Making Waves in Sensing

Surface acoustic waves (SAWs) are mechanical waves that travel along the surface of a material, with energy confined near the surface rather than spreading throughout the material. These waves typically have a slower propagation speed compared to bulk waves (waves that travel through the entire material), making them particularly sensitive to surface properties and environmental changes.

Because of this sensitivity, SAWs are widely used in sensing technologies to detect mechanical, structural, chemical, and biological changes on a surface. For example, when SAWs pass through a material with a modified surface — whether due to stress, temperature change, or the presence of chemicals — their speed and amplitude change, which can be detected and interpreted by sensors. SAWs are also useful in signal processing, where they enable high-precision filtering and signal manipulation in compact devices.

These types of devices are typically constructed using piezoelectric materials that generate and detect waves via electrical transducers. Recently, however, advancements in all-optical SAW generation have promised even more adaptable SAW applications, which could enhance photonic circuit functionalities and open up new opportunities in quantum and high-resolution sensing technologies.

Now a team led by researchers at The University of Sydney has taken all-optical SAW generation and sensing to the next level. For the first time ever, they have shown that it is possible to generate SAWs on the surface of a microchip. In addition to being compact and practical to integrate into any number of devices, using lasers rather than electricity also prevents these chips from generating excess heat. Dissipating the heat produced by traditional technologies can be very challenging.

The chip was designed to generate and measure surface acoustic wave-stimulated Brillouin scattering (SAW-SBS) by carefully optimizing the waveguide geometry and material. The waveguide core is composed of a chalcogenide glass, which offers a high refractive index and elasticity suitable for SAW-SBS. This material enables strong overlap between the optical and surface acoustic waves, which is necessary for effective Brillouin gain. The waveguide geometry is finely tuned to confine the optical mode close to the waveguide surface, allowing it to interact effectively with the surface acoustic waves. Using a rectangular cross-section with specific thickness variations, the optical field is guided near the surface in thin structures, enhancing interaction with SAWs that are confined to the surface layer of the waveguide. This geometry reduces potential scattering losses by minimizing sidewall interactions.

While the team’s work is very promising, it is still at the proof of concept stage. But with additional development, this research could open the door to the production of accurate, microchip-sized optical SAW devices.