cholesteric

Cholesteric refers to a phase of liquid crystals, also called the cholesteric or chiral nematic phase. It occurs in certain chiral molecules or mixtures, where the average orientation of elongated molecules, or the director, twists about an axis to form a helical structure. The distance over which the director rotates by 360 degrees is called the pitch, typically on the order of micrometers. The helix can be right- or left-handed depending on the molecular chirality.

Cholesteric order arises when a nematic host contains a chiral dopant or when the molecules themselves are intrinsically chiral. The pitch, and thus the color, depends on temperature, concentration of chiral material, and intermolecular interactions. The phase is distinct from plain nematic and smectic phases and is characterized by a periodically modulated director field.

Optical properties: cholesteric liquids exhibit selective Bragg reflection of circularly polarized light. The wavelength reflected is approximately n_avg times the pitch, where n_avg is the average refractive index; thus the observed color changes with pitch. Temperature or electric fields can tune the pitch, enabling tunable color and reflective displays. On polarizing microscopy, cholesteric textures often show fingerprint-like patterns.

Applications: because they reflect specific wavelengths, cholesteric liquid crystals are used in reflective, low-power displays, optical filters, sensors, and tunable photonic devices. They also serve as self-organized Bragg reflectors in various optical components or lasers.

Notes: the term cholesteric derives from the initial observation of cholesteryl derivatives.