Electric-field enhancement and modulation of permanent grating diffraction in dye and carbon nanotube-doped liquid crystal

Permanent gratings were written, without the aid of external fields, in planar nematic liquid crystal cells doped with only Methyl Red (MR) as well as cells doped with both Methyl Red and single-wall carbon nanotubes (CNTs). Grating formation based on trans-cis photoisomerism of MR followed by surface adsorption of the cis-isomer was attempted. The diffraction efficiency was easily enhanced and modulated by application of ac fields covering wide ranges of frequency and voltage; thus, allowing for tuning the diffracted signals. Cells doped with MR and CNTs had a maximum relative diffraction efficiency of 67% while cells doped only with MR exhibited a maximum of 28%. The maximum diffraction efficiency appears to be associated with the Fréedericksz threshold voltage. The permanent gratings are robust and have remained stable for over two years.