A review of the fabrication of photonic band gap materials based on cholesteric liquid crystals

Cholesteric liquid crystals (CLCs) are known to exhibit selective reflection of incident radiation due to their periodic helical structure, which makes them promising candidates for a myriad of different photonic applications. At normal incidence, CLCs reflect circularly polarized incident light of the same handedness as the cholesteric helix and of wavelength λ between n_oP and n_eP, where n_o and n_e are the ordinary and extraordinary refractive indices, respectively, of the locally uniaxial structure, and P is the pitch of the helix. Thus, the reflection bandwidth Δλ is given by Δλ = ΔnP, where the birefringence Δn = n_e - n_o. Within the bandwidth, right-circularly polarized light is reflected by a right-handed helix, whereas left-circularly polarized light is transmitted. Outside the bandwidth, both polarization states are transmitted. Therefore, Δλ depends on Δn. Moreover, Δn is typically limited to 0.3-0.4 for colorless organic compounds, and Δλ is often < 100 nm in the visible spectrum. Although a narrow reflection band is desirable for applications such as optical filters and thermal imaging, it also becomes a drawback in their applications, such as reflective displays, broadband circular polarizers and switchable mirrors. The purpose of this review is to take a closer look into how to broaden the reflection band in CLCs to overcome the above limitations for a wide variety of applications. This review covers the methodology that was used until recently, when the fabrication of photonic band gap (PBG) materials arose, based on CLCs. The mechanisms for broadening the reflection band have been reviewed.