Defect modes in non-Bragg resonant structures for guided surface water waves

Abstract

We report our experimental observation of a single and stable defect mode arising in the non-Bragg bandgap of surface water waves by introducing a defect element into a water channel with periodically corrugated sidewalls. By investigating the existence of the non-Bragg defect mode and its frequency shifting, we find that unlike the Bragg defect mode which arises within an extended region around the Bragg gap in the spectrum, the non-Bragg type defect mode exists within a much smaller region. It is further proved by the experiments that the frequency shifting property of the defect mode in the non-Bragg gap is comparable to that of the Bragg type. The results show that the location of the defect mode in the bandgap is dependent on the length of the defect element. The ability to tune the defect modes for surface water waves would guide existing and next-generation wave energy control and coastal protection technologies.