“…Bound modes are nonradiating modes: all the Floquet waves decay exponentially away from the structure (α z;pq > 0), being jβ pq j > k (outside the visible region) for all ðp; qÞ indices, so that the only decay is associated with the metal or host losses [42,47,48]. If at least one of the Floquet waves is faster than the speed of light [49] [namely, at least one spatial harmonic is in the visible region, i.e., jβ pq j < k for some ðp; qÞ], the mode is a radiating mode or leaky mode; i.e., at least one of the spatial harmonics is radiating or leaking energy into space (see, for instance, Table 12.1 in [42] for a summary of this classification of complex modes). In uniform structures, this radiation takes place in the forward direction (β pq · α > 0), and the wave grows unbounded for growing jzj.…”