||In the thesis, we used Z-scan technique to study the nonlinear optical properties of graphene saturable absorber. The light source employed in this study is frequency doubled, Q-switched and mode-locked Nd:YAG laser. As for the samples, we took advantage of CVD method to grow atomic layer graphene, and acquired different layers, which are 1-, 8-, and 16-layers.|
By injecting an ultrafast, picosecond, 532 nm wavelength Nd:YAG laser into the above-mentioned three samples, and using different laser pulse energies to measure and investigating various nonlinear optical effects of graphene. Through Z-scan experiment, we measured the nonlinear absorption and nonlinear refractive optical coefficient for different layers graphene material.
As a result, the nonlinear absorption coefficient α(I) and nonlinear refractive index n(I) are obtained as 0.468 to 3.563 (108 cm-1) and 0.733 to 5.506 (108 cm-1), respectively. Compared with other semiconductor materials, graphene grown by CVD method has large nonlinear parameters. In addition, we concluded that the nonlinear effects might not be resulted from two photon absorption. The main source of absorption is the saturation of single photon absorption. But for graphene with more layers or defects under intensity exceeds 40 GW/cm2, TPA mechanism might be involved.