The goal of this thesis is to describe development of compact terahertz time-domain spectrometer driven by a mode-locked Ti:Sapphire laser. The terahertz radiation was generated by photoconductive antenna method and detected by electro-optic detection method. In this thesis, several terahertz generation and detection method, working principle of terahertz time-domain spectroscopy and its applications are discussed. We mainly focused on working principle of terahertz time-domain spectroscopy and characterization of detected terahertz power using electro-optic detection method.

In this thesis, I present studies in the field of terahertz (THz) spectroscopy. These studies are divided into three areas: Development of a narrowband THz source, the study of carrier transport in metal thin films, and the exploration of coherent dynamics of quasi-particles in semiconductor nanostructures with both broadband and narrowband THz sources. The narrowband THz source makes use of type II difference frequency generation (DFG) in a nonlinear crystal to generate THz waves. By using two linearly chirped, orthogonally polarized optical pulses to drive the DFG, we were able to produce a tunable source of strong, narrowband THz radiation. The broadband source makes use of optical rectification of an ultra-short optical pulse in a nonlinear crystal to generate a single-cycle THz pulse. Linear spectroscopic measurements were taken on NiTi-alloy thin films of various thicknesses and titanium concentrations with broadband THz pulses as well as THz power transmission measurements. By applying a combination of the Drude model and Fresnel thin-film coefficients, we were able to extract the DC resistivity of the NiTi-alloy thin films. Using the narrowband source of THz radiation, we explored the exciton dynamics of semiconductor quantum wells. These dynamics were made sense of by observing time-resolved transmission measurements and comparing them to theoretical calculations. By tuning the THz photon energy near exciton transition energies, we were able to observe extreme nonlinear optical transients including the onset of Rabi oscillations. Furthermore, we applied the broadband THz waves to quantum wells embedded in a microcavity, and time-resolved reflectivity measurements were taken. Many interesting nonlinear optical transients were observed, including interference effects between the modulated polariton states in the sample. / Graduation date: 2012

Le développement de circuits mesoscopiques avec une architecture hybride cette dernière décennie a permis d’étudier l’interaction lumière matière dans son aspect fondamental avec des photons dans le régime micro-ondes. Ces développements permettent aujourd’hui d’étudier cette interaction dans le domaine terahertz, gamme spectrale s’étendant de 0.1 THz à 10 THz (0.4 meV-41,3 meV). L’apparition de sources performantes et de méthodes de spectroscopie efficaces telles que la spectroscopie dans le domaine temporel sont des outils utilisables pour l’étude de l’interaction lumière matière dans ce domaine spectral. Dans ce travail de thèse, nous avons développé un outil afin d’étudier cette interaction dans son aspect le plus fondamental composé d’un nanotube de carbone en régime de boîte quantique et d’une cavité térahertz. Le nanotube de carbone est un élément d’autant plus adapté que sa structure électronique est régie par des énergies dont la fréquence équivalente se situe dans le terahertz. La cavité térahertz est un "split ring resonator". Le travail s’est décomposé en deux aspects, avec dans un premier temps le développement d’un banc de spectroscopie térahertz large bande (0.3 THz-20 THz) dans le domaine temporel pour l’étude des résonateurs. En utilisant un procédé original de contrôle du front d’onde d’émission de l’antenne, nous démontrons que le champ térahertz est focalisé en limite de diffraction ce qui ouvre la possibilité d’étudier des résonateurs uniques. Dans un second temps, des mesures de transport électronique ont été effectuées afin de mettre en évidence un couplage entre le résonateur et la boîte quantique. Un couplage avec un mode bosonique est observé. La conductance de ces états est modulée par la source de photons utilisée dans cette étude. Cependant, l’énergie du mode est inférieure à celle observée par les mesures de spectroscopie ne permettant pas de conclure de manière définitive sur l’origine de ce mode. / In the last ten years, research has been devoted to the development of hybrid architecture mesoscopic circuit to study the ligh-matter interaction in the microwaves range. These improvements allow us to study this interaction in the terahertz range extending from 0.1 THz to 10 THz (0.4 meV - 41.3 meV). Moreover, new efficient sources and new spectroscopy schemes like time domain spectroscopy set-up are some tools that can be used to study the light-matter interaction in this range. In this work, we developed a paradigm to study the interaction in the fundamental aspect composed of a carbon nanotube in a quantum dot regime embedded in a terahertz cavity. Carbon nanotube quantum dot is well adapted because of its electronic levels which are separated by energy in the terahertz range. The cavity used for the study is a "split ring resonator". This thesis is decomposed in two parts. Firstly, we built an ultrabroadband terahertz time domain spectroscopy set-up (0.3 THz -20 THz) to study the terahertz resonators. We demonstrated that the terahertz field is focused at the diffraction limit by exciting the antenna with an original scheme based on a control wavefront. Then, electronic transport measurements have been done to highlight the coupling between the cavity and the quantum dot. A coupling with a bosonic mode is observed. The conductance of these states is modified by the source that we used in this work. However, the energy of the observed mode is lower than the fundamental energy mode of the cavity that do not allow us to conclude about the origin of this mode.

