The Theoretical And Experimental Studies On Thermally Induced Optical Nonlinearity Of A Cluster Polymer {WS₄Cu₄I₂(bpe)₃}_n In DMF Solution

During the laser-pulse propagation through nonlinear optical materials, thermal effect is always existent for their absorption of light. Thermal nonlinearity is one of the first discussed nonlinear optical mechanisms, and has been repaid attention recently because of their potential applications in optical limiters in nanosecond regime. In this dissertation a numerical model is implemented to research the thermal nonlinearities in liquids.The thermal index can be divided into three parts by the relation of the buildup time for the thermal nonlinearities, the laser pulse duration and the thermal diffusive time, that is steady-state regime, quasi-steady-state regime and transient regime.The acoustic-wave equation shows that the laser pulse width and beam radius determine the value of the transient thermal nonlinear refraction. Besides these two factors, the nonlinear absorption coefficient and the linear transmittance can also have influence on the transient thermal nonlinearities. These contributions are simulated using the nonlinear beam propagation equation and the Crank-Nicholson finite difference. It is found that the effect of thermal induced optical nonlinearities is enhanced by the increase of nonlinear absorption, the pulse broadening or the diminution of beam waist radius and linear transmittance.Many nonlinear mechanisms always exist simultaneously in the liquid. The thermal effect and other nonlinear effects are all taken into account to give theoretical Z-scan curves, which are also analyzed by the nonlinear beam propagation equation and the Crank-Nicholson finite difference. Then we study the dynamics of the transient thermal nonlinearities and Kerr effect. There will be a single-peak configuration of the theoretical Z-scan curves when the two effects counteract.The nonlinear optical properties of a novel cluster polymer {WS4Cu4I2(bpe)3}n in DMF solution are studied by using Z-scan technique with 4.5ns duration laser pulsed at 532 nm wavelength. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermally induced refraction. The thermal refraction is mainly due to the strong nonlinear absorption. Numerical simulations agree basically with the experimental results.