Bragg diffraction of obliquely incident light at heliconical cholesteric structures

An electric field acting on a chiral mixture of flexible dimers produces an oblique helicoidal (also called heliconical) cholesteric (Ch_OH) state with its axis parallel to the field (Xiang, J. et.al, Phys. Rev. Lett. 112, 217801 (2014); Xiang, J. et.al, Adv. Mater. 27, 3014 (2015)). Both the Ch_OH period and the conical angle depend on the applied field, which enables electrically tunable Bragg diffraction in a broad spectral range from ultraviolet to infrared. We demonstrate experimentally that oblique propagation of light results in reflection at both the half- and full pitch of the Ch_OH structure. Diffraction at the half-pitch periodicity shows bandgap triplets whose lateral peaks are polarization dependent, while the central peak is not. The full pitch bandgap is a singlet characterized by a wide bandwidth and total reflection at a large angle of oblique incidence and whose polarization characteristics are similar to those of the central peak of the half-pitch triplet. We develop a model of light propagation in Ch_OH that explains and reproduces the observed optical effects at oblique incidence of light that might be attractive for applications such as electrically tunable band-pass filters, mirrors, low-threshold lasers, etc. The work is supported by NSF grant ECCS-1906104.