The nonlinear absorption coefficient and the nonlinear refractive index of 0.2 wt% carbon nano colloidal liquids were measured using a double-beam technique. The nonlinear optical coefficients were obtained by using the open- and closed-aperture methods with a $z$-scan technique that moves the sample along the $z$-axis. The Gaussian laser beam used in this experiment was focused with a lens having a focal length of 5 cm. An Nd:YVO$_4$ laser beam ($\lambda$ = 523 nm) with a 64 mW power was used as the pump light source and a He-Ne laser beam ($\lambda$ = 633 nm) with a 4.7 mW power was used as the probe light source. To find the concentration-dependent optical property, we changed the concentration level of the carbon nano colloid from 0.076 M to 0.087 M. The result showed that the linear absorption coefficient increased as the concentration was increased. Therefore, as the concentration increases, the intensity of light at the focal point becomes weak, and the nonlinear absorptance and the nonlinear refractive index decrease.  

Keywords: Carbon nano tube, z-scan method, Nonlinear absorption coefficient, Nonlinear refractive index, Thermal lens effect