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Dissertations / Theses on the topic 'Terahertz'
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Mikhaylovskiy, Rostislav. "Terahertz magnonics." Thesis, University of Exeter, 2012. http://hdl.handle.net/10871/9803.
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The potential of terahertz time domain spectroscopy has until recently been neglected in the field of the ultrafast magnetism. At the same time this technique can serve as a useful complementary tool with respect with conventional methods to investigate ultrafast magnetization dynamics. This thesis aims to implement time domain terahertz spectroscopy to observe high frequency spin waves excited optically in different magnetic systems. This work covers several distinct phenomena related to the study of spin waves (magnonics) at terahertz frequencies. The generation of transient broadband nonlinear magnetization via inverse Faraday effect in terbium gallium garnet is described in chapter 4. We demonstrate a remarkable discrepancy of at least two orders of magnitude between the strengths of the direct and inverse Faraday effects, thereby challenging the commonly accepted understanding of their relationship. Additionally, a striking nonlocality of the optical response is found. In chapter 5 the results of THz absorption spectroscopy of the terbium gallium garnet are reported. The garnet exhibits an intricate paramagnetic state with several magnetic sub-lattices at cryogenic temperatures under the application of strong magnetic fields. Some precessional modes of these sub-lattices were measured. The components of the g-tensor of terbium ions were extracted from the data. In chapter 6 the ultrafast magnetization dynamics of thulium orthoferrite, studied my means of terahertz spectroscopy, is described. It is demonstrated that terahertz response of the orthoferrite provides crucial additional information with respect to the optical pump-probe signal. A novel exchange driven mechanism of optical manipulation of the magnetic state is demonstrated. Finally, chapter 7 is a theoretical discussion of so called planar magnonic metamaterials. It is shown that the arrays of ferromagnetic films may exhibit negative refraction index at sub-terahertz frequencies, provided the mechanism of spin wave quantization is introduced. The thesis ends with a brief conclusions chapter where a short summary of the results is given. Some possible future extensions of the conducted research are drawn as well.
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Karabiyik, Mustafa. "Terahertz Plasmonic Devices." FIU Digital Commons, 2017. http://digitalcommons.fiu.edu/etd/3185.
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Terahertz (THz) devices are designed to operate from 0.1-10 THz. The THz spectra have unique properties such as penetration through soft materials and reflecting from hard materials, which make THz technologies, a prime candidate for imaging. Plasmons are longitudinal charge oscillations in carrier rich materials. Plasmons can be generated over the channel of transistors inducing a voltage between the source-drain when conditions are satisfied. In this thesis, plasmonic devices operating in the THz region have been studied both theoretically and experimentally investigating GaN/AlGaN and Graphene based transistors.
First, we report on a detailed study of dispersion properties of uniform grating gate THz plasmonic crystals, asymmetric dual grating gate plasmonic crystals and with symmetry-breaking defect-like cavities in order to understand the physics behind THz plasmons. For the first time, we defined the dispersion of plasmons in terms of effective plasmonic index. By adding an additional grating on top of the grating gate with a different periodicity, doubles the amount of absorption. Plasmons can be excited when polarization is perpendicular to the gate. We then showed focusing and exciting of THz plasmons polarization independent using circular grating lenses. Sub-micron THz ring resonators are presented showing THz guiding in plasmonic waveguides.
So far, resonant sensing has been observed only at cryogenic temperatures since electron mobility is high enough at low temperatures to sustain resonant plasmonic excitation at the channel of the detector. Recently, graphene attracted the attention of the researchers because of its high mobility at room temperature. Room temperature detection has been attempted and achieved, however the detectors have very small responsivity with non-resonant behavior since the graphene is sandwiched and fabrication of such detectors in large scale is impossible with the methods used. Here, we present a resonant room temperature detection of THz with upside down free standing graphene FETs having more than a 400 quality factor, a record high number in the field which is up to 50 times higher than GaN detectors and hundreds of responsivity values with a maximum around 400 V/W which is record high for graphene (10,000 times higher than previously reported graphene detector).
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Peters, Luke A. S. "Surface terahertz phenomena." Thesis, University of Sussex, 2018. http://sro.sussex.ac.uk/id/eprint/76771/.
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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.
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Burghoff, David Patrick. "Broadband terahertz photonics." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/92964.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 181-190).
In recent years, quantum cascade lasers have emerged as mature semiconductor sources of light in the terahertz range, the frequency range spanning 1 to 10 THz. Though technological development has pushed their operating temperatures up to 200 Kelvin and their power levels up to Watt-level, they have remained unsuitable for many applications as a result of their narrow spectral coverage. In particular, spectroscopic and tomographic applications require sources that are both powerful and broadband. Having said that, there is no fundamental reason why quantum cascade lasers should be restricted to narrowband outputs. In fact, they possess gain spectra that are intrinsically broad, and beyond that can even be tailored to cover an octave-spanning range. This thesis explores the development of broadband sources of terahertz radiation based on quantum cascade lasers (QCLs). The chief way this is done is through the development of compact frequency combs based on THz QCLs, which are able to continuously generate milliwatt levels of terahertz power covering a fractional bandwidth of 14% of their center frequency. These devices operate on principles similar to microresonator-based frequency combs, and make use of the quantum cascade laser's fundamentally large nonlinearity to phase-lock the cavity modes. These devices will enable the development of ultra-compact dual comb spectrometers based on QCLs, and will potentially even act as complete terahertz spectrometers on a chip. This thesis also uses broadband terahertz time-domain spectroscopy to analyze the behavior of THz QCLs. By using QCLs as photoconductive switches, the usual limitations imposed by optical coupling are circumvented, and properties of the laser previously inaccessible can be directly observed. These properties include the gain and absorption of the laser gain medium, the populations of the laser's subbands, and properties of the waveguide like its loss and dispersion. Knowledge of these properties were used to guide frequency comb design, and were also used to inform simulations for designing better lasers.
by David Patrick Burghoff.
Ph. D.
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Numan, Nagla Numan Ali. "Terahertz (THz) spectroscopy." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/71690.
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Thesis (MSc)--Stellenbosch University, 2012.ENGLISH ABSTRACT: Terahertz radiation is currently used in security, information and communication technology (ICT), and biomedical sciences among others. The usability of terahertz (THz) radiation, in many of its applications depends on characteristics of the materials being investigated in the THz range. At the heart of THz usage is a THz spectroscopy system necessary for the generation and detection of the THz radiation. In this thesis, we characterise such a THz spectroscopy system. In our typical THz spectrometric system, we make use of femtosecond (fs) laser technology and pump-probe principles for emission and detection of THz radiation. Background about the principles of generation THz radiation using fs triggered antennas and the principles of the spectroscopy technique and appropriate literature references are presented. Using an assembled commercially available kit, we reproduce known spectra in order to confirm correct functionality (for calibration) of the assembled spectroscopy system and to gain experience in interpreting these spectra. By introducing a suitable x - y scanning device we construct a crude THz imaging device to illustrate the principle.
AFRIKAANSE OPSOMMING: Terahertsstraling word deesdae wyd in die sekuriteits, inligting-en-kommunikasie en biomediese sektore aangewend. Die gepastheid van terahertsstraling (THz) vir ’n spesifieke toepassings hang af van die eienskappe van die materiale wat ondersoek word. Vir die uitvoer van sulke eksperimente word ’n THz-spektroskopie sisteem benodig vir die opwekking en meting van THz-straling. In hierdie tesis word so ’n THz-spektroskopie sisteem beskou en gekarakteriseer. In die sisteem word van ’n femtosekondelaser (fs) gebruik gemaak in ’nn pomp-en-proef opstelling vir die uitstraling en meting van THz-straling. Die beginsels rakende die opwekking van THz-straling, deur gebruik te maak van ’n antenna wat deur ’n fs-laser geskakel word, asook die beginsels van die spektroskopiese tegniek, met toepaslike verwysings, word in die tesis aangebied. Deur gebruik te maak van’n kommersiële THz opstelling is bekende spektra gemeet om die korrekte funksionering (vir kalibrasie doeleindes) na te gaan en om ondervinding op te doen in die interpretasie van hierdie spektra. ’n X-Y-translasie toestel is tot die opstelling bygevoeg om THz-afbeelding moontlik te maak en sodoende hierdie beginsel te illustreer.
