The circular Bragg phenomenon

Abstract

Exhibited by structurally chiral materials—such as Reusch piles, cholesteric liquid crystals (CLCs), and chiral sculptured thin films (STFs)—due to their helical nonhomogeneity along a fixed axis, the circular Bragg phenomenon is the almost total reflection of the incident light of the co-handed circular-polarization state but very little reflection of the incident light of the cross-handed circular-polarization state. Manifesting itself in spectral regimes that depend on the angle of incidence, the structural period, and the relative permittivity dyadic, the phenomenon amounts to the formation of a light pipe that bleeds energy backward under appropriate conditions. Mild dissipation and dispersion do not significantly affect the circular Bragg regime. Every structurally chiral material of sufficient thickness is essentially a circular-polarization-sensitive band-rejection filter. Cascades of these materials with or without structural defects can be used to satisfy complex filtering requirements, such as multiband, narrowband, and ultra-narrowband filtering. A shift in the circular Bragg regime due to infiltration of a chiral STF by a fluid enables optical sensing. Sources of circularly polarized light can be fabricated by embedding emission sources in CLCs and chiral STFs.