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Azimuthal anchoring energy of liquid crystals aligned using laser-ablated gratings in polyimide

Gratings etched into polyimde thin layers are used to align nematic liquid crystals. The gratings are prepared by illumination of a 1.1 micrometer period phase mask with a KrF excimer laser at 248 nm. Fluences of 87 mJ cm-2 and 128 mJ cm-2 with one and two shots were used to ablate the gratings. Modelling of the fluence distribution behind the phase mask predicts a grating period equal to that of the phase mask and this is found experimentally. The amplitudes of the gratings are obtained from diffraction using a HeNe laser and are between 100 nm and 150 nm deep. The alignment layers are used in twisted nematic cells and the azimuthal anchoring energy is measured as a function of grating fabrication conditions. Anchoring energies of the order of 10-5 J m-2 are found in agreement with the Berreman theory.