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Jin, Chuhang. "Microstructured Terahertz Fiber." Thesis, KTH, Tillämpad fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-265667.
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Klatt, Gregor [Verfasser]. "Charakterisierung neuartiger Terahertz-Emitter mittels schneller, präziser Terahertz-Spektroskopie / Gregor Klatt." München : Verlag Dr. Hut, 2011. http://d-nb.info/1014848628/34.
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Kao, Tsung-Yu. "From high power terahertz quantum cascade lasers to terahertz light amplifiers." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/87923.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 201-208).
The terahertz (THz) frequency range (300 GHz to 10 THz, wavelength 30-1000 [mu]m), despite having many potential applications, is technologically relatively underdeveloped mainly because of the lack of suitable coherent radiation sources when compared with nearby electromagnetic radiation spectrum. The invention of the THz quantum cascade laser, a electronically-pumped semiconductor heterostructure which emits photons from electronic intersubband transitions, provides the first solidstate fundamental oscillator at the frequency range from 1.2 to 5.1 THz. Due to the subwavelength confinement nature of the metal-metal waveguide used in most of the THz QC lasers, far-field beam patterns from lasers with simple Fabry-Perot waveguides are divergent and far from ideal Gaussian beams. The first part of this thesis describes the development of single-mode THz QC lasers on metal-metal waveguides. Starting with the corrugated third-order DFB laser-a clever laser structure which utilizes end-fire array effect to achieve low divergence beam patterns-several applications using densely-packed third-order DFB laser arrays, such as frequency agile sources for THz swept-source optical tomography and local oscillators for THz heterodyne receivers with precise frequency control, have been investigated. With the improved design rules and fabrication techniques, 830 GHz single-mode frequency coverage on a monolithic multicolor DFB laser array has been achieved. The origin of the deterioration in far-field beam patterns and power outputs in long third-order DFB lasers is then identified. This finding leads to a modified third-order DFB laser structure which can achieve perfect phase-matching (PM) condition, resulting in scalable power output and even lower beam divergence when compared with that of a conventional third-order DFB laser. Radiations from up to 151 laser sectors are phase-locked to form a single-lobe beam pattern with divergence ~ 6 x 11° and ~13 mW pulsed power at the end-fire direction. This approach substantially increases the usable length of a third-order DFB laser while keeping a high slope efficiency (140 mW/A). Later development applies the concept of microstrip antenna-a structure commonly used in microwave engineering-to THz photonics devices. By coupling the microstrip antenna to each grating aperture of a perfectly phase-matched DFB laser, the radiation impedance of the laser can now be tuned to enhance the overall emission efficiency. This novel genre of DFB laser achieves > 8 mW pulsed power (10% duty-cycle) at 12 K with beam divergence as low as 12.5 x 12.5' and maximum lasing temperature Tmax = 109 K (pulsed) and 77 K (c.w.) with the highest slope efficiency (~450 mW/A) and wall-plug efficiency (0.57%) of all THz DFB laser sources. The second part of the thesis then focuses on the development of the first light amplifier in THz frequency under Fabry-Perot amplifier (FPA) scheme. Although amplification at terahertz frequency in quantum cascade structures has been demonstrated under the transient state or in a integrated platform, none of them is suitable for amplifying continuous-wave free-space THz radiations. The proposed amplifier is consisted of an array of short-cavity surface-emitting second-order distributed feedback lasers arranged in a two-dimensional grid which are operated marginally beneath their lasing thresholds. A overall system power gain of ~5.6x = 7.5 dB at ~3 THz is obtained with ~1 GHz bandwidth. The free-space THz light amplifier can be used as the pre-amplifier for a THz heterodyne receiver system to reduce the receiver system noise, or be placed on the focal plane of a THz imaging system to enhance the signal-to-noise ratio of the image and reduce the acquisition time. A new locking mechanism for two-dimensional phase-locked laser arrays based on antenna mutual-coupling is also proposed and then successfully demonstrated in the THz frequency using short-cavity DFB THz lasers. Up to 37 lasers are phase-locked to deliver 6.5 mW single-mode pulsed power (4% duty-cycle) at 3 THz with symmetric beam pattern (< 10 x 10°). This new coupling scheme can be extended to other electromagnetic systems with sub-wavelength confined elements such as plasmonic lasers and nanolasers. This thesis also reports the development of fabrication techniques required to bring the aforementioned novel THz cavity designs from concepts to reality which include a high aspect ratio (> 1:10) anisotropic reactive-ion etch on GaAs which is compatible with the metal-metal waveguide platform and the procedure to create airbridge structures by selectively removing the dielectric materials beneath the metal contacts.
by TsungYu Kao.
Ph. D.
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Skjeie, Hans Christian Bakken. "Terahertz Time-Domain Spectroscopy." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for elektronikk og telekommunikasjon, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-19214.
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The field of terahertz time-domain spectroscopy (THz-TDS) is still far from reaching its full potential, but is a very promising utility for a wide range of applications. Principle experiments have been performed in fields of drug screening, pharmaceutical, medical diagnostics, security imaging and detection of explosives. Optimized and adapted THz-TDS systems holds great promise for driving this technology further.The purpose of this thesis was to build a THz-TDS system, explore possibilities for improving this system and to perform THz-TDS measurements on semiconductors and wood. The aim of the experimental work was to build a stable and reliable system with an electric field strength of THz radiation in the order of kV/cm. The THz-TDS system used in this thesis was based upon the principles of optical rectification and free-space electro-optic sampling in zinc telluride (ZnTe) crystals using a femtosecond Ti:Sapphire amplified laser.Theoretical studies were performed on the principles of generation and detection of THz radiation. The experimental work was based on publications of similar experiments. Theoretical and experimental studies lead to several modifications and improvements of the setup first built in this thesis. Experiments were performed on disparate materials to find suitable materials for THz transmission. Results from measurements performed on semiconductors and wood, obtained by THz-TDS, were analysed to find the absorption coefficient and the refractive index of the materials. The spectroscopic information obtained by THz-TDS can also be used to find the conductivity and the mobility of these materials. THz-TDS measures the electric field and therefore provides information of both the amplitude and the phase of the THz wave. A Fourier transformation was used to obtain the frequency spectrum of the detected signal. The improvements were done by analysing the results of the detected signal to see which adjustments and modifications to the setup that had positive effects on the results. The pump power used for generation of THz radiation and the optimum azimuthal angle of the ZnTe crystals were crucial to obtain a THz-TDS system with a strong electric field. The maximum electric field strength for the THz radiation in this thesis was 13.2 kV/cm, with a signal-to-noise ratio of 43 and dynamic range of 1500.
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Williams, Benjamin S. (Benjamin Stanford) 1974. "Terahertz quantum cascade lasers." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/17012.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 297-310).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
The development of the terahertz frequency range has long been impeded by the relative dearth of compact, coherent radiation sources of reasonable power. This thesis details the development of quantum cascade lasers (QCLs) that operate in the terahertz with photon energies below the semiconductor Reststrahlen band. Photons are emitted via electronic intersubband transitions that take place entirely within the conduction band, where the wavelength is chosen by engineering the well and barrier widths in multiple-quantum-well heterostructures. Fabrication of such long wavelength lasers has traditionally been challenging, since it is difficult to obtain a population inversion between such closely spaced energy levels, and because traditional dielectric waveguides become extremely lossy due to free carrier absorption. This thesis reports the development of terahertz QCLs in which the lower radiative state is depopulated via resonant longitudinal-optical phonon scattering. This mechanism is efficient and temperature insensitive, and provides protection from thermal backfilling due to the large energy separation between the lower radiative state and the injector. Both properties are important in allowing higher temperature operation at longer wavelengths. Lasers using a surface plasmon based waveguide grown on a semi-insulating (SI) GaAs substrate were demonstrated at 3.4 THz in pulsed mode up to 87 K, with peak collected powers of 14 mW at 5 K, and 4 mW at 77 K.
