“…The specific geometrical structures for MTM waveguides reported in previous literature include a single MTM slab [5][6][7][8], grounded MTM slab [9][10][11][12], multilayered or asymmetric MTM slab [13][14][15][16], air slab with MTM clad [17], MTM channel [18], MTM fiber [19][20][21][22][23], hollow fiber with MTM clad [24], plus many more. Some of the interesting novel phenomena that have rarely been encountered in conventional metallic or dielectric waveguides are backward waves [9,24], the simultaneous propagation of forward and backward waves [8,12,24], superslow waves [12,24], the suppression of specific guided modes [8,[10][11][12][13]16], and power circulation [8,9]. Thus, innovative optoelectronic functional devices utilizing one or several of these properties in MTM waveguides (see, e.g., ref.…”