Ingeniero Civil Eléctrico / La tecnología de terahercios se encuentra en un estado de desarrollo atrasado con respecto a las tecnologías usadas en las bandas adyacentes, como la óptica infrarroja o la electróni- ca de microondas. En particular, no se poseen fuentes compactas de radiación que operen dentro esta banda logrando buenos niveles de potencia y amplios rangos de frecuencia. Las útiles propiedades de la radiación de terahercios como su capacidad de detectar moléculas complejas, buena resolución espacial y ser radiación no ionizante, hacen que el desarrollo de tecnología para esta banda sea un área con creciente interés. En el contexto del desarrollo de una nueva línea de investigación sobre espectroscopía molecular, en el Laboratorio de Terahertz y Astrofotónica de la Universidad de Chile, se realiza este trabajo que consiste en el desarrollo experimental de un sistema láser para la ali- mentación de fotomezcladores. Este sistema tiene como objetivo la generación de dos señales ópticas de alta estabilidad y coherencia, cuya diferencia de frecuencias puede ser ajustada de forma continua dentro del rango de 10 GHz a 300 GHz. Para esto, se utiliza un esquema basado en un peine de frecuencias óptico sobre el cual se enclava por inyección un láser de diodos de frecuencia sintonizable. Esto consigue tener una fuente infrarroja de alta precisión dentro de un gran rango. Además, se genera una segunda señal por medio de modulación en amplitud (AM), la cual es sintonizable dentro de un rango igual al espaciado producido por el peine óptico. En conjunto, estas señales logran abarcar un amplio espectro de frecuencias de forma continua sin perder estabilidad ni calidad de las señales. En este trabajo se logra implementar los subsistemas para la generación de cada una de las señales requeridas y se estudia la capacidad de estos para trabajar dentro del rango deseado. Para la señal generada por enclavamiento por inyección, se logra probar el concepto dentro de un rango reducido, principalmente por falta de un buen sistema de medición de altas frecuencias. Para la señal generada por modulación AM, se logran resultados positivos en todo el rango de diseño. Finalmente, se proponen modificaciones al sistema para mejorar su desempeño. / Este trabajo ha sido parcialmente financiado por Conicyt, a través de su fondo ALMA para el desarrollo de la astronomía, Proyecto 31140025, QUIMAL, Proyecto 1500010, CATA-Basal PFB06 y Fondecyt 1151213

