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Applied Mathematics and Computational Sciences

New pattern for underwater resonators

A resonator that enhances ultrasound waves in water could aid underwater communication.

Underwater metasurfaces that trap acoustic waves significantly outperform conventional resonators. © 2024 KAUST.

An underwater metasurface that performs far better than conventional resonators has been developed by Mohamed Farhat and Ying Wu from KAUST, working with colleagues from University Bourgogne Franche-Comté in France. A new pattern is helping to overcome some of the challenges of communication and sensing in water[1].

Electromagnetic waves are heavily attenuated as they pass through water, which limits their range. Instead, acoustic or sound waves provide a more viable option. But the resonators or cavities that confine and enhance acoustic waves, which are crucial for these communication systems, do not operate as well as their electromagnetic counterparts, particularly underwater, due to increased inherent losses.

“Previously, state of the art ultrasound resonators relied on conventional resonant systems, which resulted in low quality-factors or short-lived resonances that decayed only after a few cycles of oscillations. This situation is even worse underwater, where viscous damping or increased leaking create additional losses,” says Farhat.

“Our work presents a significant improvement compared to previous designs because it achieves an exceptionally high Q-factor within a simple underwater acoustic device,” explains Farhat. Waves can be trapped within a cavity and the level of confinement is quantitatively measured by its quality factor, or Q: the higher the Q, the longer the wave energy stays trapped inside.

Farhat and his co-workers developed an underwater resonator that included a metasurface: a thin silicon film imprinted with a periodic pattern that repeats over a distance shorter than the wavelength of the wave. This pattern can be engineered to control the way the metasurface interacts with the wave.

The team used a dicing machine to create an array of 0.1-millimeter slits in a silicon substrate of periodicity of 1 millimeter. The cavity consisted of two of these metasurfaces separated by a gap of 0.8 millimeter. The researchers characterized their structure by immersing it in water and using a transducer to create ultrasound waves with frequencies between 0.5 and 3 megahertz. They could then measure how the ultrasound transmitted through or reflected from the metasurface. In this way they were able to demonstrate a Q-factor of 350 for one-megahertz ultrasonic waves.

Crucial to this success was the unusual way their cavity traps the acoustic wave. In most resonators, the energy of the trapped wave needs to be less than a certain threshold. But the structure made by Farhat and team is not fully closed; the open resonator supports quasi-bound states in the continuum localized within a compact region of space even though their energy lies above the threshold.

“These so-called bound states in the continuum, firstly discovered in quantum mechanics, do not couple to the surrounding environment and hence possess a diverging quality-factor, which is a measure of the lifetime of the resonance,” explains Farhat.

“Our research advances the field of metamaterials, acoustics, and communications but also holds tremendous promise for practical applications, such as highly efficient acoustic filters, sensors, and transducers, as well as advanced communication and medical imaging systems and non-destructive testing,” concludes Wu.

Reference
  1. Farhat, M., Achaoui, Y., Iglesias Martínez, J.A, Addouche, M., Wu, Y. & Khelif, A. Observation of ultra-high-Q resonators in the ultrasound via bound states in the continuum. Advanced Science 11(33), 2402917.| article.
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