Optical properties of chiral photonic crystals with linear profiles of modulation parameters

The oblique propagation of light through a layer of chiral photonic crystal (PC) with linear profiles of modulation parameters is considered. The problem is solved by the method of Ambartsumyan layer addition. The wavelength dependences of the amplitude characteristics have been studied at different angles of incidence in two cases, i.e., in a chiral PC with a linear profile of modulation period and a chiral PC with a linear profile of modulation depth. It is shown that the photonic band gap is broadened in both cases and that omnidirectional reflection occurs under certain conditions. The nonreciprocity features are investigated for these two cases and it is shown that a chiral PC with a linear profile of modulation period can operate as an ideal optical diode in a certain frequency range. The features of the effect of change in the modulation parameters on the photon density of states, group velocity, and group velocity dispersion are analyzed. It is shown that, when the modulation parameters change in space, the photon density of states at the photonic band gap edges significantly decreases. It is proposed to use PCs with a spatially changing modulation period can be used to compress (expand) light pulses.