Phononics, phononic crystals, and beyond
Résumé
The objective of this invited paper is to present a review of the domain of phononics. A general aim is to attract new researchers to this growing field that has strong connections with ultrasonics, especially as far as applications are concerned. Along this talk, I will try and provide answers to a number of basic questions, such as: What is phononics, Who is presently involved in phononics? What is new and what is not? Why switch to phononics? What is still to be done? Are there any applications? Phononics can be defined as the science of designing artificial acoustic materials with tailored dispersion properties. Of special interest are phononic crystals, two- or three-dimensional periodically structured materials that show a number of amazing properties - but only if properly designed. For instance, strong confinement of wave energy can be obtained inside complete band gaps, i.e. frequency ranges within which no wave propagation is allowed, whatever the incidence and the polarization. Even outside band gaps, the periodic structure leads to very strong spatial dispersion, resulting in properties unavailable with usual homogeneous materials. The basic tool to understand these dispersion properties is derived from solid state physics. The so-called band structure summarizes the dispersion relations of all waves propagating in a phononic crystal. Some simple hints to understand band structures will be given. I will then highlight recent achievements in the field of phononics and use them to emphasize that much remains to be achieved. For instance, although the theory of band gaps is quite advanced, experimental demonstrations have not been that many, and technical difficulties still stand ahead. Confinement by defects (created by altering locally the otherwise perfectly periodic arrangement) has been shown to lead to efficient energy storage in cavities, to wave guiding in very narrow spaces, and more generally to simple phononic circuits; however, the practical achievement of true phononic circuits or filters is still an unsolved challenge. It is highly desirable to design phononic crystals for surface acoustic waves, and initial demonstrations have been performed, but there is still a major bottleneck to their development: efficient structural technologies for piezoelectric materials have to be obtained (for instance to obtain deep holes of sub micron aperture at the surface). The spatial dispersion properties of finite phononic crystals have been employed to design acoustic lenses, through the use of positive or negative refraction; but these lenses have shown limited efficiency, and limited frequency range (i.e. for short pulses). The geometrical dimensions of phononic crystals ultimately dictate the frequency ranges inside which band gaps appear. This relation is dependent on the acoustic velocities of the composing materials, but also on the boundary conditions imposed by the limits of the periodic parts of an actual sample. I will give a summary of achievements that have been obtained so far for bulk waves, surface waves, phononic plates, phononic layers-on-a-substrate, and true 3D phononic structures. It will be seen that the frequency range of operation has generally been limited in practice by available transducers for acoustic or elastic waves. As one possible answer to that, I will discuss the use of phononics to directly improve transducers or dramatically change their design.