Surface Acoustic Wave – Photonic Circuits in Silicon

Photonic integrated circuits in the standard silicon-on-insulator layers stack represent a key technology for large-scale data communication. Silicon photonics is also a promising platform for signal processing, sensing, and quantum science and technology. One function that remains difficult to realize in silicon photonic circuits is the true time delay of optical waveforms: the speed of light is simply too fast. Optical delay lines require excessive path lengths which are difficult to accommodate on-chip, and they are also associated with impractical propagation losses. An effective solution is known for decades in analog radio-frequency electronic circuits: the conversion of incoming signals to slow-moving surface acoustic waves. However, surface acoustic wave devices require piezo-electric interfaces.

Over the last five years, our group has introduced and implemented a new concept of surface acoustic wave – photonic devices. Rather than rely on electronic interfaces and piezo-electric materials, incoming waveforms are directly converted from the modulation of optical carriers to the form of surface acoustic waves in standard silicon. Conversion relies on the absorption of the modulated carrier in metallic structures and their subsequent thermo-elastic expansion. The signals of interest are re-converted to the optical domain through photo-elastic modulation of a second optical carrier in a resonator waveguide circuit. The devices are realized in the standard silicon-on-insulator workhorse layers stack. Applications examples include true time delays up to 170 ns on-chip, narrowband microwave-photonic filters, elastic analysis of thin layers deposition, and microwave-frequency oscillators. Lastly, the enhancement of thermo-elastic actuation through plasmon resonant absorption will be presented as well.