Thesis advisor: Willie J. Padilla / This thesis will describe the implementation of novel imaging applications with electromagnetic metamaterials. Metamaterials have proven to be host to a multitude of interesting physical phenomena and give rich insight electromagnetic theory. This thesis will explore not only the physical theory that give them their interesting electromagnetic properties, but also the many applications of metamaterials. There is a strong need for efficient, low cost imaging solutions, specifically in the longer wavelength regime. While this technology has often been at a standstill due to the lack of natural materials that can effectively operate at these wavelengths, metamaterials have revolutionized the creation of devices to fit these needs. Their scalability has allowed them to access regimes of the electromagnetic spectrum previously unobtainable with natural materials. Along with metamaterials, mathematical techniques can be utilized to make these imaging systems streamlined and effective. Chapter 1 gives a background not only to metamaterials, but also details several parts of general electromagnetic theory that are important for the understanding of metamaterial theory. Chapter 2 discusses one of the most ubiquitous types of metamaterials, the metamaterial absorber, examining not only its physical mechanism, but also its role in metamaterial devices. Chapter 3 gives a theoretical background of imaging at longer wavelengths, specifically single pixel imaging. Chapter 3 also discusses the theory of Compressive Sensing, a mathematical construct that has allowed sampling rates that can exceed the Nyquist Limit. Chapter 4 discusses work that utilizes photoexcitation of a semiconductor to modulate THz radiation. These physical methods were used to create a dynamic THz spatial light modulator and implemented in a single pixel imaging system in the THz regime. Chapter 5 examines active metamaterial modulation through depletion of carriers in a doped semiconductor via application of a bias voltage and its implementation into a similar single pixel imaging system. Additionally, novel techniques are used to access masks generally unobtainable by traditional single pixel imagers. Chapter 6 discusses a completely novel way to encode spatial masks in frequency, rather than time, to create a completely passive millimeter wave imager. Chapter 7 details the use of telecommunication techniques in a novel way to reduce image acquisition time and further streamline the THz single pixel imager. Finally, Chapter 8 will discuss some future outlooks and draw some conclusions from the work that has been done. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Willie J. Padilla / Progress in the field of metamaterials has started coming to a point where the field may finally begin to emerge as a viable solution to many electromagnetic challenges facing the community. No where is that more true then at terahertz frequencies where there lies an immense opportunity for growth. The development of mature technologies within this region of the electromagnetic spectrum would provide a valuable resource to become available for a multitude of applications. In order to achieve this, the necessary first steps of identifying viable materials and paths to integrate these with metamaterials will need to be completed. In this dissertation, we examine several different paths to achieve dynamic metamaterial electromagnetic response at terahertz frequencies, and demonstrate several paths to package these devices into imaging systems. In Chapter 1, we introduce the basic theory and design principles of metamaterials. We also describe the experimental techniques involved in the study of terahertz metamaterials. Chapter 2 presents a computational and experimental study investigating the integration of high electron mobility transistors with metamaterials allowing for high speed modulation of incident terahertz radiation. In Chapters 3 and 4, we investigate several different paths to create tunable terahertz metamaterial absorbers. Chapter 3 presents an investigation where we encapsulate a metametarial absorber unit cell with liquid crystals. We study both computationally and experimentally the tuning mechanism of the absorber as the liquid crystal refractive index is controlled as a function of the applied electric field strength and modulation frequency. In Chapter 4, we form a doped semiconducting metamaterial spatial light modulator with multi-color super-pixels composed of arrays of electronically controlled terahertz metamaterial absorbers. We computationally and experimentally study the independent tunability of each pixel in the spatial array and demonstrate high speed modulation. Chapter 5 introduces a multiplex imaging approach by using a terahertz spatial light modulator to enable terahertz imaging with a single pixel detector. We demonstrate the capability for high speed image acquisition, currently only limited by the commerical software used to reconfigure the spatial masks. We also configure the system to capture high fidelity images of varying complexity. In Chapter 6, we show how a metamaterial absorber can be implemented into a detector focal plane array for high sensitivity, low mutual coupling, and broad angle performance. Finally, we summarize in Chapter 7 the achievments of the research presented and highlight the direction of future work. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Electromagnetic radiation from the radio waves used in nuclear magnetic resonance spectroscopy through to X-rays used in crystallography have provided a wealth of knowledge about the structure, function, and dynamics of protein molecules. Terahertz waves, the topic of this thesis, are lower in frequency than radiation from the infrared, not to the frequencies of individual bond vibrations, but to the frequency range where slower longer range protein librations (low frequency vibrations) are expected to occur. The role of low frequency protein dynamics remains controversial, with some arguing that these motions are crucial for enzyme and protein function. Terahertz spectroscopy may provide key evidence to contribute to this interdisciplinary debate. In this thesis, terahertz (THz) spectroscopy has been applied in studying a number of proteins experimentally. In the first results chapter, the effect of protein concentration and ionic strength in the 0.1-2.5 THz region was investigated using Terahertz time domain spectroscopy. The results confirm the presence of terahertz excess for a number of proteins, which results from the increased absorption of THz waves when protein is introduced into the system. THz spectroscopy was then used to detect the difference between a folded protein, myoglobin, and folding intermediates, including the molten globule form, apomyoglobin. The results collected using THz spectroscopy were unable to differentiate between the folded and molten globule states. A further study was susceptible to the formation of higher order protein complexes and explored structures formed using PduA*. These experiments were primarily biochemical in nature with showing that PduA* assembles into nanotubes of 20nm diameter in vitro. The final results chapter explores the sub-THz circular dichroism signal from a vector network analyser driven by quasi-optical circuits. Wherever possible, the THz experiments were benchmarked using established analytical techniques.

