SUPerlentille à REfraction négative à base de MEtamatériaux et cristaux phononiques – SUPREME

The current trends in ultrasonic imaging are evolving towards the search for higher resolution, leading to devices functioning at higher frequencies. This increase in frequency introduces new constraints related, on the one hand, to a reduction of the penetration depth, and on the other hand, to the increase in the amount of data to be treated in real-time. The project proposes to study solutions to these problems using materials exhibiting the property of negative refraction: first by exploiting the 'superlens' effect, to increase the resolution and second by implementing a new principle of ultrasonic imaging based on the reconstruction of a real image, in contrast to traditional imaging. Two solutions are adopted to obtain the property of negative refraction. The first solution relies on the 'phononic crystals' which consist of periodic arrangements of several materials, that, under certain conditions, exhibit a stop band i.e. a frequency domain where the wave propagation is forbidden. These composite materials may present dispersion curves with a negative slope and this property can be used to focus acoustic waves. The second solution relies on the design of 'metamaterials' which are composite materials containing components on a scale smaller than the wavelength and which have an effective macroscopic behavior in close relationship with the microstructure. These non-dispersive materials possess the property of negative refraction. This project proposes to implement the theoretical models to study the negative refraction of the elastic waves and to design phononic crystals and metamaterials made of a solid matrix and exhibiting the required properties. The concept of negative refraction will enable the conception of a 'superlens' whose resolution exceeds the limit of diffraction as indicated in recent studies devoted to phononic crystals with a fluid matrix. The effect of negative refraction of certain phononic crystals or metamaterials with solid matrix makes it possible to consider an original technique of ultrasonic imaging. Indeed, by 'illuminating' a structure to be imaged with a quasi-omnidirectional source, one could find behind the solid super-lens an accurate image of this structure. This image would be materialized in a simple medium such as water. Then, the image will be 'read' with a laser probe (by exploiting the piezo-optic effect in water) or with a set of ultrasonic transducers. This would require only very simplified electronics realizing the multiplexing of the various elements such as in a numerical optical camera. The principle of such a technique of imaging will be studied here, by using a very simple method of reading such as a pin hydrophone, in order to assess its possibilities and limitations, this without developing a very expensive instrumentation.