Liquid crystal based continuous phase retarder: from optically neutral to a quarter waveplate in 200 microseconds.

摘要

Liquid crystal variable phase retarders have been incorporated into prototype devices for optical communications systemapplications, both as endless polarization controllers1,2,3, and as holographic beam steerers4. Nematic liquid crystals allowcontinuous control of the degree of retardation induced at relatively slow switching speeds, while ferroelectric liquidcrystal based devices allow fast (sub millisecond) switching, but only between two bistable states. The flexoelectro-opticeffect5,6 in short-pitch chiral nematic liquid crystals allows both fast switching of the optic axis and continuous, electricfield dependent control of the degree of rotation of the optic axis.A novel geometry for the flexoelectro-optic effect is presented here, in which the helical axis of the chiral nematic isperpendicular to the cell walls (grandjean texture) and the electric field is applied in the plane of the cell. This facilitatesdeflection of the optic axis of the uniaxial negatively birefringent material from lying along the direction of propagationto having some component in the polarization plane of the light. The device is therefore optically neutral at zero field fortelecommunications wavelengths (1550nm), and allows a continuously variable degree of phase excursion to be induced,up to 2π/3 radians achieved so far in a 40μm thick cell. The retardation has been shown both to appear, on application ofthe field, and disappear on removal, at speeds of 100-500 μs. The direction of deflection of the optic axis is alsodependent on the direction of the field, allowing the possibility, in a converging electrode “cartwheel cell”, of endlessrotation of the liquid crystal waveplate at a higher rate than achievable through dielectric coupling to plain nematicmaterials.