Wavelength Tuning of the Photonic Band Gap of an Achiral Nematic Liquid Crystal Filled into a Chiral Polymer Scaffold

We demonstrate electric-field-induced wavelength tuning of the entire photonic band gap of an achiral nematic liquid crystal (LC) filled into a chiral polymer scaffold. This chiral polymer scaffold has been formed by creating a template of a chiral nematic LC phase, which remarkably does not compromise the optical finesse of the band gap when compared to that of a conventional, polymer-stabilized chiral nematic LC. We present results on the spectral shift and temporal evolution of the photonic band gap in the presence of an external ac electric field. It is shown that initially there is a rapid (τ ≈ 1 ms) blue-shift of the long-wavelength band-edge followed by a considerably slower blue-shift (τ ≈ 6.5 s) of the entire band gap. We compare the results with those obtained for a polymer-stabilized chiral nematic LC where only a blue-shift of the long-wavelength band-edge is observed. Consequently, we find that for the templated sample the tuning range is more than a factor of 2 greater than that observed for the polymer-stabilized chiral nematic LC for the same range of electric field amplitudes. It is also found that there is little in the way of hysteresis upon increasing and decreasing the applied electric field magnitude. Finally, we present experimental evidence that suggests that the blue-shift of the entire band gap is due to an additional tuning mechanism present only for the case of the templated samples. This is believed to be caused by a contraction of the pitch that results from a translational motion of the polymer network. The greater tuning range observed in these templated samples is potentially important for the development of tunable 1-dimensional photonic band gaps and LC lasers. Furthermore, it avoids the use of dc electric fields that can lead to long-term issues regarding stability.