Ghost surface transverse waves in periodic structures

The mechanisms giving rise to the localization of surface-skimming bulk waves (SSBW) into surface transverse waves (STW) are discussed. Mass loading of the substrate by a material with a low shear-wave velocity is well known to slow down and localize the SSBW into a STW. We have studied the opposite case where the shear-wave velocity in the material layer is higher than that of the SSBW on the substrate and, consequently, uniform mass loading tends to accelerate the SSBW instead of localizing it. However, in periodic electrode gratings the higher shear velocity of the electrodes is compensated for by electric loading and the interaction between counterpropagating waves, thus enabling the existence of localized STWs. As a net effect of the competition between the mechanisms, such waves feature weaker sensitivity to the thickness variations of electrodes