Optically isotropic liquid crystal medium and optical device

A liquid crystal composition that exhibits an optically isotropic liquid crystal phase and contains achiral component T containing at least one compound selected from the group of compounds represented by formula (1) as a first component, and a chiral agent:

• wherein R1 is hydrogen or alkyl having 1 to 20 carbons, arbitrary —CH2— in the alkyl may be replaced by —O—, —S—, —COO—, —OCO—, —CH═CH—, —CF═CF— or —C≡C—, and arbitrary hydrogen in the alkyl or in a group obtained by replacing arbitrary —CH2— in the alkyl by —O—, —S—,—COO—, —OCO—, —CH═CH—, —CF═CF— or —C≡C— may be replaced by halogen;
• L1, L2, L3, L4, L5 and L6 are independently hydrogen or fluorine;
• X1 is hydrogen, halogen, —SF5 or alkyl having 1 to 10 carbons, arbitrary —CH2— in the alkyl may be replaced by —O—, —S—, —CH═CH— or —C≡C—, and hydrogen in the alkyl or in a group obtained by replacing arbitrary —CH2— in the alkyl by —O—, —S—, —COO—, —OCO—, —CH═CH—, —CF═CF— or —C≡C— may be replaced by fluorine, the liquid crystal composition shows a chiral nematic phase with a helical pitch of 700 nanometers or less in at least part of the temperature range from 70 to −20° C.