Broadband Extraordinary Acoustic Transmission for super-resolution imaging – BEAT

The project uses a combined approach:

- Numerical simulations: modeling acoustic waves in various materials to optimize structures before fabrication.

- Resonant metamaterial fabrication: sub-wavelength pillars and grooves concentrate energy and generate surface acoustic waves (SAWs).

- Experiments in solids and air: testing on model and industrial materials to measure image resolution and repeatability.

- Broadband and 2D imaging: exploring prototypes operating over a wide frequency range to reconstruct detailed topography of rigid or soft objects.

- Performance analysis: lateral and depth resolution, phase and amplitude behavior, extraction of mechanical properties from a single measurement.

These methods allow the design of innovative acoustic sensors capable of revealing details invisible to conventional instruments, enabling super-resolution imaging and new applications in detection and non-destructive evaluation.

The BEAT project has, for the first time, demonstrated that extraordinary acoustic transmission (EAT) can be used to achieve sub-wavelength imaging.

In solids, prototypes with sub-wavelength pillars and grooves achieved ?/12 lateral resolution, maintaining good signal-to-noise ratio and robustness to alignment variations.

In air, tensioned membranes reached ?/25 lateral and ?/300 vertical resolution, enabling precise mapping of object topography and mechanical properties without contact.

Devices are compact, manufacturable, and compatible with standard transducers, opening the way to broad industrial adoption.

The project opens multiple promising avenues:

- Industrial applications: non-destructive testing, material inspection, and evaluation of manufactured parts and composite surfaces.

- Development of new instruments: lensless acoustic microscopes and portable sensors for on-site measurements.

- Technique extension: adaptation to other materials, broader frequency ranges, and integration of quantitative measurements to extract mechanical properties and topography.

Wave concentration beyond the diffraction limit by transmission through subwavelength structures has proved to be a milestone in high resolution imaging. However, despite the advantages 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 using this principle could visualize the details of an object and extract its subwavelength features. In the BEAT project, we are proposing to tackle this barrier by developing an acoustic metamaterial device for sub-wavelength and broadband focusing of bulk waves at MHz frequencies.

Extraordinary transmission through small subwavelength apertures, i.e. the passage of more wave energy than expected by geometrical considerations, is a metamaterial-related phenomenon based on local resonances, not exclusive to electromagnetic waves. The BEAT project is first proposing to extend the principle of extraordinary acoustic transmission (EAT) to the MHz range by using an experimental approach combined with numerical simulations. This project has the ambition to experimentally demonstrate for the first time the EAT phenomenon for bulk waves in solids. The aim of this project is to develop a proof-of-concept device capable of reducing the focal spot below the diffraction limit and adaptable on commercial piezoelectric transducers. The physical device to be designed will be built, characterized and finally coupled to commercial piezoelectric flat transducers as primary field source.

As most of the EAT systems are based on meta-resonance, the working frequency regime is inherently narrow band, this is a major drawback in pulse-echo-based imaging. Enlarging the EAT phenomenon to a wide frequency range is a challenge to address highly resolved future sensing applications. The second objective of the BEAT project is to experimentally demonstrate broadband EAT (BEAT) for the first time. A compromise should be elucidated between the efficiency of the prototype and the fabrication feasibility. To this aim, different solutions of broadband acoustic metamaterial will be envisaged and correlated with the experimental constraints.

By proposing an experimental and broadband device, the BEAT project will lead to the first demonstration of the super-resolution imaging based on EAT process in solids. The different prototypes developed in the project will be tested in imaging context by using an experimental approach correlated with numerical models. The lateral resolution and in-depth resolution of the BEAT microscopes will be investigated. Moreover, phase and amplitude behavior of the reflection coefficient will be studied with different media samples to extract the position and the mechanical properties of the sample with a unique measurement in order to demonstrate complex topography measurement capabilities. The super-resolution capabilities will be exploited by studying the influence of a rigid-like object position in water on the amplitude and the phase pressure fields in 2D-scan mode.

By improving the spatial resolution of acoustic images under the diffraction limit with a broadband EAT-based metamaterial, the BEAT project will lead to new opportunities for non-destructive evaluation and testing, imaging and sensing.