Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - L-SID (Light and sound waves in silicon and nonlinear glass waveguides)

Teaser

The research project L-SID examines the interaction between light and sound waves. The two phenomena are around us every day. They are generally thought of as independent from one another, but this is not really the case. We can use light to generate ultra-sonic sound waves...

Summary

The research project L-SID examines the interaction between light and sound waves. The two phenomena are around us every day. They are generally thought of as independent from one another, but this is not really the case. We can use light to generate ultra-sonic sound waves, and we can use sound to scatter and modify light. Such interplay is not only intriguing from a scientific standpoint, it is also quite applicable. Components and systems that combine light and sound waves help transmit data in communication links, improve imaging deep inside tissue in biomedical diagnostics, and form the basis of structural health monitoring over kilometers of infrastructure.

More specifically, the project is dedicated to the study of light and sound waves in chips and circuits that are made of silicon and glass. Silicon forms the basis of electronics for decades. Nowadays, however, silicon is also used to process light. Optical communication data of hundreds and thousands of giga-bits per second propagate on silicon devices, alongside the more familiar electronic components. The processing of such vast volumes of information is extremely challenging, and cannot be carried out based on electronics alone. In such mukti-physics efforts for the processing of information, there are also roles for sound waves. An incident signal may be converted from light to sound, propagate as an acoustic wave on the surface of a silicon device, and undergo part of the required processing while in the realm of ultra-sound. Research tasks look to establish first proofs of principle and initial building blocks for future devices that can process data using electronics, light and sound, and bring together the relative added value of each form. The potential long-term benefit is better and more efficient data communication.

Work performed

\"In the first half of the project, our team formulated and applied models for the propagation of light and sound in a common structure, such as an optical fiber or a silicon chip. Using such models, we are able to predict and calculate precisely how light and sound waves affect each other. We already put these principles to work. In using light and sound waves together, we could map the properties of liquid media that surround optical fibers, over several kilometers. This measurements address a long-time challenge of the sensing community: optical fibers strongly confine the light that they guide, and make sure that nothing leaks outside. While this property is advantageous for data transfer, it makes the fibers \"\"blind\"\" to outside conditions. With the introduction of sound waves, we may now \"\"listen\"\" outside the optical fiber, where it is impossible to \"\"look\"\". We are now able to leverage the powerful advantages of optical fibers: long reach, small dimensions, compatibility with harsh environments and immunity to interfering radio waves, and use them for mapping of chemicals over a long range. The analysis, calculations, experimental demonstrations and applications of light and sound propagation over fibers appear in a series of publication by our group.

On the devices side, we were able to validate the main working hypothesis of the entire project. We were able to excite surface acoustic waves on a standard silicon chip, using incident light alone. Incoming light illuminates a target of metallic stripes that is patterned on the device surface for that purpose. Absorption of light leads to heating of the metals. As the intensity of incoming light is modulated to represent certain information, the metal pattern is heating and cooling, accordingly. Such changes in temperature, in turn, induce a pattern of mechanical strain to the underlying silicon. Stain then propagates away from the illuminated region towards the rest of the device, in the form of an acoustic wave. Further, we could detect and monitor the propagating surface waves using light that is also guided in the same chip. These demonstration should form the basis of a new technology for the processing of data, in the form of surface-wave-photonics.\"

Final results

Results already progress beyond the state of the art, in the following respects:

1. The generation of sound waves in standard silicon-photonic device, using light alone. There is no need for piezo-electric transducers, or for the suspension of the silicon surface in the form of membranes. The devices are made in a standard silicon foundry.

2. The sensing and analysis of liquids outside standard optical fibers, where light cannot reach

3. The extension of this sensing mechanism to the distributed mapping of liquid media, over few kilometers and with a spatial resolution of tens of meters.