Analysis of nonlinear optical properties and thermal diffusivity of carbon nano-colloids using the thermal lens effects

When a laser beam is focused on a liquid through a lens, the liquid absorbs the focused light energy, heat diffuses into the surroundings, and nonlinear absorption and refraction occur. This phenomenon is called the thermal lens effect. We measured the nonlinear optical absorption and refraction of carbon nano-colloids using a pump-probe double-beam z-scan technique, which is more efficient than the single-beam technique. We also obtained the thermal diffusivity of the liquid by measuring the characteristic time (t(c)) of the thermal diffusion and fitting the transmittance of the probe beam. The liquid sample is an aqueous solution of carbon nano-colloids with 200-nm diameter, and the nonlinear optical properties and thermal diffusivity were obtained using different concentrations.