COntrolling the ProPagation Of LAmb waves – COPPOLA

Controlling the propagation of acoustic or electromagnetic waves is of fundamental interest for many applications ranging from imaging the living and detecting hazardous components, to information processing and structural health monitoring. In the past decades, there has been many proposals in this regards, which can be separated within two frameworks. On the one hand, one can tame wave-fields in order to take advantage of the complexity of propagation media to for instance, focus waves or image various objects. On the other hand, one can force waves along desired paths through a careful design of manmade materials. This can be achieved using the concept of metamaterials, an arrangement of tailored subwavelength building blocks from which the material gains its unusual properties. The advent of such structures has given rise to fascinating concepts such as negative refraction and transformation optics.

On the one hand, negative refraction has paved the way towards the notion of perfect lens and the ability of overcoming the diffraction limit in wave imaging. It has also given rise to the notion of complementary media and the ability to cancel the propagation of waves by adjoining two mirror regions of opposite refractive indices. On the other hand, transformation optics/acoustics is a tool of choice for designing manmade metamaterials. The paths of electromagnetic or acoustic waves can be controlled by devising a material whose constitutive parameters should vary spatially in a way prescribed by coordinate transformations.

The aim of the COPPOLA project is to develop those two concepts in the context of guided elastic waves. The latter ones, referred to as Lamb waves, have recently drawn an increasing attention for the design of new electro-acoustic devices in electrical engineering or acoustic sensors in MEMS technology. They are also commonly used for non-destructive testing in the aviation, automobile or nuclear power industries. For all these applications, the control of Lamb waves is essential.

Yet Lamb waves are perfect candidates for the application of concepts such as negative refraction and the coordinate transformation tool. These modes actually exhibit complex dispersion properties. Their phase velocities depend on the ratio between the wavelength and the plate thickness. Hence, one can control the local phase velocity by playing with the thickness of the plate. Moreover, some Lamb modes display a negative phase velocity. By changing abruptly the thickness of a plate, a positive phase velocity mode can be converted into a negative-phase velocity mode, thus allowing negative refraction.

The COPPOLA project aims at designing an ensemble of devices based on negative refraction and on the notion of complementary media in order to guide, trap and focus Lamb waves. Associated with the coordinate transformation tool, the notion of complementary media will also lead to a new strategy for cloaking some parts of the space. At last, a more ambitious step is to extend this approach to evanescent Lamb modes and build a perfect lens in order to control and focus Lamb waves at the deep sub-wavelength scale.

The devices will be fabricated by the technologic platform of Femto-ST, following a well-identified process: plasma etching. As proof-of-concept, they will operate in the MHz range. They will be tested experimentally with an ultrasound laser set up. The latter one will benefit from optical wave-front shaping techniques in order to excite selectively the Lamb mode of our choice over an arbitrary frequency bandwidth.