power consumption.

Integrated optical beamforming

The large bandwidth of astronomical signals and the subsequent optical transmission of the preprocessed signal support the use of an optical implementation of the beamforming step. In the framework of the “DOME” project, an integrated optical beamformer chip was designed and produced using Si-photonics integrated optical technology.

IBM’s optical beamformer chip can bring the optical signals from four antennas into phase and combine them into one optical signal, thus reducing the effort for signal transport and further processing by a factor of 4.

Samples of these chips were packaged, wired up to control electronics and shipped to Astron in The Netherlands to become part of an array antenna demonstrator system with integrated optical beamforming processor.

Fully packaged optical beamformer chip including optical input and output fibers as well as electrical connections to the control circuitry.

Wavelength division multiplexers

CMOS integrated nanophotonics

In 2012, IBM announced its CMOS integrated nanophotonics technology. By adding a few processing modules to a high-performance 90-nm CMOS fabrication line, a variety of silicon nanophotonics components such as wavelength division multiplexers (WDM), modulators, and detectors can be integrated side-by-side with CMOS electrical circuitry. As a result, single-chip optical communications transceivers can be manufactured in a conventional semiconductor foundry, providing significant cost reduction over traditional approaches.

For integrated nanophotonics technology, IBM provides a library of predesigned and verified optical components. The team at the IBM Research lab in Zurich contributes to this library with designs for wavelength division multiplexing filters, among other things.

Wavelength division multiplexing filters

Photograph of a WDM filter. The input and outputs are labeled for use as a wavelength de-multiplexer.

In the WDM scheme for optical data transport, a set of data streams is encoded onto optical carrier signals with a different wavelength (color) for each data stream. These data streams are then combined in an optical WDM filter, which has a dedicated input for each carrier wavelength with its data stream and a single output where all wavelengths are combined (multiplexed) into one multi-wavelength data stream for further transport through a single optical link. At the receiver side of the optical data link, a WDM filter of the same type, but used in reverse, separates (de-multiplexes) the individual data streams and sends them to their corresponding optical detectors.

CMOS integrated nanophotonics transceiver chip.

The WDM filters designed in Zurich were applied in IBM’s reference design for a 100-Gb/s coarse WDM transceiver for next-generation data centers. A die photograph of this silicon photonics chip is shown in Figure 2. In this transceiver, the WDM filters multiplex and de-multiplex four data streams of 25 Gb/s each to/from a single multi-color data link. Two transceivers then connect through a single fiber per direction at an effective data rate of 100 Gb/s per direction.

Projects & collaboration

EU FP7 project Sitoga

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

Swiss project PADOMO

Plasmonic Active Devices based on Metal Oxides


PHotonic REServoir COmputing

Astron under the DOME project