No description available.

With the massive advantages of THz radiation and the current technical difficulties in mind, I have chosen to undertake research into terahertz surface phenomena, which is the focal point of my thesis. Ultrathin surface terahertz emitters have many advantages as they have an extremely thin active region, typically hundreds of atomic layers. In this framework, III-V semiconductors, such as InAs and InSb, have record-breaking conversion efficiencies per unit thickness. In addition, the phase mismatch, which commonly limits the generation of terahertz from optical crystal, is negligible and so there is an opportunity for enhancing the emitted bandwidth. My thesis is born as the core of many research interests of my research lab (Emergent Photonics), which enabled the appropriate availability of resources that made my results possible. It also created several spin-out research lines. All the work presented is my work (with the exception of the background research). Parts of chapters have been published in journals and publications which see me as the first author. The structure of this thesis is as follows. First I discuss optical pump rectification emission, and the saturation of InAs terahertz emissions. Then I introduce my work on terahertz enhancement emission through graphene. Finally, I present my work on an exotic terahertz emission mechanism, namely the all-optical surface optical rectification and I place my concluding remarks.

Chen, Mengyu. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 110-116). / Abstracts in English and Chinese. / Abstract --- p.I / Acknowledgments --- p.V / Table of Contents --- p.VI / List of Tables and Figures --- p.VIII / List of Abbreviations and Symbol --- p.XII / Chapter Chapter 1: --- Introduction --- p.1 / Chapter 1.1 --- Left-handed material --- p.1 / Chapter 1.1.1 --- Left-handed transmission --- p.1 / Chapter 1.1.2 --- Corrected physical laws and sub-wavelength imaging --- p.4 / Chapter 1.1.3 --- Methods to realized Left-handed Material --- p.5 / Chapter 1.2 --- The meaning of this work --- p.8 / Chapter 1.3 --- Organization of the thesis --- p.8 / Chapter Chapter 2: --- Basic theory and Metamaterial Devices --- p.11 / Chapter 2.1 --- Left-handed Metamaterials --- p.11 / Chapter 2.1.1 --- Metal wire arrays structure --- p.11 / Chapter 2.1.2 --- SRR structure --- p.14 / Chapter 2.1.3 --- SRR plus wire arrays structure --- p.20 / Chapter 2.1.4 --- Development and Fishnet structure --- p.22 / Chapter 2.2 --- Transmission line model --- p.26 / Chapter 2.2.1 --- Transmission line theory --- p.26 / Chapter 2.2.2 --- Transmission line model of SRR plus wire arrays structure --- p.27 / Chapter 2.2.3 --- Transmission line model of Fishnet structure --- p.29 / Chapter 2.3 --- Electric Metamaterial Devices --- p.32 / Chapter 2.3.1 --- The dielectric property of SRR --- p.32 / Chapter 2.3.2 --- Active Metamaterial Devices --- p.34 / Chapter 2.4 --- Other Metamaterial Devices --- p.36 / Chapter 2.4.1 --- Metamaterial Electromagnetic Cloak --- p.36 / Chapter 2.4.2 --- Metamaterial based Perfect absorber --- p.39 / Chapter 2.5 --- Homogenization and effective material parameters --- p.40 / Chapter Chapter 3: --- Metamaterial devices fabrication --- p.43 / Chapter 3.1 --- Introduction --- p.43 / Chapter 3.2 --- Photolithography --- p.44 / Chapter 3.2.1 --- Process description --- p.45 / Chapter 3.2.2 --- Parameter Selection --- p.50 / Chapter 3.3 --- E-beam Evaporation and lift-off --- p.51 / Chapter 3.3.1 --- E-beam evaporation --- p.51 / Chapter 3.3.2 --- Lift-off --- p.53 / Chapter 3.4 --- Flexible metamaterial and double-layer device --- p.54 / Chapter 3.4.1 --- Polyimide brief introduction --- p.54 / Chapter 3.4.2 --- Double-layer structure and fabrication --- p.57 / Chapter Chapter 4: --- Simulation and Experiment Method --- p.61 / Chapter 4.1 --- Numerical simulation methods and software --- p.61 / Chapter 4.1.1 --- Time domain solver --- p.61 / Chapter 4.1.2 --- Frequency domain solver --- p.62 / Chapter 4.2 --- High Frequency Structure Simulator (HFSS) --- p.63 / Chapter 4.2.1 --- Introduction --- p.63 / Chapter 4.2.2 --- Simulation Process --- p.64 / Chapter 4.2.3 --- Parameter Retrieve Method --- p.67 / Chapter 4.3 --- Terahertz Time Domain spectroscopy system --- p.71 / Chapter 4.3.1 --- Introduction --- p.71 / Chapter 4.3.2 --- System Setup --- p.73 / Chapter 4.3.3 --- Photoconductive Antennas --- p.75 / Chapter 4.3.4 --- Data Analysis Method --- p.77 / Chapter Chapter 5: --- Simulation and Experiment Results and Analysis --- p.82 / Chapter 5.1 --- SRR structure based Metamaterial Devices --- p.82 / Chapter 5.1.1 --- SRR based left-handed material --- p.82 / Chapter 5.1.2 --- Electric Metamaterial Devices with different substrate --- p.84 / Chapter 5.2 --- Fishnet structure based metamaterial devices --- p.94 / Chapter 5.2.1 --- Dual-band Polarization-Insensitive Left-handed Metamaterial --- p.94 / Chapter Chapter 6: --- Conclusion --- p.106 / Chapter 6.1 --- Conclusion and potential application --- p.106 / Chapter 6.2 --- Future work --- p.108 / References --- p.110 / Publication list --- p.116

