Fast optical switching using cyanine dye-doped nematic liquid crystal

A cyanine dye, 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanine perchlorate (NK-2014) was doped in 4-cyano-4′-n-pentylbiphenyl (5CB), and the mixture was sandwiched between two pieces of rubbed glass plates. The third-order nonlinear optical responses of oriented NK-2014-doped 5CB were measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique at 820 nm. The time resolution of the system was ca. 0.3 ps (FWHM). The electronic component of third-order nonlinear optical susceptibility of one of the present samples was 1.3×10−8 esu (NK-2014 concentration: 1.0 wt%). The slow DFWM response of NK-2014-doped 5CB was accelerated with increasing laser power due to amplified spontaneous emission (ASE). On the other hand, we did not observe a similar phenomenon for NK-2014-doped polyethylene glycol (PEG-400). Oriented NK-2014 molecules in nematic liquid crystal must have very high ASE efficiency. Hence the population grating disappears in ca. 3 ps. NK-2014-doped 5CB can be used as a material for very fast all-optical switching.