Diffusion in liquid crystals of two cholesterics with different pitches

Abstract

The properties of chiral photonic crystals with a pitch gradient were investigated both experimentally and theoretically. The liquid-crystalline cell was prepared by bringing two cholesteric liquid crystals (CLCs) with different pitches into contact. There are some essential features in the dynamics of diffusion between the two CLCs. The most important feature is the fact that establishing the final equilibrium pitch (for the temperatures below 17°C) takes a long time -roughly 3 months or more-which, in turn, allows us to investigate the nonstandard change in the reflection spectra during the diffusion. The two reflection curves do not directly actually get closer and merge into one peak, as usual, but the left one gradually gets smaller and almost vanishes, and the right one expands. For this reason, a theoretical model of a pitch change was developed and it was seen that this model does not coincide with the classical ideas of the pitch change. It was also assumed that due to the "freezing" effect during the diffusion process, one could have fixed the intermediate states of the pitch gradient.

Ultrabroadband gradient-pitch Bragg-Berry mirrors

“…( 3) is an idealization. Indeed, the actual pitch profile along the z axis evolves during the diffusion process [29]. Therefore, in our case, it depends on the time at which the cholesteric has been quenched in the glassy state.…”

Graded circular Bragg reflectors: a semi-analytical retrieval of approximate pitch profiles from Mueller-matrix data

“…Since the bandwidth of circular Bragg reflection of CLC is limited by their birefringence, the broadening of selective reflection has been achieved by different methods producing a varying pitch across the thickness of the sample [12][13][14][15][16][17][18][19][20][21][22]. In these cases, the graded pitch profile has been quantified from cross-section electron microscopy images.…”