  Metamaterials with an effective zero refractive index associated to their
electromagnetic response are sought for a number of applications in
communications and nonlinear optics. A promising way that this can be achieved
in all-dielectric photonic crystals is through the design of a Dirac cone at
zero Bloch wave-vector in the photonic band structure. In the optical frequency
range, the natural way to implement this design is through the use of a
photonic crystal slab. In the existing implementation, however, the zero-index
photonic modes also radiate strongly into the environment due to intrinsic
symmetry properties. This has resulted in large losses in recent experimental
realizations of this zero-index paradigm. Here, we propose a photonic crystal
slab with zero-index modes which are also symmetry-protected bound states in
the continuum. Our approach thus eliminates the associated radiation loss. This
could enable, for the first time, large-scale integration of zero-index
materials in photonic devices.
