| Summary: | 碩士 === 國立中山大學 === 光電工程學系研究所 === 104 === This research employed Z-scan technique to investigate optical nonlinearities of graphene saturable absorber with different number of layer, and used Erbium-doped fiber as gain medium in passive mode-locking to generated stable ultrashort pulses. Graphene thin-film was fabricated by Chemical vapor deposition (CVD) methods, and used polyvinyl alcohol (PVA) solution to produce high polymer thin-film for mode-locking experiment.
Our light source for Z-scan experiment was pico-second scale Nd:YAG Laser of 532 nm wavelength. Nonlinearities of graphene thin-film was investigated by changing illuminating intensity. Our research showed that both nonlinear absorption coefficient and nonlinear refractive index increased as number of layer increased, and decreased with increasing intensity. For samples with fewer layer-stacking (1-,8-layer), nonlinear absorption was resulted from the saturation of single photon absorption. However, for samples with higher number of layer, reverse saturable absorption phenomenon started to show at high illuminating intensities, indicated the involvement of two-photon absorption. This might because of the bandgap widening induced by layer-stacking in graphene material.
Our research also used graphene of different number of layer to fabricate hi-polymer thin film for passive mode-locking experiment. The experiment included the investigation of the starting pumping power and the dynamic range of stable mode-locking, as well as the optimized output pulse-width. First, the starting pumping power for stable mode-locking was 62.8 mW for 5-layer graphene sample, 62.8 mW for 8-layer, 72.1 mW for 12-layer, and 86.0 mW for 17-layer one, respectively. Secondly, as for the dynamic range, 5-layer sample had the widest dynamic range of 107.7 mW. And it was 100.2 mW for 8-layer, 81.3 mW for 12-layer, and 55.0 mW for 17-layer. The results indicated that the starting pumping power for stable mode-locking increased with the number of layer, and the dynamic range became narrower with increasing layer-stacking. The reason was that for graphene with more number of layer, the reverse saturable absorption phenomenon would be induced at high intensity, which caused instability for passive mode-locking. Lastly, the optimized pulse-width was 603 fs for 5-layer graphene thin-film, 584 fs for 8-layer, 543 fs for 12-layer, and 523 for 17-layer, respectively. The 17-layer graphene sample showed that shortest output pulse-width, indicating that the increase of layer-stacking favored the compression of mode-locking pulse.
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