Additionally, the first terahertz QCLs have been demonstrated that use metalmetal waveguides, where the mode is confined between metal layers placed immediately above and below the active region. These devices have confinement factors close to unity, and are expected to be advantageous over SI-surface-plasmon waveguides, especially at long wavelengths. Such a waveguide was used to obtain lasing at 3.8 THz in pulsed mode up to a record high temperature of 137 K, whereas similar devices fabricated in SI-surface-plasmon waveguides had lower maximum lasing temperatures due to the higher losses and lower confinement factors. This thesis describes the theory, design, fabrication, and testing of terahertz quantum cascade laser devices. A summary of theory relevant to design is presented, including intersubband radiative transitions and gain, intersubband scattering, and coherent resonant tunneling transport using a tight-binding density matrix model. Analysis of the effects of the complex heterostructure phonon spectra on terahertz QCL design are considered. Calculations of the properties of various terahertz waveguides are presented and compared with experimental results. Various fabrication methods have been developed, including a robust metallic wafer bonding technique used to fabricate metal-metal waveguides. A wide variety of quantum cascade structures, both lasing and non-lasing, have been experimentally characterized, which yield valuable information about the transport and optical properties of terahertz devices. Finally, prospects for higher temperature operation of terahertz QCLs are considered.
by Benjamin S. Williams.
Ph.D.
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Parrott, Edward Philip John. "Applications of terahertz radiation." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608716.
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Kim, Joong Hyun. "Efficient terahertz photoconductive source." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26608.
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Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.Committee Chair: Ralph, Stephen; Committee Member: Citrin, David; Committee Member: Cressler, John; Committee Member: Denison, Douglas; Committee Member: Mukhopadhyay,Saibal. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Moshirfatemi, Farnoosh. "Communicating at Terahertz Frequencies." PDXScholar, 2017. https://pdxscholar.library.pdx.edu/open_access_etds/3641.
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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.
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Al, Shanawani Mazen <1982>. "Terahertz and Millimetric Rectennas." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amsdottorato.unibo.it/9398/1/main.pdf.
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In recent years, the energy market has witnessed increasing demand on green electromagnetic energy resources to meet the next generation devices requirements. While energy harvesting in the lower gigahertz band has witnessed many improvements leading to market-ready solutions, the terahertz harvesting is, still, in an immature state.
As will be demonstrated later, the electromagnetic radiation frequency identifies the theory of operation and so the rectifiers are categorised, into lower and upper frequency bands. While the theoretical framework for the lower frequency rectifiers is more "uniform", there are many theories to explain the rectifier operation for upper frequency bands. For the latter case, Simmons and the transfer matrix method models are chosen and elaborated in more details. An optimisation framework that deploys the transfer matrix method to calculate the voltage-current relationship of a tunnelling diode and improve the relevant figures of merit will be also suggested.
New and novel techniques leading to optimized wireless energy transmission will be elaborated. In this context, the time-modulated array technique will be considered and studied, for a range of frequencies extending to 28 GHz, as a possible substitution to the lossy linear phased array control circuits. The novel frequency-diverse array technique, leading to distance-dependent radiation pattern behaviour, will be also discovered.
A market-ready solution for an efficient 2.4 GHz energy-harvesting device is presented and tailored to work in harsh electromagnetic environments. Starting from a simple and generic rectifier model, the design is upgraded to reach an end-product prototype together with its measurements in a real-world scenario.
In the end, an efficient and fast simulation method capable to calculate the received power by wireless sensors is also presented. Thanks to the integral solver simulation, the results are more accurate than typical finite difference
simulation and are obtained much faster as demonstrated in the corresponding chapter.
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Metbulut, Mukaddes Meliz. "Development Of Compact Terahertz Time-domain Terahertz Spectrometer Using Electro-optic Detection Method." Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/3/12610983/index.pdf.
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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.
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Wallauer, Jan [Verfasser], and Markus [Akademischer Betreuer] Walther. "Terahertz metamaterials : From fundamental coupling mechanisms to new terahertz metasurfaces and functional emitters." Freiburg : Universität, 2015. http://d-nb.info/1119247284/34.
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Smith, Ryan Michael. "Terahertz frequency analysis of gaseous and solid samples using terahertz time-domain spectroscopy." Diss., University of Iowa, 2012. https://ir.uiowa.edu/etd/3386.
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Developments in semiconductor and laser technologies have facilitated development of terahertz (THz)-frequency (˜2-200 cm-1) technologies. Results published in the literature as far back as the early 20th century demonstrate the utility of this frequency range for myriad applications, but the improved performance of modern THz technologies has renewed interest in THz-frequency analysis. Material presented in this dissertation focuses on three applications of terahertz time-domain spectroscopy (THz-TDS): quantitation of gas-phase molecular species, analysis of polymeric materials, and investigation of dental tissue/composite structures.
Gas phase species were quantified individually at concentrations ranging from several parts per million to several parts per thousand using various chemometric methods. Quantitative model robustness was evaluated by comparison of model precision, and partial least-squares (PLS) regressions provided the greatest precision. Species were quantified in mixtures using PLS with errors of prediction below the permissible exposure limits (PELs) set by the Occupational Safety and Health Administration. The effect of spectral broadening as a result of overall sample pressure was investigated, and species were analyzed in mixtures at various overall pressures. Errors of prediction were again near or below the PELs, demonstrating the utility of this method for atmospheric analysis. Chemical selectivity available in THz spectral features was evaluated and compared to selectivity available in infrared frequencies.
Spectral parameters measured in the THz frequency range also provide insight into structural properties of polymeric materials. In some cases, spectral peaks may be used to identify the temperature at which phase changes occur within these materials. THz refractive index spectra were found to be a sensitive and non-destructive tool for identification of phase transition temperatures.
The time-resolved measurement of THz-TDS makes it particularly useful for rapid, non-destructive analysis of layered structures. Ordinarily, the strength of bonds between dental tissues and composite materials are evaluated in the laboratory using destructive failure analyses. Transparency of dental tissues and composite materials used for restorative procedures to THz pulses allows investigation of interfaces between these materials. Refractive index spectra indicate locations in which delamination has occurred between bonded layers. These results provide an overview of unique capabilities of the THz-TDS method in real-life spectral analyses.
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Maysonnave, Jean. "Dynamique ultrarapide de lasers à cascade quantique Terahertz - le graphène comme émetteur Terahertz." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066360/document.
