Research On Thermal Optical Nonlinearity Induced By Nonlinear Absorption

Thermal induced nonlinearity always accompanys with the interaction of optics and material, and affects the experiment results directly. Study on thermal induced nonlinearities is significant for understanding the physical mechanisms of optical nonlinearities and the process of the nonlinear optics in the transient scale, and it also has positive means for consummating the knowledge of nonlinear optics.In this dissertation, the development of the research on thermal induced nonlinearity, physical mechanisms of the thermal induced nonlinearities and the photoacoustic equation which describes the thermally induced nonlinearity is summarized. In the condition of linear absorption, several approximate models of thermal induced nonlinearity in the quasi-static scale are studied, and these models are compared with the photoacoustic model in the transient scale under different pulse widths. Studies on thermal induced nonlinearity with a CW beam indicate that the refractivity change is proportional to the power of the laser beam, and it is proved by the theoretical simulation results. In the condition of nonlinear absorption, the affect of thermal effect on the Z-scan curves under different pulse widths, beam waists and nonlinear absorption coefficients is studied.The condition in which the thermal effect coexists with Kerr effect and excited-state nonlinearities is also taken into account when we studies the Z-scan curves affected by different pulse energy, pulse widths and different solvent The results of the theoretical simulation show that, it is possible for the nonlinear optical property of the material to change from self-focusing to self-defocusing or change from self-defocusing to self-focusing.. Theoretical simulation on ClAlPc/methanol solution under a single picosecond pulse and a train of picosecond pulses suggests that, excited-state nonlinearities take predominance under a single picosecond pulse and thermal accumulative effect takes predominance under a train of picosecond pluses. Finally, the theoretical study on the nonlinearities of the ClAlPc/methanol solution in the nanosecond scale indicates that ClAlPc/methanol solution will change from self-focusing to self- defocusing when the pulse width changes from 5ns to 10ns.