Third-order Nonlinear Optical Properties Of Low-dimensional Semiconductors

Third-order nonlinear optical materials have a wide range of applications in the fields of optical limiting,all-optical switching,laser mode-locking,phase modulation,and nonlinear waveguides,and are regarded as the basis of a new generation of photonics technology.In this dissertation,we fabricate new third-order nonlinear optical materials and focus on improving their nonlinear optical responses via different strategies.In recent years,low-dimensional nanomaterials such as two-dimensional materials and nanocrystals,quantum dots,etc.have flourished.The quantum confinement effect of these materials limits the movement of photoexcited carriers and greatly enhances the interaction between light and matter,which is conducive to enhance the nonlinear optical response of the materials.The size-dependence of band gaps,as well as the carrier concentrations and excited state dynamics affected by elemental doping and defects,and electron or energy transfer between composites and heterojunctions,etc.,all provide degrees of freedom for tuning the nonlinear optical response of materials.In this dissertation,we fabricated a series of two-dimensional（2D）transition metal oxides,ENZ and plasmonic materials and investigated their third-order nonlinear optical properties from the perspective of excited state dynamics.The main work includes the following parts:1.Defect engineering is utilized to regulate the third-order nonlinear optical properties of 2D transitional metal oxides.2D WO₃ nanosheets containing different oxygen vacancy defects were fabricated by controlling the precursor calcination conditions.And their third-order nonlinear absorption coefficients were determined by the Z-scan technique.The results showed that the saturation absorption properties of the WO₃ nanosheets were correlated with the concentration of oxygen vacancy defects.The decay kinetics of excited carriers in materials were studied by transient absorption and laser flash photolysis techniques.The results show that the increase in oxygen vacancies enhances the saturated absorption performance by prolonging the lifetime of excited carriers.2.The dielectric resonance characteristics of epsilon-near-zero（ENZ）materials is harnessed to enhance the third-order nonlinear optical response.Intermediate bandgap semiconductor Cu₃VSe₄ nanocrystals were synthesized by thermal injection.The Z-scan test results show that the reverse saturation absorption of the Cu₃VSe₄nanocrystals is comparable to that of C60 at 532 nm,and they also have strong reverse saturation absorption at 1064 nm where C60 is non-responsive,which indicates that the Cu₃VSe₄ nanocrystals are a broadband optical limiting material.The spectral and kinetic results obtained by transient absorption spectroscopy show that the material has plasmon-like resonance properties.Density functional theory（DFT）calculations confirmed that Cu₃VSe₄ nanocrystals are intermediate bandgap semiconductors,i.e.,natural ENZ materials.Their dielectric resonance is similar to plasmon resonance,which can be utilized to enhance the third-order nonlinear optical response.3.Plasmon resonance is used to manipulate the wavelength and intensity of the third-order nonlinear optical response of materials.Using Cu₂Se nanocrystal as the reference,the nonlinear optical properties of Cu₂SnSe₃ and CuInSe₂ nanocrystals were explored.Cu₂Se nanocrystals contain high free carrier densities due to their abundant Cu vacancies,and hence exhibit strong absorption in the visible and near-infrared region.Doping Cu₂Se nanocrystals with Sn and In elements can adjust the wavelength and intensity of the plasmonic resonance absorption,which in turn regulates the nonlinear optical response of the materials.The Z-scan results show that under532-nm laser excitation,the saturable absorption of Cu₂Se,CuInSe₂和Cu₂SnSe₃ switches to reverse saturable absorption with the increase of input energy.By contrast,under 1064-nm laser excitation,for Cu₂Se and Cu₂SnSe₃ the trend is the same but CuInSe₂maintains saturable absorption rtegardless of the input energy.Transient absorption spectroscopy show that the proportion of ground state bleaching on the long-time scale is correlated with the saturation absorption intensity of the materials.