Simultaneous observation of zero-index band and quasibound state in the continuum in all dielectric photonic crystals

Zero-index (ZI) photonic materials show the absence of spatial phase accumulation, making them a suitable candidate for many useful optical applications. On the other hand, a bound state in the continuum (BIC), having extremely high-quality (Q) factors, provides a powerful way to enhance light–matter interaction, sensitivity, and nonlinear properties of materials. We propose a simple square lattice of silicon embedded in silica with C4v symmetry through which we achieved quasi-BIC with a high Q-factor of 2.25 × 107 and simultaneously obtained the zero-index band (ZIB) with a relative 3 dB bandwidth of 3.3%–3.66% through transmission–reflection consideration around the Dirac point. The experimental verification of the ZIB in the microwave band aligns well with the simulated results. An extension study and application in the THz band are also provided. The results can have potential applications in directive antennas, high-speed connectors and devices, sensors, non-linear optics, and large single-mode lasers with an appreciable band of operating frequency.