Fast Optical Switching Using Oriented Cyanine Dye-Doped Nematic Liquid Crystal

A cyanine dye, 2-7-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5-heptatrienyl-% 1,3,3-trimethyl-3H-indolium iodide (NK-125), is doped in 4-cyano-4’-pentylbiphenyl (5CB), and the mixture is sandwiched between two pieces of rubbed glass plates. The third-order nonlinear optical properties of the oriented NK-125-5CB layers are measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique at 760nm. The third-order nonlinear optical susceptibility of one of the present samples is 5.5×10^-8esu. The slow DFWM response of the NK-125-5CB layers due to a population grating is accelerated by the increasing laser power because of amplified spontaneous emission (ASE). On the other hand, we do not observe a similar phenomenon for NK-125-polyethylene glycol (PEG-400). Oriented NK-125 molecules in nematic liquid crystals must have very high ASE efficiency. Hence the population grating in a DFWM signal disappears within about 4ps. It is expected that NK-125-5CB can be used as a material for very fast all-optical switching.