Terahertz Absorption And Dielectric Properties Of Solid-state Materials Characterized By Time-domain Spectroscopy

Research in terahertz(THz)frequencies has exploded during the last two decades owing to the superior emitters and detectors of THz radiation provided by modern ultrafast lasers coupled with novel semiconductor materials.The better understanding of light-matter interactions in THz range has been pursued by scientists from all the THz research fields.Terahertz time-domain spectroscopy(THz-TDS)has emerged as the best tool for measuring this interaction due to its ability to provide the change of propagated THz waves in both magnitude and phase for each frequency component in a broadband(0.3-3.0 THz)manner.This thesis focuses on the utilization of THz-TDS for the characterization of solid-state materials.The solid-state materials characterized in this dissertation include single component crystalline materials,pharmaceutical cocrystal assemblies as well as polymeric materials.The spectral information studied here include the absorption spectra and dielectric spectra,intrinsically caused by the magnitude and phase change respectively.The studies are conducted on the formation mechanism and application of absorption features as well as the development of method for dielectric analysis and polarizability measurement.1)The origin of the THz absorption spectra of crystalline materials was studied with vibration modes obtained from quantum-chemical calculation.Our study reveals that the crystal unit cell should be used as the initial structure during theoretical simulation of crystalline samples' THz absorption spectra which comprehensively considers not only the intra-molecular interactions but also inter-molecular interactions.Three quantum-chemical calculation schemes with different initial structures were applied to study the inter-molecular interactions' contribution to the THz absorption of L-glutamine,including monomer structure,dimer structure and crystal unit cell structure.With the most reliable simulation result from crystal unit cell calculation,we successfully assigned all three experimental absorption peaks of glutamine with overall vibration modes.2)The sensitive response to the intermolecular interaction of terahertz waves was used to characterize the co-crystal assmeblies as well as their polymorphic phenomenon.The formation of co-crystal assemblies was observed by the difference in absorption features between co-crystal products and original components.The naturally occurring crystal transformation from metastable form to stable form was monitored in an in-situ and real time manner by THz-TDS owing to its fast response.This study demonstrates the application of terahertz spectroscopy in the field of pharmaceutical co-crystal development and provids a new characterization method for crystal transformation mechanism research.3)The method for terahertz dielectric analysis of polymeric materials and their mixtures characterized by THz-TDS was established.The model was exemplified by precise determination of dielectric constants for PE and PTFE.Futhermore,the dielectric analysis model was applied to predict the dielectric response of the polymer blend according to the mixing ratio.The prediction present well consistency with the experimental values.The dielectric analysis model proposed in this study can be extended to the analysis of terahertz dielectric properties of more polymeric materials and their blends,which provides a reliable theoretical basis for the design of polymer materials for the terahertz application.4)A novel application of THz-TDS was proposed for the determination of polarizability of solid-state materials embedded in polymer binding matrix.The major novelty of proposed method is the ability to determine THz polarizability for highly absorbing materials.The methed was exemplified by measurements with the crystalline aspirin(acetylsalicylic acid)as well as three amino acids L-alanin,L-theronine and L-glutamine.A modified approach was also discussed for the solid-state materials whose crystal structure were not able to obtain,which further expands the application scope of the method.This study presents another exciting application of THz-TDS since polarizability represents a key parameter in the characterization of solid-state materials,which can be related to other fundamental properties such as stiffness,melting point and solubility.