Electrically tunable structural colors of cholesterics with oblique helicoidal director*

A cholesteric liquid crystal was recently shown (Xiang, J. et al., Phys. Rev. Lett. 112, 217801 (2014); Xiang, J. et al., Adv. Mater. 27, 3014 (2015)) to exhibit a peculiar oblique helicoidal state (Ch_OH) when acted upon by an electric field. The period of Ch_OH structure depends strongly on the applied field, which enables electrically tunable structural colors in an extraordinary broad spectral range from ultraviolet to infrared. We present experimental and theoretical studies of light reflection from Ch_OH as a function of the electric field, surface anchoring, and incident angle. Unlike the case of conventional cholecterics, Ch_OH shows tunable reflection at periodicities that correspond to both pitch (for oblique incidence) and half-the-pitch. Reflection spectra are used for the first in-situ measurements of bend elastic constant of the chiral material.