Broadband Nonlinear Optical Properties Of Fluorine-Doped Tin Oxide Film

In recent years,epsilon near zero（ENZ）materials have drawn widespread attention from researchers due to their extremely high nonlinear efficiency in the near zero permittivity region.Among them,the most typical of which is indium tin oxide（ITO）,which exhibit various nonlinear phenomena,such as enhanced nonlinear refractive index,harmonic generation with high conversion efficiency,etc.Besides,the manufacturing process of TCO（Transparent Conductive Oxide）materials is mature and compatible with CMOS（Complementary Metal Oxide Semiconductor）technology,and has good stability.Hence,ENZ material is a promising platform for nonlinear photonics.As another typical TCO material,fluorine doped tin oxide（FTO）has a region where the permittivity tends to zero,is non-toxic and more stable than indium tin oxide,and does not contain rare elements.It shows that FTO has the value and potential to be applied to nonlinear devices.Based on the previous research,this thesis mainly studies the broadband nonlinear optical response of fluorine-doped tin dioxide thin films,and analyzes the optical nonlinear enhancement of FTO in combination with ENZ effect.The specific research contents include:（1）Numerical simulations of the nonlinear response of the FTO was performed.Firstly,the transient response under femtosecond laser was calculated by the two-temperature model,and the temperature variation trend of the electron and lattice was obtained.Then,based on the non-parabolic conduction band properties of FTO,the plasma frequencies at different wavelengths and high electron temperatures were deduced,and the deviated permittivity was obtained by combining with the Drude model,thereby deriving the nonlinear response of FTO in the near-infrared band.The results show that the nonlinear response of FTO in the ENZ region will be significantly enhanced.（2）The nonlinear response of FTO is investigated experimentally and applied.The FTO film was first characterized and the ENZ region of the sample was determined.Then,the Z-scan technique was used to carry out experimental measurements,and the nonlinear absorption coefficient and refractive index coefficient of the ENZ region were obtained as-1.96×10^-9 m/W and 2.52×10^-16m²/W,respectively.Compared with the non-ENZ region,the nonlinear absorption coefficient and refractive index of the ENZ region are significantly enhanced,which is consistent with the numerical simulation conclusion.A P-scan experiment was set up to further characterize the broadband saturable absorption properties of FTO.Finally,FTO was fabricated as a saturable absorber and inserted into a fiber laser to achieve 1.5μm mode-locked pulse output.