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 (5 CB), 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
760 nm. The third-order nonlinear optical susceptibility of one of the
present samples is 5.5×10−8 esu. 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 4 ps. It is
expected that NK-125-5CB can be used as a material for very fast
all-optical switching.