Many materials such as drugs and explosives have characteristic spectral signatures in the terahertz (THz) band. These unique signatures imply great promise for spectral detection and classification using THz radiation. While such spectral features are most easily observed in transmission, real-life imaging systems will need to identify materials of interest from reflection measurements, often in non-ideal geometries. One important, yet commonly overlooked source of signal corruption is the etalon effect - interference phenomena caused by multiple reflections from dielectric layers of packaging and clothing likely to be concealing materials of interest in real-life scenarios. This thesis focuses on the development and implementation of a model-based material parameter estimation technique, primarily for use in reflection spectroscopy, that takes the influence of the etalon effect into account. The technique is adapted from techniques developed for transmission spectroscopy of thin samples and is demonstrated using measured data taken at the Northwest Electromagnetic Research Laboratory (NEAR-Lab) at Portland State University. Further tests are conducted, demonstrating the technique's robustness against measurement noise and common sources of error.

Thesis (M.E.E.)--University of Delaware, 2006. / Principal faculty advisor: James Kolodzey, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

In this thesis, I present studies in the field of terahertz [THz] spectroscopy. It covers both the generation and detection of ultra-fast broadband THz pulses, as well as the transmission properties of vertically grown multi-walled carbon nanotube forests [MWCNTs]. We have found that these vertically grown MWCNTs respond strongly to THz radiation and exhibit an anisotropic optical response. The analysis done was a simplified model that is frequency-indepedent, which led to discrepancies between the theoretical and experimental data. This suggests that the response of the CNTs is frequency-dependent, and further, more complex analysis is required to fully describe their properties. / Graduation date: 2013

No description available.

This dissertation presents research on synthesis, high-power microwave post-synthetic purification and high frequency characterization of Carbon Nanotubes (CNT). First, CNTs are synthesized using a Chemical Vapor Deposition system. The impact of substrate and methane flow rate on CNT growth is studied using Scanning Electron Microscopy, Transmission Electron Microscopy and Raman microscopy. Second, the microwave irradiation effects on purified HiPCO and CoMoCat Single-Walled CNT thin films are investigated. The measured drastic THz power transmission increase (>10 times) indicates a significant metallic content reduction after the irradiation. The Raman spectra also confirm the metallic-to-semiconducting ratio of Raman-active CNTs decreases by up to 33.3%. The observed microwave-induced effects may potentially lead to a convenient scheme for CNT demetalization. Third, Multi-Walled CNT papers are characterized from 8 to 50 GHz by rectangular waveguide measurements using a vector network analyzer. A rigorous algorithm is developed to extract the samples' effective complex permittivity and permeability from the measured S-parameters. Unlike other reported work, this method does not impose the unity permeability assumption. The algorithm is verified by finite-element simulations and the uncertainties for the characterization method are analyzed. The effective medium theory is then applied to obtain the intrinsic CNT properties. Furthermore, Terahertz Time-Domain Spectroscopy is used to characterize the samples from 50 to 370 GHz. Both transmission and reflection experiments are performed to simultaneously extract the permittivity and permeability. The extracted permittivity is fitted with a Drude-Lorentz model from 8 to 370 GHz. Finally, individual CNT characterizations at microwave frequency are studied. The impacts from impedance mismatching and parasitics on measurement sensitivity are systematically studied, revealing that the parasitic effect is possibly dominant above 10 GHz. A tapered coplanar waveguide test fixture is designed using Advanced Design System (ADS) to improve the impedance mismatching and minimize the test fixture parasitics, therefore optimize the measurement sensitivity. A de-embedding procedure to obtain the CNT's intrinsic electrical properties is presented and demonstrated with ADS simulations. In addition, the test fixture fabrication process is discussed, which is an ongoing research work. At the end, the conclusions of this dissertation are drawn and possible future works are discussed.

