The switching properties of chiral nematic liquid crystals using electrically commanded surfaces

In this paper, we describe in detail the electrical photonic band gap (PBG) tuning properties of a chiral nematic liquid crystal (N*LCs) using electrically active ferroelectric liquid crystals (FLCs) to generate surface mediated switching. The tuning of the PBG occurs as a result of a contraction of the helix which is induced passively by the in-plane rotations of the N*LC molecules at the FLC/N*LC interface. We provide detail on the preparation of the samples and discuss how the tuning range is altered when the number of switching surfaces increases from just one surface to both top and bottom surfaces. The effect is found to be completely reversible and there is no apparent change in the quality of the structure during tuning. It is observed, however, that the time required to reach equilibrium once an electric field is applied or removed is much longer than the response time of the FLC confined between conventional polyimide rubbed substrates. This is due to the elastic restoring force and viscosity of the N*LC. Nevertheless, this approach to tuning the band gap is of potential use in biomedicine and sensing applications whereby a tunable band-pass filter and/or tunable lasers are required.