Fast and large refractive index switching of optically isotropic liquid crystal films

Polymer Dispersed Liquid Crystals (PDLCs) consist of low molecular weight liquid crystal (LC) droplets embedded in a polymer matrix. Nano-PDLCs are a special type of PDLCs where the LC droplet sizes are in the nanometre range. For normal incidence of light, the nano-PDLC has a voltage dependent, isotropic refractive index that could be used to make polarisation independent tunable lenses. Here, we report the effect of chiral dopant on haze, switching time and field-induced variation of the refractive index δnE of PDLC materials having a high birefringence (Δn=0.38 at 550nm) nematic liquid crystal dispersed in a polymer matrix. As an effect of the chiral dopant, δn decreases slightly from 0.05 to 0.037 at E=10V/μm, however the relaxation time and the haze decrease to a much larger extent from 2.5ms to 0.3ms, and from 50% to less than 10% at 550 nm, respectively. These improvements are due to the sub-micrometer helical pitch in the LC droplets that reduces the size of the LC droplets well below 1 µm, leading to nano-PDLC with sub-millisecond switching time and low haze.