Highly stable optically induced birefringence and holographic surface gratings on a new azocarbazole-based polyimide

An azocarbazole-based polyimide was synthesized from an azocarbazole diamin e monomer and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) by a two-step polycondensation reaction. Birefringence experiments have been performed on the pure polyimide and on the polyimide mixed with small azo c hromophores. Because of its high T-g, the polyimide has a very stable induc ed birefringence (only 14% loss during the relaxation process). The stabili ty of the mixtures is also very high. Surface relief diffraction gratings w ere inscribed on these systems. The measured diffraction efficiency was abo ut 0.25% for a diamine-doped polyimide exposed for 1 h to an irradiance of 200 mW/cm(2). The surface profile studied by atomic force microscopy showed an amplitude of 30 nm and a regular spacing of 700 nm on a 255 nm thick fi lm. The photoinduced gratings had a high stability without further surface deformation after baking at 240 degrees C for 1 h in air.