Bis(tetrabutylammonium)bis(4,5-dithiolato-1,3-dithiole-2-thione)copper (BCDT) was synthesized and its third-order optical nonlinearity was characterized using picosecond Z-scan at 1064 nm. The Z-scan spectra reveal a large negative nonlinear refraction coefficient n(2) as high as 4.0 x 10(-18) m(2)/W and a slight two-photon absorption beta, which is determined to be 3.4 x 10(-12) m/W. The molecular second-order hyperpolarizability gamma was calculated to be 6.5 x 10(-32) esu. All these results suggest that this material has potential for the application of all-optical switching. (C) 2005 Elsevier Ltd. All rights reserved.