Defect modes in one-dimensional comblike photonic waveguides

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Abstract
We investigate the existence of defect modes in the photonic band structure of a comblike waveguide geometry made of dangling side branches grafted periodically along an infinite monomode waveguide. This one-dimensional photonic crystal exhibits forbidden bands that originate both from the periodicity of the system and the resonance states of the grafted branches. The defect modes result from the presence of defective branches in the comb and may occur in these stop bands. The localized states appear as very narrow peaks in the transmission spectrum of finite comblike waveguides composed of a finite number of grafted branches. The behavior of the localized states is analyzed as a function of the length, of the position, and of the number of the defective branches. These theoretical results are confirmed by experiments using coaxial cables in the frequency range of a few hundreds of MHz.