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La gamme des ondes terahertz (THz) se situe à l'interface des domaines électronique et optique. Malgré un potentiel d'applications élevé, elle souffre d'un manque de dispositifs performants. Dans ce cadre, cette thèse se concentre sur l'étude fondamentale et la réalisation de nouvelles fonctionnalités associées à différentes sources THz, en utilisant la spectroscopie THz dans le domaine temporel (TDS). Cet outil puissant permet de mesurer le profil temporel d'un champ électrique THz et est utilisé pour explorer l'émission THz de lasers à cascade quantique (LCQ) et de graphène.Dans une première partie, la réponse ultrarapide de LCQs est étudiée. Un contrôle de la phase du champ électrique de LCQs THz via la technique "d'injection seeding" est réalisé puis optimisé. Il nous permet de mesurer le profil temporel de l'émission laser. A l'appui de cette expérience et de simulations, une description quantitative de la dynamique du gain est faite. Ces informations sont critiques pour la production d'impulsions courtes. Une modulation rapide du gain de LCQ est ensuite réalisée et conduit à la génération d'impulsions courtes (durée ~ 15 ps) en régime de blocage de modes. Ces études permettent notamment d'envisager les LCQs comme sources puissantes pour la TDS. Dans une seconde partie, nous montrons que le graphène peut émettre un rayonnement THz sous excitation optique par un effet non linéaire d'ordre 2. Cette émission résulte d'un transfert de quantité de mouvement des photons aux électrons du graphène ("photon drag"). Elle permet ainsi d'explorer des propriétés subtiles du graphène, telles que de très faibles différences de comportement entre les électrons et trous photogénérésThe terahertz (THz) range is a region of the electromagnetic spectrum which lies at the limit between the electronic and optical domain. Currently, THz applications suffer from the lack of sources and detectors. In this context, this thesis focuses on the fundamental study and the development of new functionalities of different THz sources, usingTHz time-domain spectroscopy (TDS) as a base. This powerful tool enables to acquire the temporal profile of a THz electric field and is used to explore the THz emission properties of quantum cascade lasers (QCLs) and graphene.In the first part, the ultrafast response of QCLs is investigated. A phase control of the electric field of THz QCLs via injection seeding is realised and optimised. This enables the measurement of the amplitude and temporal profile of the laser emission. Throughthese experiments and simulations, a quantitative description of the gain dynamics can be accessed. This information is critical for modelocking. Finally, a fast modulation of the gain of QCLs is realized and leads to short pulses generation (15 ps) in a modelocked regime. These studies open the way for using QCLs as powerful sources in TDS.In the second part, THz radiation generation from graphene under optical excitation is demonstrated by a second order non-linear process. The THz emission results from themomentum transfer from the photons to the electrons of graphene (photon drag). As well as broadband THz generation, novel bandstructure properties of graphene can be explored such as the different dynamics between the photogenerated electrons and holes
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Maysonnave, Jean. "Dynamique ultrarapide de lasers à cascade quantique Terahertz - le graphène comme émetteur Terahertz." Electronic Thesis or Diss., Paris 6, 2014. http://www.theses.fr/2014PA066360.
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La gamme des ondes terahertz (THz) se situe à l'interface des domaines électronique et optique. Malgré un potentiel d'applications élevé, elle souffre d'un manque de dispositifs performants. Dans ce cadre, cette thèse se concentre sur l'étude fondamentale et la réalisation de nouvelles fonctionnalités associées à différentes sources THz, en utilisant la spectroscopie THz dans le domaine temporel (TDS). Cet outil puissant permet de mesurer le profil temporel d'un champ électrique THz et est utilisé pour explorer l'émission THz de lasers à cascade quantique (LCQ) et de graphène.Dans une première partie, la réponse ultrarapide de LCQs est étudiée. Un contrôle de la phase du champ électrique de LCQs THz via la technique "d'injection seeding" est réalisé puis optimisé. Il nous permet de mesurer le profil temporel de l'émission laser. A l'appui de cette expérience et de simulations, une description quantitative de la dynamique du gain est faite. Ces informations sont critiques pour la production d'impulsions courtes. Une modulation rapide du gain de LCQ est ensuite réalisée et conduit à la génération d'impulsions courtes (durée ~ 15 ps) en régime de blocage de modes. Ces études permettent notamment d'envisager les LCQs comme sources puissantes pour la TDS. Dans une seconde partie, nous montrons que le graphène peut émettre un rayonnement THz sous excitation optique par un effet non linéaire d'ordre 2. Cette émission résulte d'un transfert de quantité de mouvement des photons aux électrons du graphène ("photon drag"). Elle permet ainsi d'explorer des propriétés subtiles du graphène, telles que de très faibles différences de comportement entre les électrons et trous photogénérésThe terahertz (THz) range is a region of the electromagnetic spectrum which lies at the limit between the electronic and optical domain. Currently, THz applications suffer from the lack of sources and detectors. In this context, this thesis focuses on the fundamental study and the development of new functionalities of different THz sources, usingTHz time-domain spectroscopy (TDS) as a base. This powerful tool enables to acquire the temporal profile of a THz electric field and is used to explore the THz emission properties of quantum cascade lasers (QCLs) and graphene.In the first part, the ultrafast response of QCLs is investigated. A phase control of the electric field of THz QCLs via injection seeding is realised and optimised. This enables the measurement of the amplitude and temporal profile of the laser emission. Throughthese experiments and simulations, a quantitative description of the gain dynamics can be accessed. This information is critical for modelocking. Finally, a fast modulation of the gain of QCLs is realized and leads to short pulses generation (15 ps) in a modelocked regime. These studies open the way for using QCLs as powerful sources in TDS.In the second part, THz radiation generation from graphene under optical excitation is demonstrated by a second order non-linear process. The THz emission results from themomentum transfer from the photons to the electrons of graphene (photon drag). As well as broadband THz generation, novel bandstructure properties of graphene can be explored such as the different dynamics between the photogenerated electrons and holes
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Yeh, Ka-Lo. "The generation of high field terahertz radiation and its application in terahertz nonlinear spectroscopy." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/49748.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2009.Includes bibliographical references (p. 147-155).
In this thesis research, I implemented a terahertz generation scheme that enables high-field near-single-cycle terahertz (THz) pulse generation via optical rectification in a LiNbO3 (LN) crystal. I also developed a method for the non reconfigurable generation of high-intensity multiple-cycle THz fields aimed to more efficiently deliver THz energy to resonant samples. A novel free-space THz-pump/THz-probe setup enabled time-resolved measurement of ultrafast nonlinear electronic responses in doped bulk semiconductor samples. The ability to spectrally and temporally resolve the response of the semiconductor sample using a THz probe allowed us to uncover both the dynamics of impact ionization and interesting phonon-plasma interactions in indium antimonide (InSb) for the first time. Nonlinear vibrational responses in LN were ob-served first in a dual THz beam setup on an integrated LN waveguide platform with optical probing to reveal the generation of terahertz second harmonic signals and also in THz transmission measurements in cooled LN where THz intensity-dependent self-phase modulation and harmonic generation were observed. An outlook and preliminary results toward implementation and observation of high-field THz-driven responses in ferroelectric materials are given in the final chapter of this thesis.
by Ka-Lo Yeh.
Ph.D.
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Upadhya, Prasanth Chandrashekara. "Coherent generation and detection of Terahertz radiation : time domain Terahertz spectroscopy of molecular crystals." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614771.
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Hossain, Zakir. "Planar Antennas for Terahertz Detectors." Thesis, Högskolan i Gävle, Avdelningen för elektronik, matematik och naturvetenskap, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:hig:diva-13213.
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The use of the millimeter-wave and terahertz (THz) systems are becoming very important in various scientific and military applications. In many applications i.e. radio-astronomy, THz imaging, remote sensing, secure communication links, THz radars, plasma diagnostics etc. the use millimeter-wave and THz technologies are increasing rapidly and in these applications, integreted lens antennas are porividing attractive solutions. But perfect modeling and simulation of the lens antenna and beam pattern calculation is still a challenge for the antenna engineers. In this thesis project thorough investigation has been done to optimize the simulation technique in the EM simulator CST MWS to get reliable and accurate simulation results which may replace the time consuming, complex and expensive measurements in sub-millimeter/THz frequency range. Different experiments are done by retriving from the publications related to this project for the result comparison. Later on with this optimized simulation process an extended hemispherical lens antenna integrated with log-spiral feed is designed for THz frequency range. Most of the desired goals are achieved during the design process as the design goal is to produce a constant radiation beam and polarization over the a broad bandwidth (500GHz-750GHz)
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Stead, Robert A. "Advances in terahertz frequency combs." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/43450.
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Following a review of the theory of terahertz generation via optical rectification in nonlinear crystals, a method for enhancing the nonlinear conversion efficiency of this process is proposed. A nonlinear crystal is placed at the intracavity focus of a passive optical resonator, which is seeded by an ytterbium-doped fibre laser. Models of this arrangement indicate that an enhancement of the optical field of several orders of magnitude is possible. As the difference frequency radiation produced through the optical rectification process results from a mixing of the spectral components of the optical field, one expects a corresponding increase in the terahertz field. We present a design of optical resonator that compensates for the large group velocity dispersion of the nonlinear crystal. Our experimental results indicate that below bandgap absorption in the crystal severely limits the resulting enhancement of the optical field, and hence the terahertz field one would expect from this nonlinear process.
