Polarization-induced quasi-bound states in zero-index photonic crystals by angular perturbation for terahertz sensing

Photonic crystals (PhCs) with zero effective refractive index, permittivity, and permeability are commonly described by Dirac-like cone dispersion at the Γ point of the Brillouin zone center. These parameters govern the electromagnetic (EM) behavior of light, enabling phase-free propagation and maintaining spatial coherence. Bound states in the continuum (BICs), with their high-quality factors, provide an excellent platform for exploring the light-matter interaction. However, integrating conventional PhCs having zero effective index with BIC properties onto photonic circuits remains challenging due to their large footprint, high losses, and restricted design parameters. In this work, by breaking the σz-symmetry while preserving the C4v symmetry through angular perturbation (θ = 45º to 0º) of the zero-index (ZI) PhC, we enable the conversion of incidental (I)-BIC into quasi-BIC via destructive interference. We also demonstrated a low-loss (~0.22 dB/mm) microscale ZI PhC with a high Q-factor of ~1.5×107 in the far-infrared region. This novel design offers a facile fabrication advantage, facilitating the large-scale integration of ZI materials in photonic devices. Consequently, it reduces radiation losses for both in-plane and out-of-plane transmission and has linear behavior in far-field polarization along with high refractive index sensing sensitivity of 12.24 µm/RIU or 400 GHz/RIU, making it suitable for applications in structured light, directive antennas, multiplexing/demultiplexing, high-performance sensing, linear optics, quantum optics, and photon-based twistronics.