Acoustic energy focusing

diffraction limit, focusing, extraordinary acoustic transmission, EAT, sub-wavelength, metamaterials, Fabry-Pérot resonances, acoustic microscope, imaging

Resume

Overcoming diffraction limit and geometrical considerations for energy focusing has been reveal to become a real challenge the past last decades. The passage of more wave energy than would be expected from geometrical considerations through a small sub-wavelength aperture is called Extraordinary transmission (ET).

In this research topic, we are developing new architectures based on metamaterial mechanisms to reduce the size of the focusing spot with acoustic waves : this phenomenon is called Extraordinary Acoustic Transmission (EAT). Reducing the size of the focusing spot should prove useful for future imaging and sensing applications.

The acoustic energy is focusing through an subwavelength pillar. Grooves are adding for concentrating more energy and to modify the output pattern for imaging application.

Previous Works

During my post-doctoral fellow, we proposed a new architecture of EAT as well as a proof-of-concept of imaging application with an EAT system.

Extraordinary Acoustic Transmission at GHz regime

EAT architectures containing a nanowire and, in two cases, additional sets of grooves.

In the first project of my post-doctoral fellow, we demonstrate extraordinary bulk-wave acoustic transmission in a solid-solid structure. The basic geometry consists of a tungsten nanowire joining two tungsten blocks, which is not able to exhibit EAT efficiencies with only FP resonances. However, when concentric grooves of the optimally calculated dimensions are added to the input block, thereby introducing surface wave resonances, we show that the EAT efficiency can reach a value of 220. We also show that when grooves are added to the output block, the output acoustic field intensity can be concentrated in the normal direction without a loss in efficiency — as previously demonstrated in liquid-solid systems — which may be useful for future imaging and sensing applications.

Sub-wavelength acoustic microscope based on extraordinary transmission

Despite the potential advantage of extraordinary transmission in concentrating wave energy to tiny regions, its potential in sub-wavelength imaging has never been demonstrated. A device capable of generating an image by capturing evanescent waves with this principle could visualize the small details of an object and extract its sub-wavelength features without huge signal losses.

In the second part of this post-doctoral fellowship, we show how extraordinary acoustic transmission (EAT) with a zero-mass metamaterial can be used to make sub-wavelength images using airborne sound. This EAT-probe can image topography or acoustic impedance of objects in two dimensions at kHz frequencies with a lateral resolution of λ/24 and a depth resolution of λ/300. Moreover, owing to the circular symmetry of the sound excitation and collection system, the detection sensitivity is independent of the scanning direction, i.e. isotropic in the lateral plane, yielding undistorted images without contact with the sample.