Surface-mediated mechanical coupling of pillar pairs
Résumé
Interaction between guided waves and mechanical resonators has been widely studied in the field of phononic crystals or elastic
metamaterials, where many works have been devoted to periodical arrangement of local resonators. Most of the literature however deals
with the collective behavior of such physical objects. In this work, we focus on the intrinsic properties of isolated resonators coupled to
surface acoustic waves (SAW), with the aim of controlling propagation at a subwavelength scale. We investigate the interaction and
coupling between a substrate excited by SAW and a pair of phononic resonators. The influence of the orientation of the source as well as of
the distance between the two neighboring resonators is experimentally studied.
Pairs of platinum pillars were grown using focused ion beam induced deposition on a Y-cut lithium niobate substrate. Their diameter and
height were set at 4.4 μm and 4 μm respectively, leading to a resonance frequency of the order of 70 MHz for the first flexural mode. SAWs
were launched on the substrate surface using interdigital transducers. The elastic energy distribution at the surface of the pillars and of the
substrate was measured using optical interferometry, giving access to the frequency responses for each pillar and to the orientations of the
modes as a function of the incident SAW wave vector.
The experimental observation of frequency splitting of the pillar pair response, which is not present for an isolated pillar, is an evidence of
coupling. The characteristics of this splitting depend on the orientation of the source and on the distance between the resonators. Two
coupling regimes can be identified: a weaker coupling, where the two pillars resonate at different frequencies, respectively blue and red
shifted compared to the individual pillar natural response, and a stronger coupling regime, where the two pillars present the same behavior
and two separated modes, as shown by the obtained split frequency response. Preliminary investigations in addition show that non-
linearities can be observed at higher drive power for particular configurations. These results prove that, by choosing appropriate
configurations, the combination of SAW and mechanical resonators can be used either to control the elastic energy distribution at the
microscale, or, reciprocally, to manipulate pillar ensembles by changing the excitation conditions.