We first investigate the existence of defect modes in
the photonic band structure of a comb-like waveguide geometry made of
dangling side branches grafted periodically along an infinite
monomode waveguide. This one-dimensional photonic crystal exhibits
forbidden bands which 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 comb-like
waveguides composed of a finite number of grafted branches. The
behaviour of the localized states is analyzed as a function of the
length, of the position and of the number of the defective branches.
The influence of the dissipation inside the waveguides on the
amplitude of the localized states is also studied.
We
then consider the tunneling between two monomode waveguides through a
monomode coupling device made out of one-dimensional comb-like
photonic waveguides. We present the conditions, in absence or
presence of dissipation, for selective transfer of a single
propagating state from one continuum to the other, leaving all other
neighbor states unaffected. These conditions are then applied to
coupling devices of peculiar geometry. Complete channel drop
tunneling in this system is due to localized states situated within a
gap of the coupling device.
The theoretical results on defect modes and resonant
tunneling are confirmed by experiments using coaxial cables in the
frequency range of a few hundreds of MHz.