Terahertz (THz) radiation has become increasingly important in many scientific and commercial fields in recent years. It possesses many remarkable features resulting in an increased use of THz radiation for various applications, like biomedical imaging, security screening, and industrial quality control. The capability of these applications depends directly on the availability of powerful THz sources and high-responsivity, fast THz detectors. Current commercial products used to detect THz radiation, like Golay cells and pyroelectric detectors, have only slow detection rates and poor sensitivities. Other commercial THz detectors, like bolometers, are more sensitive but require liquid helium cooling. In this thesis, two types of room-temperature high-responsivity graphene-based THz detectors are presented, relying on the unique properties of graphene and the function of plasmonic antenna arrays which boost the interaction between THz waves and graphene. Graphene has been demonstrated as a promising material for THz detection. However, the challenge is its insufficient light absorption that largely limits the responsivity. The first design is based on an array of planar antennas arranged in series and shorted by graphene squares. Highly efficient photodetection can be achieved by using the metallic antenna to simultaneously improve both light absorption, as resonant elements, and photocarrier collection, as electrodes. The device has been characterized with quantum cascade lasers, yielding a maximum responsivity of ~2 mA/W at 2 THz. The second detector is based on an array of interdigitated bow-tie antennas connected in parallel and shunted by graphene squares. The arms of the bow-tie antennas were made of two metals with different work functions to create a built-in electric field and improve the responsivity. The device has been characterized and yields a maximum responsivity of ∼34 μA/W at 2 THz. Efficient THz imaging is presented by integrating the detector in a QCL-based THz imaging system.

Engine oil is critical to tractor engine performance. Engine designers recommend farmers change engine oil depending on recommendations by engine manufacturers. Engine manufacturers did not take into account different tasks often performed by tractors in fields, like tillage or seeding. Farmers do not have certain criteria to determine when engine oil must change. The only criteria to change engine oil is the physical /chemical method, which takes at least one week to obtain oil results. It is a waste of time to wait one week to get the results. There will be a lot of mechanical engine problems if oil is not changed. These engine oil problems cost farmers a lot of money. The aim of this research is to use new technology that could be contributed to solving these technical difficulties. Terahertz technology was used to determine engine oil characteristics by measuring refractive index and absorption coefficient on different conditions. Four experiments were performed to identify the ability of terahertz technology on various engine oil grades, engine oil types, and engine oil contaminants by using terahertz time-domain spectroscopy (THz-TDS). The first experiment was classifying gasoline engine oils of various viscosities by using THz-TDS. The range of 0.5–2.0 THz was evaluated for distinguishing among gasoline engine oils of three different grades (SAE 5W-20, 10W-40, and 20W-50) from the same manufacturer. ANOVA results confirmed a highly significant difference (p<0.0001) in refractive index among each of the three oils across the 0.5–2.0-THz range. Linear regression was applied to refractive index data at 0.25-THz intervals from 0.5 to 2.0 THz to predict kinematic viscosity. The refractive indices of these oil samples were promising for identification and distinction of oil grades. The second research identified three levels of water contaminants 0.0%, 0.1% and 0.2% inside diesel engine oils, grade SAE 15W-40, by utilizing THz-TDS in the range of 0.5 to 2.0 THz. The 0% water contamination level had the lowest absorption coefficient, while 0.2% water had the highest absorption coefficient. The refractive index of 0% water was the lowest and 0.2% water was the highest across the THz range. The refractive indices of these oil samples were promising for discrimination of water contamination. The third experiment demonstrated the possibility of identifying gasoline in engine oil (SAE 5W-20) which was contaminated with four rates (0%, 4%, 8% and 12%) of gasoline fuel and were measured by using THz-TDS. For both refractive index and absorption coefficient of the single cuvette method, ANOVA and Fisher results illustrated that there were highly significant differences (p < 0.0001) among each of the gasoline contaminant levels across the 0.5-2.5 THz range. The 2.5 THz frequency was the best to predict fuel contamination based on refractive index, and 0.5 THz was the best frequency for absorption coefficient. The fourth experiment illustrated the potential of THz-TDS to detect viscosity at 40 °C and TBN changes in gasoline engine oil (SAE 5W-20) due to thermal oxidation (TO). For refractive index, ANOVA and Fisher results showed that there were highly significant differences (p < 0.0001) among each of the TO times across the 0.51 - 2.48 THz range. Refractive index was used to predict TO time, and the 1.25 THz frequency was best to predict viscosity at 40°; for TBN, 2.25 THz was best.