Third-order Nonlinear Optical Properties Of Ge-As-Te Chalcogenide Glasses In Mid-infrared

Chalcogenide glasses（Ch Gs）have excellent infrared transparency,low phonon energy and high linear and nonlinear refractive indices,thus are promising for various mid-&far-infrared photonic applications,such as wavelength conversion,environmental monitoring,biomedical sensing,integrated photonic devices,and supercontinuum generation.For these applications,it is essential to investigate nonlinear optical properties of chalcogenide glasses for optimized compositions in the mid-infrared.At present,In the three groups（sulfide,selenide and telluride）of Ch Gs,the nonlinear optical properties of some binary or ternary S-based and Se-based Ch Gs have been extensively studied.Meanwhile,some empirical semiconductor models have been developed to predict the correlation of optical nonlinearity with the other material properties such as band gap,lone pair electrons,and linear refractive index.However,these studies are mainly concentrated in the near-infrared wavelength below 1.5μm,and the short infrared cut-off wavelengths of these materials（S-based is about 12μm,Se-based is about 15μm）and low nonlinear refractive index have been difficult to meet the needs of the longer wavelength range in the future.Therefore,it is particularly important to explore new materials that not only have wider infrared cut-off wavelengths,but also have higher third-order nonlinear optical property.Compared with S and Se elements,Te element has a larger relative atomic mass and a smaller phonon energy,which results in Te-based chalcogenide glasses have farther infrared cut-off wavelengths（over 20μm）.These make Te-based chalcogenide glasses the most promising glass material in the mid-&far-infrared.In addition,Te-based chalcogenide glass also has a higher linear refractive index and a nonlinear refractive index,which is conducive to the strong light confinement of the photonic circuit,making it possible to further reduce the size of the device and the realization of all-optical signal processing.However,there are few systematic studies on the optical properties of Te-based glasses.Therefore,in this thesis,we firstly prepared three groups of S-based,Se-based and Te-based Ch Gs glasses using the melt-quenching method.The optical and physical properties of these glasses were characterized and analyzed in detail.According to the comprehensive consideration of linear refractive index,band gap,transmittance,etc.,the component with the highest non-linearity was selected,and a series of Ge-As-Te glasses were prepared based on this component.Finally,the nonlinearity of this series of glass is measured and discussed from the internal mechanism,and the experimental results are compared and analyzed with various models,so as to seek the best guidance for designing new materials with better nonlinear properties.The details are as follows:（1）Study on the optical properties and thermal stability of Ge-As（Sb）-S（Se,Te）chalcogenide glasses.By measuring the transmission spectrum of these samples in the visible and near-infrared ranges,it was found that Te-based Ch Gs not only has a good transmittance,but extends the transmission spectrum to more than 20μm in the long wavelength region.The linear refractive index of Ge-Te based Ch G as high as 3.63-3.67 at 2.5μm,which indicates that Te-based glasses have excellent light-limiting ability.Therefore,based on the two aspects of wide transmission wavelength range,and high linear refractive index,it is concluded that Ge₁₀As₂₀Te₇₀ and Ge₁₀As₂₅Te₆₅ can have the highest nonlinear composition.Based on this composition,a series of Ge₁₀As_xTe_90-xglass samples were prepared.（2）Study on optical properties,thermal stability and network structure of Ge₁₀As_xTe_90-x series chalcogenide glass.The ellipsometer was used to test the linear refractive index of samples.The results show that there is a clear threshold behavior for linear refractive index,with a maximum at As=30（MCN=2.5）and a minimum at As=50（MCN=2.7）.The glass transition temperature T_gand the crystallization temperature T_xof the glass were obtained by differential scanning calorimeter（DSC）analysis,in order to determine a glass composition that can withstand high temperatures and has thermal stability.Third-order nonlinear optical properties of Ge₁₀As_xTe_90-x chalcogenide glasses were investigated utilizing Z-scan method at the mid-infrared wavelengths of 2.5 and 3.0μm,and the effects of sample structure changes on the nonlinear optical performance were further analyzed according to MCN and XPS results.Finally,a composition with the best optical nonlinear property can be determined by comprehensively analyzing the experimental results.（3）Comparison of the third-order nonlinearity of Ge₁₀As_xTe_90-x chalcogenide glass with various empirical semiconductor models.By comparing the third-order nonlinearity of Ge₁₀As_xTe_90-xchalcogenide glass with Miller’s rule,the nonlinear dispersion behavior is explored,which is important for the application of chalcogenide glass in photonic devices.