Surface Coupled Phononic Resonators
Résumé
Mechanical resonators are ubiquitous physical systems that have been lying at the core of a resurging interest over the past few years. In the field of micro- and nano-electromechanical systems, the possibility to fabricate resonators at the micro- or nano-scale with tailorable properties has opened exciting perspectives, ranging from quantum mechanics to sensing [1-4]. In acoustics or phononics, investigations of the interaction between a propagating wave and a collection of so-called local resonators have contributed to the rise of acoustic metamaterials, a class of composites exhibiting a behavior conditioned by the local properties of its constitutive, sub-wavelength unit cells [5]. The concept has been successfully extended to surface acoustic waves (SAW), and it was for example shown that pillars deposited on a surface could hybridize with SAW to form band gaps conditioned by the pillar resonance frequencies [6-8].Interaction of isolated resonators with a supporting surface however remains usually considered in the light of micro-mechanics where it is seen as detrimental and usually carefully avoided. In this work, we propose to exploit the interaction of SAW with isolated, micron-scale mechanical resonators to strongly couple, confine and potentially store elastic energy at will [9]. Confinement of the elastic energy in cylindrical pillars exhibiting dimensions at least ten times smaller than the excitation wavelength is experimentally reported, with a ten-fold field enhancement compared to the free surface. The elastic field behavior can be further controlled through resonator-to-resonator coupling as revealed by an investigation of the characteristic response of pairs of pillars. Much similarly to what has been achieved in plasmonics, such an approach could make it possible to conceive phononic chains capable to carry the elastic energy along the most twisted paths.