Sensitivity versus the guiding layer thickness for the proposed sensor (/, , and nm) with a metamaterial layer (red solid line) and without a metamaterial layer (blue dotted line): (a) normal symmetry ( and ) and (b) reverse symmetry ( and ). Sensitivity versus the guiding layer thickness for TM mode () (red solid line) and TE mode () (blue dotted line) ( nm, /, and ): (c) normal symmetry ( and ) and (d) reverse symmetry ( and ) [20].