Electromagnetically tunable cholesterics with oblique helicoidal structure [Invited

Abstract

Cholesteric liquid crystals form a right-angle helicoidal structure with the pitch in the submicrometer and micrometer range. Because of the periodic modulation of the refractive index, the structure is capable of Bragg and Raman-Nath diffraction and mirrorless lasing. An attractive feature of cholesterics for optical applications is that the pitch and thus the wavelength of diffraction respond to temperature or chemical composition changes. However, the most desired mode of pitch control, by electromagnetic fields, has so far been elusive. Synthesis of bent-shape flexible dimer molecules resulted in an experimental realization of a new cholesteric state with an oblique helicoidal structure, abbreviated as ChOH. The ChOH state forms when the material is acted upon by the electric or magnetic field and aligns its axis parallel to the field. The principal advantage of ChOH is that the field changes the pitch but preserves the single-harmonic heliconical structure. As a result, the material shows an extraordinarily broad range of electrically or magnetically tunable robust selective reflection of light, from ultraviolet to visible and infrared, and efficient tunable lasing. The ChOH structure also responds to molecular reorientation at bounding plates and optical torques. This brief review discusses the recently established features of ChOH electro-optics and problems to solve.