On-Chip Tightly Confined Guiding and Splitting of Surface Acoustic Waves Using Line Defects in Phononic Crystals

Phononic crystals (PnCs) exhibit acoustic properties that are not usually found in natural materials, which leads to the possibility of new devices for the complex manipulation of acoustic waves. In this article, a micron-scale phononic waveguide constructed by line defects in PnCs to achieve on-chip, tightly confined guiding, bending, and splitting of surface acoustic waves (SAWs) is reported. The PnC is made of a square lattice of periodic nickel pillars on a piezoelectric substrate. The PnC lattice constant, pillar diameter, and pillar height are set to 10, 7.5, and 3.2 µm, respectively, leading to a complete bandgap centered at 195 MHz. Interdigitated transducers are monolithically integrated on the same substrate for SAW excitation. The guiding, bending, and splitting of SAWs in the phononic waveguide are experimentally observed through measurement of the out-of-plane displacement fields using a scanning optical interferometer. The combination of destructive interference due to the Bragg bandgap and the interaction of the propagating wave with the pillars around the channel results in a tight confinement of the displacement field. The proposed phononic waveguides demonstrate the feasibility of precise local manipulation of SAW that is essential for emerging frontier applications, notably for phonon-based classical and quantum information processing.