Experimental elucidation of nematic host-driven modulation of Bragg reflection in cholesteric liquid crystals

Abstract

This experimental study explores the critical role of the host nematic liquid crystal environment in modulating selective Bragg reflection in cholesteric liquid crystals, which possess a self-organized helical superstructure capable of reflecting circularly polarized light within a specific wavelength band. By incorporating 2.45 wt% of the chiral dopant R5011 into five distinct NLC hosts (ML-0648, E7, RDP84685, MLC-2053, and MLC-2048), we systematically investigate the influence of host-specific parameters (such as birefringence, refractive index, and molecular compatibility) on key optical features, including central Bragg reflection wavelength, bandwidth, and helical twisting power. Our findings reveal a tunable Bragg reflection range from 499 nm (ML-0648) to 754 nm (MLC-2048), strongly governed by the cholesteric pitch and host refractive index. The reflection bandwidth spans 23–86 nm, with broader spectra observed in hosts exhibiting higher birefringence. Notably, ML-0648, chemically similar to the chiral dopant, achieves the highest helical twisting power value of 123.88 μm⁻¹, underscoring the importance of host–dopant molecular affinity. Furthermore, doping E7 with varying S5011 concentrations (2.30–3.42 wt %) produces a pronounced blue shift in central Bragg reflection wavelength, alongside an inverse correlation between reflection bandwidth and dopant concentration. These results elucidate the tunability of optical properties in cholesteric liquid crystals via strategic host selection, offering valuable insights for designing advanced optical devices such as tunable filters, reflective displays, and chiral photonic structures.