A scanning-delay terahertz time-domain spectrometer has been constructed, using a gallium phosphide guiding structure to increase the interaction length of the optical and terahertz fields, thereby increasing the terahertz power produced. Our experiment demonstrates 20 dB signal to noise ratio over the spectral range of 0.5-1 THz. We propose a method for increasing the spectral resolution, whilst simultaneously reducing the required data acquisition time of such a terahertz spectrometer, through the use of two femtosecond optical frequency combs. One of these fields drives the nonlinear optical rectification process, whilst the second serves as a sampling local oscillator field to probe the terahertz field via electro-optic sampling in a second nonlinear crystal. By precisely controlling the relative pulse repetition rates of the two oscillators, we show that the full spectral content of the terahertz field can be acquired at rf frequencies, and without the slow mechanical delay lines associated with conventional terahertz time-domain spectroscopy. Finally, we present experimental efforts towards the demonstration of this technique, and show that, to be effective, steps must be taken to increase the strength of the expected rf signal over that of the measurement noise floor.
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Singh, Rohit. "Modulated orientation sensitive terahertz spectroscopy." Thesis, State University of New York at Buffalo, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3565883.
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The energies of protein correlated motions lie in the far infrared or THz frequency range (λ = 1 cm – 50 mm, f = 0.03 – 6 THz). The existence of correlated motions has been confirmed by neutron and inelastic x-ray scattering measurements. These techniques require large sample volumes and specialized facilities, limiting their application to systematic studies of changes in correlated motions with functional state and allosteric interactions. Standard terahertz time domain spectroscopy measurements have shown sensitivity to protein-ligand binding, oxidation state, conformation, temperature and hydration. However, the response is broad, in part from the large vibrational density of states and in part from the dielectric response contribution from surface water and side-chains.
As an overall strategy to measure the correlated structural motions in protein, we use anisotropic and birefringent behavior of molecular crystals to develop a new technique called MOSTS (Modulated Orientation Sensitive Terahertz Spectroscopy). We achieve high sensitivity and mode separation, by using single molecular crystal such as sucrose and oxalic acid, and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample. By locking into the signal at the rotation frequency, we determine the polarization sensitive signal and map out the optically active vibrational resonances. To illustrate the technique, we compare our measured spectra with the calculated, and find a close agreement.
We measure dielectric properties of oxalic acid, sucrose and protein crystals and polycarbonate sheet using standard terahertz time domain spectroscopy. We determine the absorbances in oxalic acid and sucrose crystals, using MOSTS technique. We compare the resonances in these two distinct methods. Then, we develop a protein model sample by sticking together two thin plates of sucrose and polycarbonate. We carry out standard THz-TDS and MOSTS measurements on the protein model sample. We show that we are able to isolate the vibrational modes from glassy background in protein model sample by using MOSTS.
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Othman, Mohd Azlishah. "Sub-Terahertz : generation and detection." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13375/.
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Nowadays, there has been an increasing interest in Terahertz (THz) radiation for application across scientific disciplines including atmospheric sensing, medical diagnosis, security screening and explosive detection. The limitation of THz generators and detectors has gained interest from scientists and engineers to explore the development of both sources and detectors. With the advantages of low cost, low power consumption, high reliability and potential for large-scale integration, sub-THz generator and detector can be developed using CMOS process technology. In this thesis, an IMPATT diode acts as a sub-THz generator, HEMTs and MOSFETs act as sub-THz detectors, which are developed in AMS 0.35 μm CMOS technology and UMC, 0.18 μm CMOS technology. The size of the IMPATT diode was 120 μm x 50 μm with the target resonant frequency at 30 GHz. The experiment results show that the operating frequency of the IMPATT diode was between 12 GHz up to 14 GHz. Then by using HEMTs with 0.2 μm gate length and 200 μm gate widths, sub-THz radiation detection has been demonstrated. Experimental results show that the photoresponse depends on the drain current and the gate to source voltage VGS. In addition, photoresponse also depends on varying frequencies up to 220 GHz and fixed the drain current. Furthermore, the HEMT also give an indication of response by varying the input power of microwave extender. MOSFETs from two types of CMOS technology; AMS 0.35μm and UMC 0.18 μm technology with different gate length ranging from 180 nm up 350 nm were demonstrated. These results provide evidence that the photoresponse increases with the drain current and the RF input power, but inversely to the frequencies. These results also provide evidence that the MOSFETs are able to work as low cost and sensitive sub-THz detector.
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Walsh, David A. "Intracavity terahertz optical parametric oscillators." Thesis, University of St Andrews, 2011. http://hdl.handle.net/10023/1713.
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This thesis describes the design and implementation of several novel, nanosecond pulsed, intracavity optical parametric oscillators for the generation of terahertz radiation. The application of the intracavity approach in the context of terahertz optical parametric oscillators has been demonstrated for the first time, and the pump wave energy required was thereby reduced by an order of magnitude. The terahertz wave was tunable from under 1THz up to 3THz with a free running linewidth of ~50GHz and pulse energies up to ~20nJ (pulses were a few nanoseconds in duration). The terahertz beam profile was of Gaussian shape and could be focussed down to 2.3 times the diffraction limited spot size (M² values of 2.3 and 6.7 in the components of the beam parallel and perpendicular to the silicon prism array output coupler respectively). Developments of this intracavity source with regard to the linewidth are also reported. Implementation of etalons in the optical (laser and OPO) cavities was shown to be a promising technique that brings the terahertz linewidth down below 1GHz (close to the transform limit of nanosecond pulses) while retaining the tuning range and beam characteristics of the free running system. Close to Fourier transform limited pulses were obtained (<100MHz linewidth) via an injection seeding technique, although with significantly increased system complexity. A deleterious effect caused by the mode beating of a multimode host laser was also discovered, in that sidebands were induced on the seeded downconverted wave. This has wider implications in the field of intracavity OPOs. Finally, quasi-phasematching techniques implementing periodically poled lithium niobate were investigated as a way to lower the downconversion threshold energy requirement (by collinear propagation of the optical waves), and also to extract the terahertz wave rapidly from the (highly absorbing in the terahertz region) lithium niobate crystal. The existence of two phasematching solutions arising from the bidirectionality of the grating vector was identified as a serious design constraint in the context of an OPO where either solution can build up from noise photons, and so prefers the solution with the lowest walkoff of the downconverted waves - possibly resulting in unextractable terahertz radiation. Quasi-phasematching with an orthogonal grating vector (with identical but opposite phasematching solutions) was demonstrated and cascaded downconversion processes observed and characterised. These cascaded processes are permitted by the collinearality of the optical waves and may allow efficiency improvements through overcoming the quantum defect limit. This research has resulted in four peer reviewed papers in respected journals, and the intracavity terahertz OPO has been licensed to a company who have commercialised the technology (M Squared Lasers, Glasgow).
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Li, Dazhang. "On-chip pulsed terahertz systems." Thesis, University of Leeds, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.531524.
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Freeman, J. R. "Heterogeneous terahertz quantum cascade lasers." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599209.
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Although terahertz quantum cascade lasers (QCLs) are promising devices for cheap, compact and coherent sources of terahertz radiation, the full potential of these lasers has not been realised. In this work we shall examine the active region design of THz QCLs and study the behaviour of heterogeneous active regions, where there are several quantum cascade designs combined in the same active region. We will present a study of the systematic design of THz QCLs and show that it is possible to systematically tune the frequency of these devices by simple modifications to the active region design. We then present some active region designs, together with results for some high-performance designs, operating around 2 THz. Two of these designs have shown high efficiency and high temperature performance at this frequency range. From this we move on to the subject of heterogeneous active regions, and present electrically switchable emission from heterogeneous THz QCLs; devices which emit at different, widely separated frequencies depending on the electrical conditions. This is the first reported electrically switchable emission from THz QCL devices without a magnetic field.
