Quantitatively Characterizing Optical Anisotropy Of Low-Symmetry Materials And Its Applications In Polarization-Modulation Devices

Optical anisotropy in low-symmetry materials brings brand new physical properties and offers an extra degree to modulate the optical property,which cannot be realized by isotropy materials.However,more recent research attentions have been paid to qualitatively identifying their optical anisotropic phenomena.Optical modulators can dynamically modulate light by introducing external stimuli,accompanied by the complex design of micro-nano structure.However,such optical modulators extremely reduce the modulate efficiency.According to the background above,this thesis will accurately acquirie the complete and reliable dielectric tensors and complex refractive index tensors of the low-symmetry materials by Mueller matrix ellipsometry to quantitatively characterize the optical anisotropy at first,and then propose a novel type of optical modulators based on low-symmetry materials,namely polarization-modulation devices（including polarization-regulating and polarization-regulated devices）,to dynamically modulate light with reconfigurability and high efficiency.The main works and innovations are as follows:Accurately characterizing the dielectric tensors and the complex refractive index tensors of the representative low-symmetry materials for the first time,and proposing a method that reveals the origin of optical anisotropy.To quantitatively characterize the optical anisotropy,the dielectric tensors and the complex refractive index tensors as well as the derived dichroism and birefringence of the representative low-symmetry materials,including Zr Te₅,GeSe,SnSe,and（S-PEA）₂SbI₅,have been accurately acquired by Mueller matrix ellipsometry for the first time.Connected with the electric anisotropy calculated by first-principle,this thesis also provides a method combining anisotropic critical points analysis and anisotropic interband transitions analysis to reveal the optical anisotropy origin of low-symmetry materials like Zr Te₅.Polarization-regulating devices based on low-symmetry materials have been simulated and fabricated.Taking the chapter 3’s perfert absorber based on GeSe-Si O₂-Si multilayer structure as an example,both simulation and experiment results suggest that such perfert absorber can dynamically modulate the absorbing wavelength more refined in near-infrared region（800～1300 nm）with reconfigurability and high efficiency.Moreover,the absorption wavelength can be also linearly tuned by the thickness of GeSe,and the absorption at the interface of GeSe leeds to the perfert absorption.Such multilayer structure makes the simulation and fabrication more simple and easier.The polarization-regulated devices based on low-symmetry materials have been simulated or fabricated.This thesis takes the chapter 4’s simulations of polarizer based on low-symmetry SnSe multilayer structure and the fabrication of phase retarder based on the（S-PEA）₂SbI₅in chapter 5 as examples.In chapter 4,the SnSe-Si O₂-Si multilayer structure can be modulated by the incident angle and the thickness of multilayer leading to tunable wavelength and broadband（1279～1562 nm）polarizer with the extinction ratio as high as71 d B and insertion loss of only 1.1 d B.The centimetre-sized（S-PEA）₂SbI₅ single crystal with giant birefringent（about 0.13）and suitable bandgap has been spinned coated into thin film to regulate the phase retardance in chapter 5.By altering the concentration of precursor solution and rotate speed of spinning coated,the phase retardance can be tuend and such thin film can be even served as one-eighth wave plate.Besides no need for external stimuli,the proposed optical modulators that regulating polarization state also possess the advantages of high modulation speed.Moreover,these polarization-modulating devices are easy to be simulated or fabricated due to its multilayer structure.The way to quantitatively characterizing the optical anisotropy and revealing its origin in this thesis not only offers a new method to characterizing the optical anisotropy and promote the comprehension of the optical anisotropy,but is also suitable for other novel low-symmetry materials.Besides,the proposed polarization-modulation devices based on low-symmetry materials open up a new thinking way for designing optical modulators.