Time-domain response of Parity-Time symmetric structures: Causality, stabilitv and bandwidth considerations

Parity-Time (PT) symmetry has been recently introduced in quantum mechanics as a condition allowing non-Hemiitian systems to support real eigenvalues, and as a result to be physically permissible. Although PT symmetry in quantum mechanics is still under dispute, it has attracted significant attention in classical electromagnetic and acoustical systems, where it is manifested by spatially balanced amounts of loss and gain, and leads to unique phenomena, such as unidirectional invisibility, laser-absorber modes, and negative refraction. To date, most of the studies on PT symmetric systems have focused on their steady-state time-harmonic response. However, since such systems are active, they are potentially unstable, and investigation of their time-domain properties is crucial in order to determine whether such systems are physically possible.