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Zhou, Yun. "Reconfigurable Terahertz Integrated Architecture (RETINA)." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.509509.
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Drysdale, Timothy D. "Passive devices for terahertz frequencies." Thesis, University of Canterbury. Electrical and Electronic Engineering, 2003. http://hdl.handle.net/10092/7721.
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Terahertz technology is a relatively new field of electromagnetic study and interest is rapidly growing in the wake of dramatic imaging demonstrations. Other applications are expected to follow, and they will need passive devices with functionality already found in more familiar microwave and visible regions of the electromagnetic spectrum, but presently missing in the terahertz region. Two fundamental devices in particular are variable polarisation compensators, and tunable frequency-selective filters.
This work represents the first demonstration of a variable polarisation compensator using subwavelength patterned features (artificial dielectrics). Following on from the original proposal, this work contains a complete and thorough investigation including the development of a bulk silicon micromachining fabrication process, full characterisation of the device performance in the W-band (70 – 110GHz) and comprehensive simulations of the device, including detailed simulation of three distinct new designs with improved performance (continuously-variable retardance with maximum in excess of quarter- and half wave). The third of the three designs is capable of extremely low insertion loss (<0.6 dB) and overcomes a difficulty of the original design that prevented zero retardance in a practical device.
Secondly, a new tunable photonic crystal filter is proposed and demonstrated. Easily accessible external control surfaces integrated into the interlocking plates of a layer-by-layer photonic crystal allow unprecedented contol over the number and type of defects within the structure, all of which may be tuned "on-the-fly". Devices are initially investigated with a full-vector electromagnetic finite-difference time-domain technique, to reveal the influence of the design dimensions on the band gap as well as the effect of the defects. A two-plate metal device having four layers of rods is constructed and measured in the W-band. In good agreement with the simulations, it is experimentally determined that a moveable passband is centered at 81 GHz, with a quality factor of 11, and a tuning shift of 1.7 GHz for a plate movement of 450 µm.
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Li, Di. "Investigation of Terahertz photoconductive antennas." Thesis, University of Liverpool, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526799.
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Terahertz (THz) frequency range usually refers to the electromagnetic spectrum between 100 GHz and 10 THz, which is between the millimetre and infrared regions. THz research has received a lot of attention because of its wide potential applications for such as high-speed wireless communications, medical imaging, remote sensing and security scanning. Photoconductive antenna is the most popular device used to generate and detect THz waves. However, there are still many challenges in this area, for example, how to improve its radiation efficiency and how to increase its directivity to the desired direction. In this dissertation, firstly four methods are proposed to improve the generation efficiency of photoconductive antennas. The first method is to adjust the gap of the photoconductive antenna to an optimum value which is dependent on the input laser power and the material properties of the substrate. The second method is to focus the laser beam on a very small area rather than the whole gap and the generated power can be increased by more than 5 times. The third method is to increase the bias voltage, which can strengthen the photo-induced current. The final method discussed is to use the indentation configuration instead of the conventional dipole shape to enhance the electric field in the gap which can result in about two times stronger power radiation. Secondly a THz hom structure is introduced to improve the directivity and the radiation efficiency of the photoconductive antenna. The conventional photoconductive antenna cannot provide high directivity, but this horn antenna can if it is designed and constructed properly. It consists of two main parts: a photoconductive emitter and a THz conical horn. A computer aided design approach has been adopted, and the simulation results show that the THz conical horn antenna with the proposed feeding structure can radiate more THz power in desired directions than conventional antenna. The directivity of this structure is proved to be 10 dB greater than the conventional photoconductive antennas. It should be pointed out that the THz horn antennas are not the same as the conventional microwave horn antennas. The major difference is on the feeding structure. In addition, the effects of the substrate on THz photoconductive antennas are also investigated theoretically and numerically, some very interesting results are obtained.
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Hussain, Ali. "Ultrabroadband time domain terahertz spectroscopy." Thesis, University of Bath, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.431734.
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Qui, Junyi. "Terahertz spectrometry applied to proteins." Thesis, Queen Mary, University of London, 2017. http://qmro.qmul.ac.uk/xmlui/handle/123456789/24777.
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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.
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Natrella, M. "Photonic terahertz emitters and receivers." Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1470214/.
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The research work presented in this thesis is concerned with the design, fabrication and characterisation of Continuous Wave (CW) Photonic Terahertz (THz) Emitters employing antenna-integrated Uni-Travelling-Carrier Photodiodes (UTC-PDs), based on the Indium Phosphide (InP) materials system. The solution employing photonic techniques for the generation of sub-millimetre and THz waves, via photomixing of lasers operating at 1550 nm, is a major candidate for the realisation of tuneable, power efficient, compact and cost effective THz sources operating at room temperature. The availability of sources endowed with such properties would make many important applications possible in this frequency range, such as ultra broad band wireless communications, spectroscopic sensing and THz imaging. The UTC-PDs enable high optical to electrical (O-E) conversion efficiency and are key components for the realisation of a photonic terahertz emitter. In this thesis the fabrication and characterisation of test vertically illuminated UTC-PDs, achieved using materials grown by Solid Source Molecular Beam Epitaxy at UCL and the fabrication of high performance waveguide UTC-PDs are reported, as milestones towards the development of a simple, repeatable and high yield fabrication process. A comprehensive study of UTC-PD impedance and frequency photo response, carried out using experimental techniques, circuit analysis and 3D full-wave electromagnetic modelling, is presented. The results of this investigation provide valuable new information for the optimisation of the UTC-PD to antenna coupling efficiency. New THz antenna and antenna array designs, obtained by means of full wave modelling, are also presented, and shown to be suitable for integration with both standard silicon lenses and a novel solution employing a ground plane. The accurate antenna design, along with the results of the UTC-PD impedance investigation, enables the prediction of the power radiated by antenna integrated UTC-PDs, not only in terms of trend over the frequency range but also of absolute level of emitted power. 3D full-wave modelling has also been used at optical frequencies, to address the problem of the optical fibre-to-chip coupling efficiency, which is another fundamental factor for the optimisation of a photonic THz emitter. Among other features, this analysis enables a better understanding of how the light is absorbed throughout the device structure and provides key information for future realisation of travelling-wave photodetectors. An additional experimental tool for the analysis of THz emission, namely the sub-wavelength aperture probe, has been modelled and characterised, revealing interesting properties for the characterisation of antenna far-fields and near fields, and hence providing a valuable tool for THz antenna analysis and design.
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Swithenbank, Matthew. "Terahertz spectroscopy in microfluidic systems." Thesis, University of Leeds, 2017. http://etheses.whiterose.ac.uk/18515/.
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Spectroscopic measurements in the terahertz-frequency-range can offer insight into the picosecond dynamics, molecular conformation, and biological function of chemical systems. Despite the recent emergence of terahertz-frequency time-domain spectroscopy as a tool for the measurement of dry, solid samples, the investigation of liquid analytes is complicated by the strong attenuation of terahertz-frequency signals in aqueous environments. The integration of microfluidic systems with on-chip waveguides offers a potential solution as picosecond pulses confined to a waveguide can interact with nano- or microlitre liquid sample volumes over a distance of several millimetres, with significantly reduced attenuation compared to free-space techniques. Specifically, the single-wire planar Goubau line waveguide has attracted attention in recent years owing to the relatively large extent of the supported evanescent field, enabling sensitive interaction between a propagating electric field and nearby samples. In this work, the first on-chip microfluidic spectrometer, capable of measuring the complex permittivity of liquids in the terahertz-frequency range is introduced. The fabrication of planar Goubau line devices with integrated photoconductive switches for the generation and detection of terahertz-frequency electric fields is discussed in detail. Given the importance of maximising the signal-to-noise ratio in spectroscopic measurements, an investigation of the signals excited from these switches is conducted, and factors such as the pump-power, generating beam polarisation, and switch geometry are found to have a significant impact on signal generation efficiency and noise. In addition to problematic signal noise, waveguide geometries can introduce artefacts that complicate further analysis. To simplify later modelling of these structures, the sources of unwanted reflections and propagation modes are identified, and prevented by design. The integration of microfluidic systems with on-chip waveguides presents several interesting challenges. Intimate contact between the waveguide and analyte allows for sensitive measurement of the sample properties, yet the electronic circuitry required to generate and detect a probing terahertz field must be isolated from the risk of a short-circuit presented by the potentially conductive liquid. A device structure is proposed that simultaneously overcomes these design limitations, and comprises a geometry that can be accurately modelled. Given the lack of analytical models with which the planar Goubau line can be described, numerical modelling techniques are used to create an accurate simulation of the device structure. A method is then introduced that allows interpretation of experimental data, such that the complex permittivity of unknown liquid samples can be calculated. This new technique is used to measure the complex permittivity of a selection of well-studied polar alcohols, and the results are found to compare well to those available in literature. A free-space terahertz spectroscopy system is then used to measure liquid samples that have not been published in order to verify the results of the on-chip spectrometer when used to measure a wider range of liquid samples.
