Liquid crystal Bragg filters

Bragg filters or gratings have the advantages of spectrally selective reflection and high diffraction efficiency, which make them useful for a variety of applications. Liquid crystal Bragg gratings possess an additional interesting and useful feature in that they are switchable or tunable. In switchable filters the reflection notch can be switched on and off, while in a tunable filter it can be scanned through a broad spectral range. We have explored these types of filters for several years and present a review of some of their more intriguing aspects. Two types of filter have been studied: holographic polymer-dispersed liquid crystals and cholesteric liquid crystals. We describe the Bragg diffraction of these two types of filter and explore their similarities and differences. Here we will focus on switching and tuning by external stimuli such as electric fields as well as thermal and mechanical mechanisms. We further describe the physics of these devices and point out some new features we have observed as well as open questions concerning their behavior.