Metasurfaces could offer possibility of 'cloak of silence'

Researchers from Pohang University of Science & Technology in South Korea have designed a metasurface which could control acoustic and elastic waves. The material could have applications in hiding submarines from sonar and shielding buildings from earthquake damage.

Metamaterials are materials engineered to have unnatural properties, created by precisely combining and arranging composite materials to form repeating structures. This can allow them to manipulate electromagnetic waves – leading to metamaterial surfaces being compared with 'invisibility cloaks' – and other types of waves.

When waves encounter the boundary between two substances, it usually refracts in a certain direction (positive direction); metamaterials can manipulate waves such that they are refracted in the opposite direction or such that they are wholly absorbed or transmitted.

The Pohang researchers hypothesised that a metamaterial could be designed to significantly absorb sound waves, without reflecting them, by tailoring the resonance of sound. They designed a thin metasurface using layers of split-orifice-conduit (SOC) hybrid resonators to reduce reflectance and maximise absorption of sound waves in the 14-17kHz range.

They also confirmed that it is possible to manipulate elastic waves, such as seismic waves, very dramatically. They proposed a methodology for precisely controlling elastic waves on a curved plate based on principles similar to those of general relativity, which states that the path of light changes in the curvature of space time.

For instance, a metamaterial lens of near-zero thickness can be used to demonstrate a version of an Eaton lens – a lens with a refractive index varying from zero to infinity – for elastic waves in the 15-18kHz range on thin, curved plates. This had previously been considered impossible to demonstrate.

“Until now, metamaterial research has focused on light and electromagnetic waves, but we have confirmed that it can be applied to sound waves and seismic waves,” said Professor Junsuk Rho, a renowned metamaterials researcher. “We anticipate that [our methodology] will be applicable to building untraceable submarines or nuclear power plants that can stay intact even during earthquakes.”

Their proposed methodology could be explored using astronomical phenomena, such as the extreme gravitational fields created around black holes.

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