  Coupled-resonator optical waveguides (CROWs), which play a significant role
in modern photonics, achieve waveguiding through near-field coupling between
tightly localized resonators. The coupling factor, a critical parameter in CROW
theory, determines the coupling strength between two resonators and the
waveguiding dispersion of a CROW. However, the original CROW theory proposed by
Yariv et al. only demonstrated one value of coupling factor for a multipole
resonance mode. Here, by imaging the tight-binding Bloch waves on a CROW
consisting of designer-surface-plasmon resonators in the microwave regime, we
demonstrate that the coupling factor in the CROW theory can reverse its sign
for a multipole resonance mode. This determines two different waveguiding
dispersion curves in the same frequency range, experimentally confirmed by
matching Bloch wavevectors and frequencies in the CROW. Our study supplements
and extends the original CROW theory, and may find novel use in functional
photonic systems.
