Unusual Optical Properties of Helical Metallic Photonic Crystals and Chiral Channels in Dielectric Photonic Crystals

In this chapter, we will show that a photonic crystal comprising an array of subwavelength metallic helices has a unique photonic dispersion that allows for the realization of negative refraction for frequencies both above and below the polarization gap for electromagnetic waves propagating along the helical axis. The helical photonic crystal can also serve as a broadband wave plate for waves propagating perpendicular to the helical axis of the metallic photonic crystal. In addition, we show that robust transport of light can be achieved using chiral photonic guided modes in a channel of a dielectric photonic crystal. The light transport is immune to the scattering of isotropic homogenous impurities and the phenomenon bears some phenomenological similarity to robust transport of electrons in topological insulators, although the mechanism is not the same. In particular, the system is time-reversal invariant and the robust one-way transport does not require an external field, which is distinct from the previous strategies employing two-dimensional magnetic photonic crystals with explicit time reversal breaking by an external field.