Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 2/25/2010). Advisor and committee chair names found in the thesis' metadata record in the digital repository. Includes bibliographical references.

Terahertz spectroscopy shows promise in applications including quality control, security and medical imaging, but remains limited by slow data acquisition. This also poses an impediment to the study of samples undergoing irreversible transitions, as typical schemes rely on the assumption that results are consistent from pulse to pulse. In this work, we propose a high-speed terahertz detection technique based on chirped-pulse encoding that can enable single-shot measurements up to laser repetition rates in the MHz. An all-normal dispersion optical fiber is used to create a broadband probe spectrum, onto which the terahertz pulse waveform is encoded as a phase modulation. The sampling process makes use of the dispersive Fourier transform, a technique which maps the spectral features of a pulse into the time domain, in this case via the dispersion of a long commercial optical fiber. The elongated pulse can subsequently be detected with a high-speed photodetector and oscilloscope. We show steps toward implementing the technique by characterizing the components required to shape the probe pulse and providing some proof-of-concept measurements. In addition, fiber optic simulation procedures are detailed including complete coupled generalized nonlinear Schrödinger equations to provide insight into polarization effects that occur during highly nonlinear processes such as supercontinuum generation.

Biomolecules function only in aqueous environments and their dynamics are strongly influenced by physiological conditions including the temperature and the presence of co-solutes. The presence of biomolecules in aqueous solutions will change the dynamics and structure of water, and as a response, water will form hydration layers around biomolecules. The dynamics of hydration water, as well as hydrated proteins, lead to translation, rotation, and oscillating dipoles that, in turn, give rise to absorption in the megahertz-to-terahertz frequencies. However, the strong absorption of water in this frequency range leads to a significant challenge in obtaining terahertz dielectric spectra of aqueous biomolecular solutions. In response, I have employed a high sensitivity terahertz frequency-domain spectroscopy to overcome these issues on a large range of frequencies from 10 MHz to 1.12 THz. The high dynamical range of the system combined with a variable-path-length cell allows precise measurement of the complex dielectric response of the solutions. Employing Debye and Lorentzian approximations, I have decomposed contributions of the dielectric response of the solutions. The structure and dynamics of hydration shells and hydrated biomolecules have been identified. Performing experiments on a number of biomolecules have verified the certainty of the methods, thus, enriching the knowledge of the biological science of dynamics and functions of biomolecules. / Doctor of Philosophy / Biomaterials are essential for life, including all elements present in cells and organisms, and contribute to the living biological processes. Biomaterials, consisting of a diverse range of biomolecules, have traditionally been characterized in a wide range of approaching methods based on biological, chemical, and physical methodologies. This study investigates the molecular dynamics of biomolecules in native living environments to explore physics- and mechanics-based insights into their biological functions. Biomaterials together with water molecules perform their functions through molecular translations, rotations, and collective motions. To explore these dynamics, a home-built terahertz spectroscopy with high sensitivity has been utilized to characterize the dynamics of biomolecular aqueous solutions in the frequency range from megahertz to terahertz. The collected complex dielectric responses of the solutions have been examined through physical models to map out structures and dynamics of hydration shells and, then, the dynamics of hydrated biomolecules have been determined. The successfully investigating results in the dynamics of solvents from three different types of proteins and ionic solutions reveal critical information on hydrated biomolecular dynamics and biomolecule–water interactions, which impact the biochemical functions and reactivity of biomolecules.

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This thesis concerns the study of equilibrium and photo excited carriers in thin-film semiconductors using terahertz time-domain spectroscopy (THz- TDS) and optical- pump terahertz-probe (OPTP) spectroscopy. In particular, it focuses on the develop- ment and application of a MATLAB program to extract the sample complex refractive index (and thereby the complex conductivity) from THz-TDS data. The parameter extraction procedure was developed to be effective in the case of thin-film samples for which the transmission function contains a substantial contribution from multiple internal reflections in the sample. The program's effectiveness was demonstrated in the case of a 30 nm-thick film of gold deposited on silicon; the program successfully extracted the complex refractive index while the original method failed. The program was applied in a study of thin-film nanoporous indium phosphide (InP). The frequency-dependent complex conductivity-obtained via THz- TDS meas- urements-was not Drude-like, as observed for bulk InP, but was qualitatively con- sistent with a plasmonic response. The time-resolved photoconductivity was obtained from OPTP spectroscopy measurements, and revealed that the presence of pores substantially increases the photo carrier lifetime compared with the value measured for bulk InP. This behaviour is attributed to the presence of surface-bound electronic states that pin the Fermi level at the pore surface, bending the electronic bands upwards. This creates a depletion region which contains the carriers away from the recombination sites at the surface. The significance of the role played by surface states in determining carrier be- haviour is further evidenced by a study of carrier dynamics in etched and surface- passivated semi-insulating gallium arsenide (SI-GaAs). Compared with an untreated sample, surface-passivated SI-GaAs exhibited a greater change in photoconductiv- ity upon generation of photocarriers and an increased photo carrier lifetime. These observations are attributed to an increased photo carrier mobility and a decreased recombination rate resulting from the removal of surface states.

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.

Huang, Shengyang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 100-108). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

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.

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