Exceptional points and lines and Dirac points and lines in magnetoactive cholesteric liquid crystals

We investigated the properties of cholesteric liquid crystals (CLCs) being in external static magnetic field directed along the helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, and in the presence of absorption new features appear in the optics of CLCs. We have shown that in this case new photonic band gaps (PBGs) appear. This new PBG is sensitive to the polarization of the incident light. But if the chirality sign of the polarization of the incident diffracting light for the basic PBG (which exist also at the absence of external magnetic field) is determined only by the chirality sign of the CLC helix, then for the second one it is determined by the external magnetic field direction (i.e., on whether the directions of the external magnetic field and the incident light are parallel, or they are antiparallel). We have shown that in this case besides Dirac points there appear also Dirac lines as well as exceptional points and exceptional lines. And moreover, near some of these points and lines the narrow bands or wide bands of magnetically induced transparency, and magnetically induced absorption appear, and near the others a wide coherent perfect absorption band appears that is insensitive to incident light polarization. And finally at some wavelength the same reflection, transmission and absorption takes place for any polarization of incident light, although it is not Dirac point nor exceptional point. This system can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.