Nondestructive Evaluation of Aircraft Composites Using Terahertz Time Domain Spectroscopy

Terahertz (THz) time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Material properties of glass fiber composite were measured using both transmission and reflection configuration. The interaction of THz with a glass fiber composite was then analyzed, including the effects of scattering, absorption, and the index of refraction, as well as effective medium approximations. THz TDS, in both transmission and reflection configuration, was used to study composite damage, including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by THz TDS transmission and reflection imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the mail pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging

Studies of atmospheric effects on free-space THz pulse propagation and applications

Within THz frequency range, free-space propagated EM wave and the related applications have attracted a lot of attention, due to promising solutions for new types of remote sensing, wireless communications and security applications. However, the characteristics of free-space THz wave are degraded by several atmospheric conditions, such as water vapor absorption and pulse distortion.In this thesis, a comprehensive study on atmospheric effects on free-space THz signal and its potential applications by using the state-of-art Long-Path THz-TDS system is presented. Two types of absorptions from the ambient water vapor have been investigated: the resonant absorption which is associated with strong phase shifts and the continuum absorption. The molecular response theory is used as the lineshape function based on parameters from JPL and HITRAN databases for simulation to water vapor resonant absorption and phase shift. Not only were the THz digital bit observed for potential wireless communications, but the refractivity of water vapor was also obtained by line-by-line summation.A series of accurate experiments of water vapor continuum absorption have been carried out by using the 170 m Long-Path THz-TDS system. With precise experimental results and MRT simulation, the parameters for general model of water vapor continuum absorption have been successfully obtained within several THz transparent windows. As another kind of atmospheric condition, artificial fog fully filled the 137 m long sample chamber, in order to investigate absorption and phase shift of the atmospheric diffusive scattering mediums.With all the quantitative understanding of the atmospheric effects, a comprehensive atmospheric model for the free-space THz signals has been established, including the humidity, temperature and distance. This model can be used to provide a theoretical verification for all of the free-space THz applications.To demonstrate another free-space THz application, a series of experiments of THz-TDS remote detection of small molecules vapor have been performed. Samples of CH3CN, D2O and HDO have been successfully detected using 170 m Long-Path THz-TDS system. Moreover, the reaction ratio of the transition from H2O and D2O to HDO has been monitored in time

Silicon Doping Profile Measurement Using Terahertz Time Domain Spectroscopy

Doping profiles in silicon greatly determine electrical performances of microelectronic devices and are frequently engineered to manipulate device properties. To support engineering studies afterward, essential information is usually required for physically characterized doping profiles. Secondary ion mass spectrometry (SIMS), spreading resistance profiling (SRP) and electrochemical capacitance voltage (ECV) profiling are mainstream techniques for now to measure doping profiles destructively. SIMS produces a chemical doping profile through the ion sputtering process and owns a better characterization resolution. ECV and SPR, on the other hand, gauge an electrical doping profile from the free carrier detection in microelectronic devices. The major discrepancy between chemical and electrical profiles is at heavily doped (\u3e1020 atoms / cm3) regions. At the profile region over the solubility limit, inactive dopants induce a flat plateau and only being detected by electrical measurements. Destructive techniques are usually designed as stand-alone systems for the remote usage. For an in-situ process control purpose, non-contact approaches, such as non-contact capacitance-voltage (CV) and ellipsometry techniques, are currently under developing. In this dissertation, novel terahertz time domain spectroscopy (THz-TDS) is adopted to achieve an electrical doping profile measurement in both destructive and non-contact manners. For this brand new application, everything has been studied from bottom-up. Firstly, the measurement uncertainty from the change of a bulk wafer thickness and the recognition of the doping profile dissimilarity were proven experimentally. The phosphorus refractive index from 1.2×1015 cm-3 to 1.8×1020 cm-3 levels was then generated physically for the modeling of the complex THz transmission and its shift to the Drude Model prediction is explained two scientific mechanisms. Through the experimental demonstrated of the proactical degeneracy, relative strategies were proposed to shrink or break it. The doping profile measurement was finally performed by both methods. We conclude that THz-TDS can be designed as either an either in-situ or stand-alone system to estimate a doping profile in semiconductor materials

Study of propagation and detection methods of terahertz radiation for spectroscopy and imaging

