In this master thesis, optical nonlinear properties of solidified silk fibroin are investigated by using a Z-scan and an optical Kerr gate method. Silk fibroin is the adhesive proteins in silkworm and has been newly focused in diverse application areas due to its mechanical robustness, optical transparency, biocompatibility, controllable degradation, and easy functionalization. Furthermore, it has been demonstrated that silk fibroin could be utilized for a base material of nanophotonic devices. In this study, three different types of silk fibroin samples, pure and water- and methanol-treated silk fibroin samples, respectively, are prepared in order to study optical nonlinear properties of silk fibroin depending on the silk structure. From Z-scan and optical Kerr gate measurements, values of the third order susceptibility of silk fibroin samples are measured to be in the order of 10^-10~10^-9 esu in a near infrared wavelength range, which are exceptionally large values for an inorganic material and comparable to that of single-walled carbon nanotubes. Especially, the methanol-treated silk fibroin samples show generally a factor of two larger third order susceptibility than the water-treated samples, which have also 2~3 times larger values than pure silk fibroin samples. Such increased nonlinearities of the water- and methanol-treated samples can be estimated from the enhanced van der Waals interaction and hydrogen bonding of molecules and improved cooperative interaction between electric transitions of aromatic acids in solidified silk fibroin. Silk fibroin, along with the biocompatibility and suitability to nanofabrication, will open new ways for nonlinear bioimaging and biosensing applications.
