High refractive-index sonic material based on periodic subwavelength structure

We show that a steel plate with periodic array of subwavelength slits or holes can serve as a material of tunable refractive index for acoustic waves. The effective refractive index is inversely proportional to the filling factor of the slits or holes, which enables the realization of high refractive-index material for acoustic waves. An acoustic wave focusing lens based on this material is exemplified.