Scanning phononic lattices with surface acoustic waves

We have examined the propagation of surface acoustic waves across structures with periodically varying elastic properties. These so-called phononic lattices include a) alternating layers of aluminum and polymer in a multilayer structure, b) a 2-d hexagonal lattice of holes drilled in an aluminum substrate and filled with polymer, and c) a 2-d hexagonal array of Al posts surrounded by polymer. A simple method is employed for probing the elastic properties of these periodic structures with ultrasound in the 0.5–5 MHz range. Ultrasonic surface waves are introduced and detected by water-immersion transducers, cylindrically focused to a line on the sample surface. Transmission and reflection patterns are observed by continuously scanning the wavevector angle with respect to the symmetry axes of the lattice. By Fourier transforming the transmitted signals using a broadband source, we obtain the frequency responses of the lattices as a function of propagation angle. Phononic band gaps, waveguide channeling and Scholte-like interface modes are observed. The results are compared to models of the propagation of transverse waves through analogous bulk structures, for which theories are available. These preliminary models explain the basic features of the experimental results, such as frequency gaps and wave channeling. A number of observations, however, remain to be explained. The angle-scanning technique provides a powerful probe of these modulated structures.