Extraordinary Fast Forward and Backward Light in Transparent Non-Hermitian Systems

The phenomenon of fast forward and backward light pulse propagation in effectively transparent non-Hermitian photonic systems is reported. Specifically, the propagation eigenmodes are analyzed in several parity-time-symmetric coupled waveguides. In contrast to a previous finding that showed slow light due to an exceptional point in such systems, a critical point in its close proximity that displays a diverging dispersion is identified. Superluminal light pulse propagation in the forward and backward directions is then found and observed numerically on the two sides of the critical point, respectively. This abnormal phenomenon can be understood by a conservation relation of group velocities, which can be extended to other transparent non-Hermitian photonic systems with different symmetries. More importantly, the abrupt transition across the critical point between the fast forward and backward light can be tuned, indicating the possibility of detecting atom-scale movement of the waveguides with little time delay. In addition, bandwidth broadening and a passive superluminal propagation configuration are discussed.