Hybrid longitudinal-transverse propagating electric fields in photonic crystal waveguides

Strong longitudinal fields in tightly focused beams of light enable many novel phenomena and applications, including single-molecule detection, near-field imaging, and high-resolution photolithography. However, in traditional guided wave platforms, it has not been possible to carefully tailor the spatial and spectral distribution of the longitudinal electric (LE) field component of the electromagnetic field. Here, we demonstrate both theoretically and experimentally that breaking the in-plane mirror symmetry of photonic crystal (PhC) waveguides enables unprecedented control of guided LE fields through the hybridization of LE and transverse electric (TE) modes and the opening of a geometry-induced photonic bandgap. The LE-field contribution in the hybrid LE-TE modes and the width of the bandgap can be precisely tuned by rotating the asymmetric PhC unit cell. These findings provide a pathway for advanced control of light, enabling applications in polarization engineering, dipole coupling in quantum systems, and integrated photonics.