Title:  The Nondestructive Evaluation of Terahertz Imaging for Structural and Material Characterization

Committee: 

Dr. Citrin, Advisor

Dr. Locquet, Co-Advisor

Dr. Voss, Chair

Dr. Declercq

Abstract: The objective of the proposed research focused on the development of theories and techniques to enhance axial resolution for the promotion of THz technique among industries and the improvement of accuracy when characterizing thick samples. For coating layer with optical thin thickness, the temporal reflected THz echoes will overlap partially even totally, and thus no visually distinct. In order to resolve the structure of measured samples, advanced signal postprocessing techniques are essential. In this thesis, three different deconvolved techniques, Frequency wavelet-domain deconvolution, Sparse deconvolution based on an iterative shrinkage algorithm, and Autoregressive spectral extrapolation are employed to characterize the optical-thin mill scale layer on steel substate. The high consistency between THz deconvolved results and destructive cross-sectional imaging demonstrates the effectiveness of THz technology serving as a complementary modality on structural characterization among steel industries. When propagating within thick plastic samples, due to the frequency-dependent attenuation and dispersion, THz echoes will broaden as the propagation distance increases, degrading the capabilities of deconvolution techniques. In order to overcome these disadvantages, the frequency-dependent absorption coefficient and refractive index are characterized with THz transmission spectra. Through incorporating the model accounting for dispersion to Cross-correlation technique, a factor-of-two improvement is achieved compared with Frequency wavelet-domain deconvolution as well as Cross-correlation technique ignoring dispersion, enabling commercial pulsed THz systems to measure thicknesses of plastics within few -cm range.