Electrically tunable and reversible selective reflection due to the transition between simple cubic and tetragonal blue-phase liquid crystals

In this paper, we present experimental results on electrically tunable reflected wavelength in the full visible spectral range from 460 nm to 620 nm in one simple cubic blue-phase cell due to the simple-cubic-tetragonal lattice transition, confirmed by Kossel lines. The reflected wavelength is switched less than few hundreds milli-second and it is reversible. From the experimental observations in the reflection spectra and Kossel lines, we can see the following phenomena as increasing the applied field: the lattice-plane realignment in the simple-cubic blue phase (BPII), the transition between BPII and tetragonal blue-phase liquid crystal (BPX), and the lattice deformation of the BPX. Among them, the lattice deformation of the BPX shows larger electrostriction coefficient to shift the wavelength in a wider range, but it takes the longest response time. Because the reflected wavelength in each applied field exhibited extraordinarily sharp photonic bandgaps (less than 20 nm) with high reflectivity, the BP cell here shows a great potential to be a tunable photonic device.