Terahertz Polarization Spectroscopy And Applications

The constitutive relations associated with Maxwell's equations contain all the information to describe interaction of electromagnetic fields with matter.Electromagnetic responses of linear media are usually determined by permittivity and permeability.In general,there exist two independent eigen modes for electromagnetic waves propagating in media,which are particular solutions of Maxwell equations satisfying corresponding dispersion relations and have specific polarization states keeping constant during propagation.In the sense of a propagating field with properties fully characterized,only eigen modes are allowed by media.An arbitrary field can always be regarded as a linear superposition of the eigen modes,which leads to rich propagation phenomena and is the foundation for development of functional devices manipulating propagation of electromagnetic fields.Therefore,analyzing propagation characters of electromagnetic waves in a given medium is essential to study interaction of electromagnetic fields with different materials.In this thesis,Eigen modes and electromagnetic response in terahertz frequency of two kinds of non-magnetic media are studied:?1?Bi-isotropic media,also called chiral media or materials with optical activity,which are isotropic and composed of chiral molecules or structures.Due to additional magnetoelectric coupling effects,the eigen modes in this kind of media are right and left handed circularly polarized waves.Different dispersive relation and polarization state of two eigen modes results in optical activity,circular dichroism and so on.?2?Monoclinic crystals,whose complex permittivity tensor cannot be diagonalized by spatial rotation transformation.Consequently,eigen modes are elliptically polarized waves and the major dielectric axis direction is dispersive with frequency.Both media change the polarization state of transmitted fields,if incident field is linearly polarized.The terahertz electromagnetic responses of the two media mentioned above are studied by experiment and theory in this thesis.Main research contents and obtained results are summarized as follows:?1?For acquiring polarization states of transmitted terahertz electric fields from samples,a polarization sensitive terahertz time domain spectroscopy system is built and optimized.Through programming control,projections of the terahertz electric field in two orthogonal directions can be recorded sequentially in each step during scan of the translation stage.The noise caused by moving error of the stage in multiple measurements can be greatly reduced in this way.Measuring accuracy of the polarization direction of electric fields can reach up to p/200.?2?The dispersive relations and corresponding electromagnetic eigen modes of bi-isotropic media are analyzed in theory.Complex transmissivity and reflectivity at the interface of isotropic and bi-isotropic medium for normally incident electric fileds are deduced.A procedure for extracting chiral parameters of bi-isotropic media is proposed using terahertz polarization sensitive measurements.The developed measurement and analysis scheme are applied to study terahertz electromagnetic responses of polycrystalline saccharides powders.?3?The dispersive relations and corresponding electromagnetic eigen modes of monoclinic crystals are analyzed in theory.Properties of eigen modes are discussed in detail.Specific data acquisition and processing scheme are developed for extracting elements of the nondiagonal permittivity tensor.The electromagnetic response parameters of BaGa₄Se₇ crystal are obtained by polarization sensitive terahertz time-domain spectroscopy.?4?Phonon polariton excitation and propagation in BaGa₄Se₇ crystal are observed at room temperature in frequency ranging from 1.7 to 2.5 THz.To our best knowledge,it is the first time to directly observe bulk polariton waves generated by a linear excitation process and propagating in a crystal.The polarization selected Raman spectral data and terahertz generation spectra using optical excitation at 800nm also confirm our observations.Also,it is found that only one eigen mode can effectively propagates in the crystal due to the large difference in dissipation rates of the two modes,which is highly related to the energy decay channels and dephasing processes of the different phonon excitations.