No description available.

archives@tulane.edu / The electronic, spin, phonon and magnetic behavior govern the electrodynamics of solid materials. The different compositions and symmetries mix all the degrees of freedom leading to varieties of interesting phenomena such as metal-to-insulator transition, nonreciprocal directional dichroism and topological states. The study of the behavior of electrons, spins and phonons is crucial to reveal the physics behind the mysterious phenomena. The nature of terahertz time domain spectroscopy (THz TDS) which has low photon energy and contains phase information makes this technique very powerful to probe the physics of spins, electrons, phonons and magnons where the resonance energy is in the THz range. The multiferroic materials are studied by using THz-TDS with strong dc magnetic field. Multiferroic material is one of complex materials that simultaneously contain ferroelectricity and magnetism. Many fascinating physical phenomena are discovered in multiferroics, including magneto-dielectric effect and nonreciprocal directional dichroism. The magneto-dielectric effect, change in dielectric function in applied magnetic field, is studied in multiferroic CaBaCo4O7. We analyze the dynamics of phonons to clarify the individual phonon contribution to the magneto-dielectric effect. We observe giant nonreciprocal directional dichroism in the multiferroic material FeZnMo3O8, which is defined as the difference in absorption coefficient for linearly polarized light waves travelling in the opposite direction. A spin excitation is determined as the origin of nonreciprocal effect in the multiferroic FeZnMo3O8 by using THz-TDS. The nonreciprocal effect from magneto-chiral dichroism is also observed in BaCoSiO4 crystal where the material simultaneously possesses the chiral structure and magnetization. The polarimetry of transmitted THz light through BaCoSiO4 is carefully analyzed. We attribute the change in polarization in the zero magnetic field to the chirality of the structure. Nonlinearity of semiconductor InSb due to intense THz electric field is investigated quantitatively by using THz-TDS. The effective mass approximation breaks down when the intense THz pulse is applied to the semiconductor. We develop a predictive model that replaces the effective mass with a realistic band structure and retains the Drude parameters, the electron density and scattering rate, to accurately calculate the experimental observations (saturable absorption and amplitude-dependent refractive index) in InSb. / 1 / Shukai Yu

The number of users who get access to wireless links is increasing each day and many new applications require very high data rates. The increasing demand for higher data rates has led to the development of new techniques to increase spectrum efficiency to achieve this goal. However, the limited bandwidth of the frequency bands that are currently used for wireless communication bounds the maximum data rate possible. In the past few years, researchers have developed new devices that work as Terahertz (THz) transmitters and receivers. The development of these devices and the large available bandwidth of the THz band is a possible solution to this ever increasing demand. However, THz communication is still in its infancy and more research needs to be done to bring THz technology into every day life. In this research, we study wireless THz communication systems. As the first step, we conducted detailed channel measurements to study and analyze the characteristics of THz signals under different channel conditions. These propagation models mimic the behavior of THz signals in real applications. Then we use these models to study appropriate modulation methods for directional and omni-directional THz channels. We also use pulsed THz signals in wireless communication channels to send data at a very high rate. We have developed rate adaptation algorithms to allow multiple users to share the same THz channel for downlink applications while fairness is maintained among them.