Broken phase of parity-time symmetry enables efficient superluminal pulse transmission

Abstract

Parity-time (PT) symmetric Bragg gratings (PTBGs) exhibit unique band characteristics compared to their traditional counterparts. Notably, when the PT symmetry is broken, the initial bandgap closes, and the upper and lower branches coalesce. We demonstrate that this believed to be novel band dispersion supports fast light, also known as the optical superluminality. A light pulse can propagate through a fiber PTBG with broken PT symmetry, achieving high transmission efficiency (comparable to, and even exceeding, unity) while maintaining its Gaussian shape. This effect offers a significant advantage over superluminal tunneling, where the transmission coefficient is typically very small. We also analyze the transmission of optical precursors and show that they cannot be superluminal, consistent with the principle of causality. This work presents a mechanism for realizing superluminality with some possible applications and underscores the vast potential of non-Hermitian optics.