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Brother, Louis Reginald. "Terahertz optical frequency comb generation." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10746.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (leaves 191-195).
by Louis Reginald Brothers, Jr.
Ph.D.
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Ashworth, Philip Carr. "Biomedical application of terahertz technology." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609421.
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Salissou, Yacoubou. "Détecteurs de radiation terahertz pulsée." Mémoire, Université de Sherbrooke, 2005. http://savoirs.usherbrooke.ca/handle/11143/4700.
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Dans ce mémoire, nous présentons une étude portant sur la détection de rayonnement terahertz (THz) à l'aide d'antennes photoconductrices et par électroabsorption dans les structures à puits quantiques. Nous présentons tout d'abord les résultats d'une étude exploratoire sur la technique de détection basée sur l'électroabsorption par effet Stark quantique. Dans cette étude, nous examinons le potentiel d'utilisation de cette technique et dégageons le type de structure à puits quantiques le plus adapté à la détection de rayonnement terahertz. Par la suite, nous présentons les résultats d'une étude de caractérisation d'une série d'antennes photoconductrices utilisées comme détecteur de rayonnement THz. Ces antennes ont été fabriquées sur substrat de GaAs semi isolant implanté. Le principe de la mesure consiste à utiliser le champ électromagnétique THz pour déplacer les photoporteurs générés par une impulsion optique sonde, et ainsi induire un photocourant qui dépend de l'amplitude instantanée de ce champ THz. Afin d'optimiser les performances de détection en termes de résolution temporelle et de sensibilité, il est important de réduire le temps de vie des photoporteurs tout en préservant une bonne mobilité électronique. La recherche du meilleur compromis nous a conduit à réaliser des études de l'effet des différents paramètres d'implantation (espèce chimique, dose et profondeur d'implantation). L'étude de l'effet de la forme géométrique de la région photosensible sur la sensibilité de détection sera également présentée. Finalement, des simulations de l'effet de la durée de vie des porteurs sur la largeur de bande de détection sont également présentées.
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Pan, Mingming. "Terahertz wave-guided reflectometry system." Thesis, Bordeaux, 2020. http://www.theses.fr/2020BORD0062.
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Ce travail vise à construire un système compact et facile à mettre en œuvre de réflectométrie guidée par ondes térahertz (TGR) en tirant parti de l'émetteur-récepteur térahertz et des guides d'ondes pour diverses applications. Par rapport aux systèmes conventionnels utilisant une méthode quasi-optique, le nouveau concept a une configuration beaucoup plus simple et permet des applications de sondage à distance.Après des études sur le développement de la technologie térahertz, un émetteur-récepteur à double ACP à pompage optique et deux émetteurs-récepteurs radar à ondes continues modulées en fréquence (FMCW) ainsi que des guides d'ondes à parois minces à noyau creux sont sélectionnés pour mettre en œuvre le premier système TGR en mode impulsionnel et en mode FMCW. Les deux expériences et la simulation 3D pleine onde sont exploitées pour étudier les comportements de propagation des ondes à l'intérieur du système et pour évaluer les performances du système. Les systèmes TGR créés font l'objet d'une démonstration à des fins d'imagerie et de détection. Grâce à la capacité de guidage du guide d'ondes, ces installations présentent un potentiel dans certaines conditions de mesure difficiles, comme dans un environnement étroit et semi-fermé ou dans le liquide. En particulier, la lentille d'immersion solide insérée à l'extrémité du guide d'ondes s'est avérée être une méthode efficace pour améliorer la capacité d'imagerie du système TGR en mode FMCW, donnant lieu à une résolution de sous-longueur d'onde dans la bande de fréquences autour de 100 GHz.En plus des études sur les systèmes TGR, un système de réflectométrie sur plaquette exploitant des sources à pompage optique est également proposé pour bénéficier de la large bande de fréquences des sources photoniques. Comme première tentative, des sondes RF associées à un émetteur-récepteur à double ACP sont utilisées pour délivrer le signal d'impulsion dans des échantillons sur la tranche et les signaux temporels obtenus sont analysés. D'autres propositions sont faites pour pousser plus loin cette étudeThis work aims to build up a compact easily-implemented terahertz wave-guided reflectometry (TGR) system by taking advantage of the terahertz transceiver and waveguides for diverse applications. Compared to conventional systems using a quasi-optical method, the new concept has a much simpler configuration and allows for remote probing applications.After reviews on the development of terahertz technology, an optical-pumped double-PCA transceiver and two frequency-modulated continuous-wave (FMCW) radar transceivers together with hollow-core thin-wall waveguides are selected to implement the first TGR system in pulse mode and in FMCW mode. Both experiments and 3D full-wave simulation are exploited to investigate the propagation behaviors of waves inside the system and to evaluate the system performance. The created TGR systems are demonstrated for imaging and sensing purposes. Thanks to the guiding capacity of the waveguide, these setups show potential in some difficult measurement conditions, such as in a narrow semi-enclosed environment or the liquid. In particular, the solid immersion lens inserted at the end of the waveguide has been proved as an efficient method to enhance the imaging capacity of the TGR system in FMCW mode, giving rise to a subwavelength resolution at the frequency band around 100 GHz.In addition to the studies of TGR systems, an on-wafer reflectometry system exploiting optical-pumped sources is as well proposed to benefit from the wide frequency band of photonic sources. As the first attempt, RF probes in association with double-PCA transceiver are used to deliver the pulse signal into samples on-wafer and the obtained time signals are analyzed. More propositions are given to push further up this study
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Burrow, Joshua Anthony. "4-Gap Asymmetric Terahertz Metasurfaces." University of Dayton / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=dayton150091131322781.
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Shen, Hao. "Compressed sensing on terahertz imaging." Thesis, University of Liverpool, 2012. http://livrepository.liverpool.ac.uk/8457/.
