Characterization of Materials Properties by Polarization-Resolved Time-Domain Therahertz Spectroscopy

Abstract: Recently, the advance in terahertz (Thz) technology have offered unique opportunities in the application of THz frequency range in spectroscopy, sensing and imaging. Large progress has been achieved in this area and some birefringent materials have been applied for THz applications, for instance, the liquid crystals are used as THz wave plates, and the crystals of lithium niobate (LiNbO3) have been employed in the THz wave polarizer. The optical components conduction at the THz frequency range are still not widely demonstrated due to some technical issues on THz. Recently, some doped optical crystals have been found with higher laser damage threshold, thermal values and remarkable features like pure YVO4 crystal and the currently used LiNbO3crystal, for instance, or Neodymium (Nd)-doped YVO4. Besides, several parameters, such as temperature-dependent electro-optical constants, thermos-optical coefficient and Sellmeier formula parameters, which are vital to utilize the features in the THz polarizers and wave plates, are rarely reported in the published articles. For instance, the uniaxial optical crystal yttrium vanadate (YVO4), a well-developed laser material, has strong anisotropy in thermal and optical properties with birefringence in the THz range. Yet, the thermal effect and optical crystal yttrium vanadate (YVO4), a well-developed laser material, has strong anisotropy in thermal and optical properties with birefringence in the THz range. Yet, the thermal effect and optical constants are still not elaborately investigated. In addition, the comparison between ordinary and extraordinary refractive indices of YVO4 to thermos-optical coefficients and optical constants would be of interest and useful for applications as polarizers in the THz frequency range. Furthermore, numerous modern materials with interesting properties in the THz range (e.g. superconductors, charge density waves, ferrot and antiferromagnets) are often grown on birefringent substrates (e.g. sapphire). Therefore, unveiling birefringent optical properties of different substrate materials is also important, when analyzing material’s electronic and lattice properties, such as the conductivity and phonon vibrations, using THz spectroscopy. To summarize, in order to characterize physical properties in the THz range, investigation of birefringent materials is required, and accurate analysis methods need to be applied within the THz range.

The purpose of this thesis is to characterize THz components and properties of different substrates and optical crystals. To achieve the aim, we carried out the following experiments. Firstly, we built the (polarization-sensitive) time domain terahertz spectroscopy (THz-TDS) system in our lab. Different cuts (a, r-cut) of sapphire single crystal substrate were investigated to collect birefringent properties in the THz range. Secondly, c-cut magnesium fluoride (MgF2) single crystal as a substrate and c-cut LiNbO3 single crystal as THz generator as well as 3% Nd-doped YVO4 were examined to collect the parameters to compare the properties of different (birefringent) optical crystals. On the other hand, MgF2 and LiNbO3 did not show any birefringence. Finally, both, graphene and metallic antiferromagnet Mn2Au were investigated in the THz range.