Engineering waveguide-cavity resonant side coupling in a dynamically tunable ultracompact photonic crystal filter

We employ a plane-wave-based transfer-matrix method in combination with a Bloch-mode scattering model to scrutinize the optical performance of an ultracompact photonic crystal (PC) all-pass optical filter made from a single-mode PC waveguide side coupled with latitudinal optical microcavities. A series of geometrical configurations of resonant microcavities side coupled with the PC waveguide, such as single one-side and two-side, periodically cascaded one-side and two-side arrangement of microcavities, and different quantities of the latitudinal and longitudinal cavity scales, cavity-cavity distance, and cavity-waveguide distance have been systematically investigated. The calculated reflection spectra exhibit a continually splitting feature of resonant frequency when the cavity size is increased. The resonant frequency shifts toward higher frequencies when we reduce the indirect coupling coefficient by increasing the waveguide-cavity distance or by introducing a symmetric two-side structure of microcavities. Optical monomode operation of the reflection pulse can be achieved for appropriate distance between two horizontal microcavities. The resonant peaks gradually evolve into a distinct rectangular shape when we introduce periodically cascaded side-coupled microcavities along the waveguide direction, implying the creation of a forbidden minigap within the guided mode continuum. The optical filter in the two-side configurations has a better filtering performance than in the one-side constructions. The cascaded side-coupled cavity-waveguide structure can act as a high-performance optical delay line.