Large Optical Torque Enhancement By Oligothiophene Dye In A Nematic Liquid Crystal Host

We have studied the nonlinear optical response of the nematic liquid crystal E7 doped with the oligothiophene dye TR5. Z-scan measurements showed a large positive(self-focusing)nonlinearity, originating in director reorientation. The oligothiophene dye enhances the nonlinearity of the host, in a similar manner to that of azo- and anthraquinone dyes in guest-host systems which exhibit the Janossy effect. The enhancement factor is unusually large; our TR5-E7 system showed as large a dye-induced optical torque enhancement factor as any observed in a guest-host system. Here we present our experimental results, determine the enhancement factor, and hypothesize that the dominant mechanism involves a photo-induced decrease in the orientational mobility of the dye molecules.