“…They belong to a kind of burgeoning artificial composites usually consisting of multiple elements, and can exhibit some novel properties beyond those of conventional materials. PCs and metamaterials have been successfully adopted to control flexural waves [1][2][3][4][5][6][7][8][9][10][11][12][13], e.g., prohibiting their propagation based on the Bragg scattering [1][2][3]8,12] or the local resonances [7,10], localizing vibrations by a point defect [6], manipulating the phase difference with the aid of metasurface [11,13], cloaking of hidden objects by a specific distribution of material parameters [5], focusing energy via a particular gradient-index (GRIN) PC [4,9], and so forth.…”