Broadband quasi-PT-symmetry sustained by inhomogeneous broadening of the spectral line

Abstract

It is shown that inhomogeneous broadening of the spectral line of active impurities may sustain simultaneously parity ($\mathcal{P}$) and time ($\mathcal{T})$ symmetries of a medium, in a finite range of field frequencies, which is forbidden by the causality principle in media without broadening. If a spectral width of a propagating pulse is less than the inhomogeneous broadening, the medium for such a pulse becomes quasi-$\mathcal{PT}$-symmetric. The effect of the broadband quasi-$\mathcal{PT}$-symmetry in a finite frequency domain, is illustrated by examples of unidirectional diffraction of pulses in Bragg and Laue geometries, propagating in photonic crystals.