Electrically Tunable Structural Colors of Cholesteric Liquid Crystals

Cholesteric liquid crystals are soft materials capable of selective reflection and transmission of light thanks to a spatially varying refractive index associated with the helicoidal twist of the optic axis. An attractive feature of cholesterics for optical applications is that the pitch and, thus, the wavelength of reflected and transmitted light can be tuned by temperature or chemical composition. However, the most desired mode of pitch control, by electromagnetic fields, has been elusive. This presentation reports on a cholesteric liquid crystal with an oblique helicoidal structure, abbreviated as Ch_OH. In the heliconical Ch_OH structure, the local director twists about a single axis but remains tilted to this axis (rather than being perpendicular to it, as in a conventional cholesteric). The Ch_OH forms when a chiral liquid crystal with a low bend elastic constant is acted upon by an electric (or a magnetic) field. The field tunes the pitch and the conical tilt angle without destroying the single-harmonic mode of periodic variation of the effective refractive index. As a result, a simple device in the form of a thin Ch_OH slab confined between two plates with transparent electrodes shows an extraordinarily broad range of electrically tunable robust selective reflection with structural colors reversibly shifting from ultraviolet to visible and infrared. For the oblique incidence of light, the slab shows total reflection with a tunable wavelength. The electric field controls both the bandwidth and the wavelength of reflection and transmission. The structure can also be used for electrically tunable retarders. The work is supported by the National Science Foundation grants ECCS-1906104 and ECCS -2122399.