Point-Defect Localized Photonic Bound States in the Continuum*

Photonic crystals (PhCs) with defect modes are known to exhibit exceptional confinement and transport properties enabling devices such as high-Q resonators, PhC fibers and spectral filters. Typically, these properties require the spectral isolation of defect modes within a bandgap which requires the use of materials with a high dielectric contrast. However, it has been shown in various systems that confinement can occur even in the absence of bandgaps using bound states in the continuum (BICs), which are states that remain perfectly bound to the system despite residing in a continuum of radiating states. BICs localized to point defects could enable the construction of devices such as fibers and ultra-high-Q nanocavities with gapless PhC designs that employ low-index materials.

In this work, we propose bound states in the continuum (BICs) in 2D PhCs that are exponentially localized to point-defects despite the lack of a photonic bandgap. The defect cavity and the host PhC are carefully designed such that radiation leakage is suppressed by a symmetry mismatch between the defect mode and the continuum states of the PhC. We also demonstrate how these BICs can be used as propagating modes with arbitrary small group velocity in a low-contrast slow-light PhC fiber.