Fig. 8
a–c are adapted from [43]
Space-coiling metamaterials. a Scheme of a single curled unit (left-hand side). It consists of thin plates (length L, thickness d) arranging in channels of width d and lattice constant a. The zigzag arrows denote a path of waves in the second quadrant inside curled channels. X-shaped blue region shows a simple view of the path of the waves through the curled channels (right-hand side). b Pressure field of the space-coiling metamaterials (left-hand side) and the effective medium (right-hand side) which has same conditions without coiling. It shows both are well matched and the negative refractive index is obtained. c Pressure field for the cases of a hard solid plate (above) and coiling metamaterials surrounding a hard plate (below). High transmission with no reflection by coiling metamaterials is obtained.