Experimental study of band gaps and defect modes in a two-dimensional ultrasonic crystal

We report on the experimental observation of the existence and the interaction of localized defect modes in a full acoustic band gap in a two-dimensional lattice of steel cylinders immersed in water. The confinement of defect modes and the splitting of their resonance frequencies are observed and are explained by their evanescent coupling. A new type of waveguiding in a phononic crystal based on the evanescent coupling of defect modes is proposed and demonstrated experimentally. The finite-difference time-domain (FDTD) method is used to interpret the experimental data and it is found that theoretical predictions properly account for the observed spectra.