In this paper, we present the linear and nonlinear optical properties of an organic dye, 4,8,12-tri-n-octyl-4,8,12-triazatriangulenium tetrafluoroborate. Transmission Z-scan setup was used to measure the third-order nonlinear optical absorption and susceptibility. Open aperture configuration reveals a concurrence of saturable absorption peak and reverse saturable absorption valley. The SA/RSA curves were interpreted on the basis of single-photon and two-photon absorption respectively. The closed aperture configuration shows an asymmetrical peak-valley indicating a negative refractive index due to self-defocussing phenomena.
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