Nonlinear optical properties of (CH₃)₄NAu(dmit)₂ using Z-scan technique

The third-order optical nonlinearities of (CH_3)_4NAu(dmit)_2 (dmit = 4,5-dithiolate-1,3-dithiole-2-thione) at 532 nm and 1064 nm are investigated using the Z-scan technique with pulses of picoseconds duration. The Z-scan spectra reveal a strong nonlinear absorption (reverse saturable absorption) and a negative nonlinear refraction at 532\,nm. No nonlinear absorption is observed at 1064 nm. The molecular second-order hyperpolarizability \gamma for the (CH_3)_4NAu(dmit)_2 molecule at 532 nm is estimated to be as high as (2.1 \pm 0.1) \times 10^-31 esu, which is nearly three times larger than that at 1064\,nm. The mechanism responsible for the difference between the results is analysed. Nonlinear transmission measurements suggest that this material has potential applications in optical limiting.