Surface wave trapping in isolated mechanical resonators
Résumé
ackground, Motivation and Objective
Phononic crystals waveguides have often been presented as a promising path towards control of surface acoustic wave propagation. Yet, although their applicative potential is unquestionable, experimental demonstrations remain scarce. The development of phononic devices exhibiting Bragg band gap has been hindered by technological issues as it requires micro-machining of piezoelectric materials with high enough aspect ratios to ensure wave confinement at the surface. Line-defect managed inside locally-resonant phononic crystals can be seen as a way to reduce the propagation loss as guided modes will appear at lower frequency, limiting phase-matching to bulk waves. Here we propose to take this approach a step further by using the intrinsic confinement induced by isolated resonators to trap the elastic energy at the substrate surface.
Statement of Contribution/Methods
The resonators were excited using integrated surface wave sources consisting in chirped annular transducers deposited atop a Y-cut lithium niobate substrate. The electrode shape follows the Rayleigh wave slowness curve to allow focusing of the elastic energy at the center (Figure 1b). The finger pitch is chirped, allowing to cover an emission bandwidth ranging from 50 to 110 MHz. Several resonators were fabricated at the center of the transducer using ion beam assisted deposition (Figure 1a). The resulting material is composed of carbon, platinum and gallium (about 55 at %, 30 at % and 15 at % respectively, depending on the process parameters). The resonator dimensions varies from 3 to 5 µm with an aspect ratio between 0,75 and 1,2. The elastic energy distribution was experimentally measured by laser scanning interferometry.
Results, Discussion and Conclusions.
The resonance of the first flexural mode was clearly observed and occurs at frequencies between 55 and 95 MHz depending on the pillar geometrical parameters (Figure 1c). Amplitudes of displacement as high as 9 nm were measured at the pillar surface. The quality factor of the resonator remains low, of the order of 20, which can be accounted for by the poor mechanical properties of the deposited Pt compound. This could be improved by switching to another material, carbon in particular, that should exhibit higher Young moduli and lower vicoelastic losses. These preliminary measurements confirm the ability of the fabricated resonators to trap the elastic energy and open encouraging prospects for surface wave confinement and guiding at the micron-scale.