Optical Constants And Their Compositional Dependence In Chalcogenide Glasses In The Mid-Infrared

Chalcogenide glass refers to the inorganic glass material containing one or more the elements of Group VIA in the periodic table like S,Se and Te,which are covalently bonded with other network-forming elements such as As,Ge,Sb and so on.It has a wide infrared transmission window,fast optical response time,and high refractive index.Optical constants such as refractive index（n）and extinction coefficient（k）in chalcogenide bulk glasses or thin films are important parameters in optical design.Due to limitations of test techniques and their poor accuracy,it is difficult to design optical systems in the infrared band.In this paper,a series of Ge-Se-based chalcogenide glasses with different chemical compositions and different topological network structures（characterized by the average coordination number MCN）were prepared and characterized by ellipsometry.In particular,we concentrated on how to use the elliptical polarization technique to accurately measure the refractive index of the mid-infrared band of the glass.We further analyzed the relationship between refractive index and the composition,and its implication on the competition between the chemical and topological order.The contents in the thesis are summarized as follows:The first point outlines the definition,discovery process and application fields of chalcogenide glass,and introduces in detail the background,research significance,research content and research ideas of the topic.By comparing the different methods of obtaining optical constants,it is difficult to accurately measure the optical constants of the mid-infrared band by using an ellipsometer.The principle of ellipsometry and the data acquisition program WVASE32 of IR-VASE Mark II variable angle spectroscopic ellipsometry（IR-VASE）are briefly introduced by Fresnel reflection function and mean square error function.The dispersion model is analyzed,and the data analysis process is illustrated from the structural model and the dispersion model.Finally,the main factors affecting the experimental results are analyzed in detail and the solutions are proposed.The second point is the analysis of the experimental results of the Ge-Se bulk glasses.Firstly,the preparation of a series of the Ge-Se bulk glasses,the test method of the ellipsometer and the model established during the fitting process are described.The refractive index changes of the mid-infrared band of Ge-As/Sb-S/Se glass are found.It decreases with increasing wavelength,but there is a transition threshold in the relationship between refractive index and MCN,which reflects the competition between topological order and chemical order.Studies have shown that in the series of Ge-As/Sb-S/Se（As/Sb content is 10% and 20%,respectively）,replacing As with Sb or replacing S with Se,the refractive index increases.Ge-As-Se has an approximately perfect covalent bond,and the refractive index transition point is at MCN=2.67;however,the Ge-As-S and Ge-Sb-S/Se series transition points appear at the stoichiometric ratio.Among them,as the MCN increases,the Ge-As-S series elements are replaced by each other,the volume is reduced,the density is increased,and the density change curve masks the refractive index transition threshold.The third point describes the preparation of Ge_11.14As_25.98Se_62.88 film sample and the test results of ellipsometer.The software fitting process of the refractive index in the mid-infrared band and the final fitting result are analyzed.The research shows that the four-layer physical structure is established and the Cauchy dispersion model is used to obtain the ideal fitting result.The minimum mean square error is 4.612.In the last part,a summary of the whole thesis is given,and possible improvements and the important content of this research work in the future are pointed out.