Research case study: Microphononic crystals

Our research area is the use of phononic crystals for surface acoustic waves at MHz frequencies. Surface acoustic waves are elastic waves that travel along the surface of a material and are widely used in electronic components for signal processing, sensing and increasingly for lab-on-a-chip applications. Phononic crystals can control the propagation of acoustic waves, enabling improved performance of existing device concepts as well as providing a route to more advanced applications.

Recently we developed a new phononic crystal design consisting of periodic arrays of finite depth annular holes, as shown in figure 1. Through simulations and experiments we demonstrated that this phononic crystal can open bandgaps, which prohibit the propagation of surface acoustic waves at certain frequencies. These bandgaps are induced by local resonances and can be tuned using the depth and radius of the holes.

Importantly, this design improves upon the previously used pillar phononic crystal design by drastically enhancing the surface acoustic wave attenuation within the bandgap, even for relatively shallow features. A comparison of bandgap attenuation of our hole phononic crystal compared to conventional pillar phononic crystals is shown in figure 2. This work transforms the ability to exploit phononic crystals for developing novel SAW device concepts.

Following this work, we are currently exploring the use of phononic crystals for acoustic superlensing. We aim to do this by fabricating negative index materials which can resolve images beyond the diffraction limit of conventional lenses.

Figure 1a. Section of a 3 mm x 80 µm square array of annular holes with depth 6.4 µm, inner radius 5.1 µm and pitch 10.9 µm.

Figure 1b. Cross section depth profile of an individual annular hole with platinum within the hole to provide image contrast.

Figure 2. Simulated surface acoustic wave bandgap transmission for annular holes and pillars against number of elements in propagation direction.