CHOLESTERIC LIQUID-CRYSTALLINE SILOXANES WITH AZO-DYE - GENERATION OFADDITIONAL REFLECTION BANDS WITH LINEARLY POLARIZED-LIGHT

Photosensitive cholesteric polysiloxanes, which contain an azo dye, we re irradiated with linearly polarized light. The cholesteric samples w ere oriented in the Grandjean texture. Before irradiation they reflect ed circularly polarized light in the near infrared region. For perpend icular incidence, only one order of reflection was observed. Upon irra diation with linearly polarized light, which is absorbed by the azo dy e, additional reflection bands appeared in the visible part of the spe ctrum. It turned out that the additional reflection is caused by a new Bragg type grating which shows higher reflection orders. The formatio n of the grating is based on the periodic deformation of the helical o rdering of the molecules. The deformation is periodic, as due to photo selection, only dye molecules in equidistant layers with a suitable or ientation absorb radiation. For low exposure, the grating reflects lin early polarized light. After continued irradiation, the reflection ban ds disappear almost completely. High birefringence, strongly dichroic dye absorption and the loss of the reflecting properties prove that a planar nematic texture has developed. The formation of this texture fr om the Grandjean texture is a new example for photoinduced rotational diffusion.