Determination Of Crystal Birefringence Properties By Critical Angle Method

As the basic properties of crystals,refractive indices and birefringence are of great significance for the study of mineralogy,materials science,optoelectronics,etc.Not only are the characteristics of basic guiding value for the identification of mineral crystal types,but they are also related to the development of new crystals and the improvement of polarized optical devices.Being the property of anisotropic crystals,birefringence is determined by the direction of the material's surface direction and the principal refractive indices.However,previous studies on the measurement of birefringence have certain limitations in the measurement of the principal refractive indices and surface direction.Based on the total internal reflection critical angle method,this thesis proposes a rotating scanning measurement method,which realizes the measurement of the principal refractive indices and the surface direction of the crystal,which lays the foundation for the application of this method in the identification of optical mineralogy and the study of optical crystals.The thesis first discusses the existing birefringence measurement methods,analyzing and comparing the birefringence measurement methods used in different fields,and proposes the potential advantages of improving the TIR critical angle method to rotate and scan the refractive index ellipsoid.Explained are the principles,the mode of the refractive index curve,and then the feasibility of retrieving the crystal parameters from the refractive index curves is demonstrated through simulation.Starting from the principle and requirements of the rotating scanning method,an experimental setup is designed,and the devices,including the light source,detection module,and receiver,are selected.In order to extract information from the light spot reflected by the birefringent crystal with two TIR critical angles,the image processing algorithm and the peak-finding algorithm for the extraction of the two boundaries are designed to eliminate the wrong critical angle positions caused by noise and boundary overlap.For the measured refractive index curve data,a retrieval method is used to retrieve the parameters,and the uncertainty transfer matrix of this retrieval is given.Finally,verification experiments are carried out for two types of samples of uniaxial crystal and biaxial crystal.The samples in the experiments are the quartz crystal pieces with customized surface directions and the natural gypsum and muscovite crystal pieces with cleavage surfaces.The experiments not only prove that the rotating scanning method can measure the principal refractive indices,birefringence,and surface direction of uniaxial and biaxial crystals,but also demonstrate the feasibility of using these parameters for the identification of crystal type.