Soft Matter Macro and Micro-Photonics workshop

Electro-optics of chiral heliconical and ferroelectric nematics

Oleg D. Lavrentovich1,2,3
1 Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA

2 Department of Physics, Kent State University, Kent, OH 44242, USA

3 Materials Science Graduate Program, Kent State University, Kent, OH 44242, USA

Cholesteric and ferroelectric nematic liquid crystals are soft materials offering novel electro-optical effects. This presentation reports on two effects, (1) electrically tunable selective reflection of light by an oblique helicoidal cholesteric and (2) light-induced Fréedericksz transition in a ferroelectric nematic.

1. A cholesteric liquid crystal with an oblique helicoidal structure, abbreviated ChOH, forms in a chiral nematic formed by flexible dimers. The low bend elastic modulus enables a heliconical state in the presence of an electric field, in which the molecules twist around a helicoidal axis but remain titled rather than perpendicular to it as in a conventional cholesteric. The electric 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 ChOH 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.

2. A ferroelectric nematic demonstrates an extraordinary sensitivity to a laser beam in the visible and ultraviolet part of the spectrum, as revealed by an optically induced Fréedericksz transition. Upon light irradiation, the polarization and optic axis realign in the plane of the cell.

The work is supported by the National Science Foundation grant DMR-2341830 and ARO MURI grant 84914-SM-MUR.