The applications of terahertz (THz, 1 THz is 1012 cycles per second or 300 pm in wavelength) radiation are rapidly expanding. In particular, THz imaging is emerging as a powerful technique to spatially map a wide variety of objects with spectral features which are present for many materials in THz region. Objects buried within dielectric structures can also be imaged due to the transparency of most dielectrics in this regime. Unfortunately, the image quality in such applications is inherently influenced by the scattering introduced by the sample inhomogeneities and by the presence of barriers that reduces both the transmitted power and the spatial resolution in particular frequency components. For continued development in THz radiation imaging, a comprehensive understanding of the role of these factors on THz radiation propagation and detection is vital. This dissertation focuses on the various aspects like scattering, attenuation, frequency filtering and waveguide propagation of THz radiation and its subsequent application to a stand-off THz interferometric imager under development. Using THz Time Domain spectroscopic set-up, the effect of scattering, guided THz propagation with loss and dispersion profile of hollow-core waveguides and various filtering structures are investigated. Interferometric detection scheme and subsequent agent identification is studied in detail using extensive simulation and modeling of various imaging system parameters

Near-field imaging with terahertz pulses

High spatial resolution imaging is implemented with a novel collection mode near-field terahertz (THz) probe. Exceptional sensitivity of the probe allows imaging with spatial resolution of few microns using THz pulses with spectral content of 120 to 1500 microns. In the present study, the principle of the probe operation as well as the probe design and characteristics are described. The probe performance is related to effective detection of radiation coupled into the probe aperture. Propagation of short single-cycle electromagnetic pulses through apertures as small as 1/300 of the wavelength is experimentally and numerically studied. Finite-difference time-domain method is used to model propagation of THz pulses through the probe aperture in order to optimize the probe design. It is shown that the probe sensitivity is significantly improved if the detecting antenna measures electric field coupled through the aperture in the near-field zone rather than in the far-field zone. Effects of temporal and spectral pulse shaping are described by frequency-dependent transmission at the near- or below cutoff regimes of the aperture. Imaging schemes, properties, and artifacts are considered. The technique provides the best to date spatial resolution capabilities in the THz range of the electromagnetic spectrum

Design, Fabrication and Measurement of a Plasmonic Enhanced Terahertz Photoconductive Antenna

Generation of broadband terahertz (THz) pulses from ultrafast photoconductive antennas (PCAs) is an attractive method for THz spectroscopy and imaging. This provides a wide frequency bandwidth (0.1-4 THz) as well as the straightforward recovery of both the magnitude and phase of the transmitted and/or reflected signals. The achieved output THz power is low, approximately a few microwatts. This is due to the poor conversion of the femtosecond laser used as the optical pump to useable current inside the antenna semiconducting material. The majority of THz power comes from the photocarriers generated within ~ 100 nm distance from the antenna electrodes. However, the optical beam covers larger spot size, therefore much of the absorbed optical photons do not contribute to the THz power. The goal of this work is to advance the design, fabrication, and measurement of THz-PCAs to generate significantly improved output power. This work proposed a plasmonic enhanced thin-film photoconductive antenna to enhance optical carrier generation in the PCA. The electromagnetic wave equations were solved in order to compute the enhanced plasmonic field in the semiconductor. The Poisson’s and the drift-diffusion equations were solved in order to compute the carrier dynamics inside of the semiconductor. A parametric optimization was implemented in order to design the plasmonic nanodisks and the thickness of the ultrathin photoconductive layer. These solutions and optimizations were achieved using the commercial package COMSOL® Multiphysics model. The PCAs’ fabrication was accomplished using the electron beam lithography for patterning the plasmonic nanostructures, the molecular beam epitaxy for the sample growth, the lapping/selective etching for the epitaxial liftoff, and standard microfabrication practices for patterning the antenna and device packaging. The PCA was characterized utilizing a tunable pulsed laser system with a 100 fs pulse width for the optical excitation and a Gentec-EO pyroelectric power detector for measurement of the output THz power. Also, the spectral characterization of the PCA was conducted, in collaboration with Teraview LTD in their site at UK, using a THz time-domain spectroscopy experimental set-up. The results demonstrate the enhancement in the output THz power of the plasmonic thin-film PCAs in comparison with conventional THz-PCAs