Near-Zero Index Photonic Crystals with Directive Bound States in the Continuum

Near-zero-index platforms arise as a new opportunity for light manipulation with boosting of optical nonlinearities, transmission properties in waveguides, and constant phase distribution. In addition, they represent a solution to impedance mismatch faced in photonic circuitry offering several applications in quantum photonics, communication, and sensing. However, their realization is limited to the availability of materials that could exhibit such low index. For materials used in the visible and near-infrared wavelengths, the intrinsic losses annihilate most of near-zero index properties. The design of all-dielectric photonic crystals with specific electromagnetic modes overcomes the issue of intrinsic losses while showing effective mode index near zero. Nonetheless, these modes strongly radiate to the surrounding environment, greatly limiting the devices applications. Here, a novel all-dielectric photonic crystal structure is explored that is able to sustain effective near-zero-index modes coupled to directive bound-states in the continuum in order to decrease radiative losses, opening extraordinary opportunities for radiation manipulation in nanophotonic circuits. Moreover, its relatively simple design and phase stability facilitate integration and reproducibility with other photonic components.