Optical nonlinearities of (TBA)2Ni(dmit)2:: Saturable-absorption and negative third-order nonlinear refraction due to charge-transfer transtion

The optical nonlinearity of (TBA)(2)Ni(dmit)(2) (TBA = tert-n-butanyl-amonimium, dmit = 1,3-dithiol-2-thione-4,5-dithiolate) at high incident irradiance was investigated using Z-scan technique with 1064 nm picosecond laser pulses. This molecule shows a strong absorption band ranging from 900 to 1600 nm. Under excitation by 1064nm laser pulses, this molecule exhibits typical saturable-absorption (SA) due to this transition, and shows a negative third-order contribution to nonlinear refraction (NLR). This result indicates that it shows a third-order self-defocusing. In addition, a positive fifth-order contribution to NLR, and the influences of aggregation and high excited-state absorption on nonlinear absorption were also observed in this molecule.