Shear direction switching phenomena in twisted nematic liquid crystal cells for novel differential interference contrast imaging systems

A lateral shear occurs between ordinary and extraordinary rays in general optical uniaxial crystals, and we can observe a double image through the crystal, indicating a typical birefringence phenomenon. Small lateral shears occur in typical liquid crystal (LC) cells under small voltage applications, where the molecular alignment direction is generally inclined. The small lateral shear is useful for differential interference contrast (DIC) imaging systems, although it must be negligible in a typical LC display panel. In particular, unique lateral shearing properties are exhibited by twisted nematic (TN) LC cells, which are popular and important LC display modes. The 45 degrees-oblique lateral shear occurs according to the direction of the surface molecular alignment, and the inclination direction (+45 degrees or -45 degrees) depends on the molecular twist direction. If it is possible to switch the twist direction in the TN cell, the lateral shear direction can be switched by 90 degrees, and a full DIC imaging system can be attained because the normal DIC system can obtain one-directional derivative information along the shear direction. In this work, twist direction switching phenomena in TN cells are investigated by applying a magnetic field in the direction parallel to the cell surface.