From UV to IR: electrically controlled optical filters based on oblique helicoidal cholesterics

Oleg D. Lavrentovich
Kent State University, Ohio, United States

Keywords: electrically tuned optical filters, cholesteric liquid crystals, UV-IR range

Cholesteric liquid crystals (CLCs) exhibit selective light reflection due to the Bragg reflection on their right-angle helicoidal periodic structure. For practical applications, an electric field should align the orientation of helical axis along the field and tune the periodicity. However, in standard CLCs the field realigns the helicoidal axis perpendicular to the field and then untwists the helicoid. We create a new class of CLCs with the extremely small bend elastic constant by adding chiral dopants to the mixtures of dimer materials that exhibit nematic twist-bend phase below working temperature. In these CLCs the electric field creates an oblique helicoidal structure along the field with the decreasing pitch and cone angle when the applied field increases. As a result, reflection of light can be tuned by the electric field in the range from ultraviolet throughout the visible and up to infrared part of the spectrum. We demonstrate that the proper surface alignments improve the quality of oblique helicoid and make the CLCs cells the prototype quality adjustable optical filters with ultra-wide range and the intensity of selective light reflection close to the 50% theoretical limit for unpolarized light. The work was supported by NSF grants DMR-1410378 and IIP-1500204.