Carbon nanotube enhanced diffraction efficiency in dye-doped liquid crystal

We report on the optical properties of nematic liquid crystal cells containing the liquid crystal E7 doped with the azo dye Methyl Red at percentages ranging from 0.4% to 0.8% and 0.002% single-wall carbon nanotubes. From polarized absorption measurements an order parameter of S = 0.605 was obtained, showing that the dye molecules align themselves very well with the liquid crystal's director axis. Diffraction efficiencies of 532-nm pump and 670-nm probe beams were measured. Efficiencies and rise times were found to increase with dye concentration. A maximum efficiency of 5.8% was found for cells doped with carbon nanotubes, while cells without carbon nanotubes had a maximum efficiency of 3.2%. Therefore the presence of carbon nanotubes enhanced the diffraction efficiency by a factor of 1.8. The nonlinear index coefficient, n₂, was calculated to be 18 × 10^?3 cm²/W. The data are consistent with a grating formation based on trans-cis photoisomerism of the dye molecules that leads to a reorientation of the liquid crystal phase. These photonic devices are functional without the application of any external field, are easily prepared, and have lifetimes in excess of two years without any indication of degradation when stored at ambient temperatures.