Fig. 1: Schematic diagram of an ultra-low-loss on-chip zero-index PhC slab based on a bound state in the continuum (BIC).

a Zero-index PhC slab without BICs whose radius, pitch and height are r, a and h. A photonic dipole mode forming the zero index results in out-of-plane radiation, dramatically increasing the propagation loss of the material. b Zero-index PhC slab with a BIC. At a particular height, all the upward/downward out-of-plane radiation destructively interferes