Universal anti-reflection blocks for 4-port periodic microresonator structures

We introduce universal "anti-reflection blocks" for periodic microresonator sequences consisting of 4-port unit cells. When added to the end of a finite periodic sequence, the anti-reflection block can eliminate the reflectivity at a given frequency ωref and significantly reduce it in a frequency range around ωref. These anti-reflection blocks are universal: By adjusting only their characteristic parameters, they can be made applicable to any 4-port periodic sequence, regardless of the detailed inside structure of its unit cells, at any frequency within the photonic bands.