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Most terahertz (THz) time-domain (pulsed) imaging experiments that have been performed by raster scanning the object relative to a focused THz beam require minutes or even hours to acquire a complete image. This slow image acquisition is a major limiting factor for real-time applications. Other systems using focal plane detector arrays can acquire images in real-time, but they are too expensive or are limited by low sensitivity in the THz range. More importantly, such systems cannot provide spectroscopic information of the sample. To develop faster and more efficient THz time-domain (pulsed) imaging systems, this research used random projection approach to reconstruct THz images from the synthetic and real-world THz data based on the concept of compressed/compressive sensing/sampling (CS). Compared with conventional THz time-domain (pulsed) imaging, no raster scanning of the object is required. The simulation results demonstrated that CS has great potential for real-time THz imaging systems because its use can dramatically reduce the number of measurements in such systems. We then implemented two different CS-THz systems based on the random projection method. One is a compressive THz time-domain (pulsed) spectroscopic imaging system using a set of independent optimized masks. A single-point THz detector, together with a set of 40 optimized two-dimensional binary masks, was used to measure the THz waveforms transmitted through a sample. THz time- and frequency-domain images of the sample comprising 20×20 pixels were subsequently reconstructed. This demonstrated that both the spatial distribution and the spectral characteristics of a sample can be obtained by this means. Compared with conventional THz time-domain (pulsed) imaging, ten times fewer THz spectra need to be taken. In order to further speed up the image acquisition and reconstruction process, another hardware implementation - a single rotating mask (i.e., the spinning disk) with random binary patterns - was utilized to spatially modulate a collimated THz. After propagating through the sample, the THz beam was measured using a single detector, and a THz image was subsequently reconstructed using the CS approach. This demonstrated that a 32×32 pixel image could be obtained from 160 to 240 measurements. This spinning disk configuration allows the use of an electric motor to rotate the spinning disk, thus enabling the experiment to be performed automatically and continuously. To the best of our knowledge, this is the first experimental implementation of a spinning disk configuration for high speed compressive image acquisition. A three-dimensional (3D) joint reconstruction approach was developed to reconstruct THz images from random/incomplete subsets of THz data. Such a random sampling method provides a fast THz imaging acquisition and also simplifies the current THz imaging hardware implementation. The core idea is extended in image inpainting to the case of 3D data. Our main objective is to exploit both spatial and spectral/temporal information for recovering the missing samples. It has been shown that this approach has superiority over the case where the spectral/temporal images are treated independently. We first proposed to learn a spatio-spectral/temporal dictionary from a subset of available training data. Using this dictionary, the THz images can then be jointly recovered from an incomplete set of observations. The simulation results using the measured THz image data confirm that this 3D joint reconstruction approach also provides a significant improvement over the existing THz imaging methods.
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Lucyszyn, Stepan. "Millimetre-wave and terahertz electronics." Thesis, Imperial College London, 2009. http://hdl.handle.net/10044/1/6974.
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Overview: The basic thesis for the advancement of millimetre-wave and terahertz electronics is represented in four sections: Signal Processing, Component Design and Realization, Modelling and Materials, and Paradigm Shift. The first section is at system and circuit levels and reports on complex signal process functions that have been performed directly on the millimetre-wave carrier signal, intended for realizing low-cost and adaptive communications and radar systems architectures. The second section is at circuit and component levels and reports on techniques for the design and realization of low-loss passives for use at millimetrewave frequencies. The third section is at component and material levels and reports on modelling techniques for passives for use at both millimetre-wave and terahertz frequencies. Finally, the fourth section introduces a revolutionary new technology that represents a paradigm shift in the way millimetre-wave and terahertz electronics (i.e. components, circuits and systems) can be implemented. As found with the new generation of mobile phone handsets, a fusion of two extreme technologies can take place; here, complex signal processing operations could be performed both directly on the carrier signal and with the use of a spatial light modulator. Based on a selection of 20 papers (co-)authored by the candidate †b, and published over a period of 15 years, it will be seen that a coherent theme runs throughout this body of work, for the advancement of knowledge in millimetre-wave and terahertz electronics.
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Salissou, Yacoubou. "Détecteurs de radiation terahertz pulsée." [S.l. : s.n.], 2005.
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Russell, Christopher. "Broadband on-chip terahertz spectroscopy." Thesis, University of Leeds, 2013. http://etheses.whiterose.ac.uk/5911/.
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An enhancement of on-chip terahertz time domain spectroscopy (THz-TDS) systems for the analysis of polycrystalline materials has been made. An in depth review of planar Goubau lines is presented for which there are no analytically defined terms for transmission line parameters such as characteristic impedance and effective permittivity. Using a simulation package, Ansoft HFSS, the transmission line is optimised for spectroscopy applications and the bandwidth enhanced using a variety of methods. The theory for calculating the effective permittivity of the transmission line is derived based on a two dimensional interpretation of electromagnetic field patterns and cross-section transmission line geometry. The resulting formulae have a significant impact on both bandwidth and resonant filter designs. The excitation of the planar Goubau line’s quasi transverse magnetic mode, which is typically excited using coplanar to planar Goubau line transition, has been modified in favour of a novel all-planar Goubau line on-chip spectroscopy system utilising photoconductive generation and detection methodologies. In doing so, the frequency resolution of the system is heightened enabling a narrow line width system to be resolved. The planar Goubau lines are fabricated on a quartz substrate with epitaxial transferred low-temperature-grown GaAs based photoconductive switches for both THz generation and detection. The bandwidth of the planar Goubau line is enhanced using a substrate thinning methodology to 2 THz for a 1-mm-long planar Goubau line. Using the enhanced bandwidth, THz-TDS spectroscopy using a planar Goubau line is demonstrated for the first time, where spectra of polycrystalline lactose monohydrate is obtained with a 3.75 GHz frequency-resolution over variable temperature range (4 – 298 K). The THz-TDS spectra are compared with spectra found using alternative THz spectroscopy systems to highlight the improved benefits of using this device. The theoretical development of a narrow bandstop filter design is presented, with the analytical terms defined. This novel filter enables the dielectric sensing of overlaid materials at a multiple of predefined frequencies to be pushed away from the transmission lines, which would otherwise reduce the bandwidth of the system. Results presented in this thesis present a strong candidacy for planar Goubau lines to be utilised in a broad range of applications which hold information in the THz regime.
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Kloosterman, Jenna Lynn. "Heterodyne Arrays for Terahertz Astronomy." Diss., The University of Arizona, 2014. http://hdl.handle.net/10150/319878.
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The clouds of gas and dust that constitute the Interstellar Medium (ISM) within the Milky Way and other galaxies can be studied through the spectral lines of the atoms and molecules. The ISM follows a lifecycle in which each of its phases can be traced through spectral lines in the Terahertz (THz) portion of the electromagnetic spectrum, loosely defined as 0.3 - 3 THz. Using the high spectral resolution afforded by heterodyne instruments, astronomers can potentially disentangle the large-scale structure and kinematics within these clouds. In order to study the ISM over large size scales, large format THz heterodyne arrays are needed. The research presented in this dissertation focuses on the development of two heterodyne array receiver systems for ISM studies, SuperCam and a Super-THz (>3 THz) receiver. SuperCam is a 64-pixel heterodyne imaging array designed for use on ground-based submillimeter telescopes to observe the astrophysically important CO J=3-2 emission line at 345 GHz. The SuperCam focal plane stacks eight, 1x8 mixer subarrays. Each pixel in the array has its own integrated superconductor-insulator-superconductor (SIS) mixer and Low Noise Amplifier (LNA). In spring 2012, SuperCam was installed on the University of Arizona Submillimeter Telescope (SMT) for its first engineering run with 32 active pixels. A second observing run in May 2013 had 52 active pixels. With the outliers removed, the median double sideband receiver temperature was 104 K. The Super-THz receiver is designed to observe the astrophysically important neutral atomic oxygen line at 4.7448 THz. The local oscillator is a third-order distributed feedback Quantum Cascade Laser operating in continuous wave mode at 4.741 THz. A quasi-optical hot electron bolometer is used as the mixer. We record a double sideband receiver noise temperature of 815 K, which is ~7 times the quantum noise limit and an Allan variance time of 15 seconds at an effective noise fluctuation bandwidth of 18 MHz. Heterodyne performance is confirmed by measuring a methanol line spectrum. By combining knowledge of large array formats from SuperCam and quasi-optical mixers, initial tests and designs are presented to expand the single pixel 4.7 THz receiver into a quasi-optical 16-pixel array.
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Otter, William James. "Technologies for terahertz frequency sensing." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/46159.
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Electromagnetic sensors provide information about the environment around us. Historically, THz has been used for astronomy and other scientific insturments. Within this thesis, the aim is to investigate a variety of technologies that have the potential to bring THz technology to a wider market, by creating either low cost devices or technologies that can been monolithically integrated. Chapter 1 provides an introduction to THz and definitive material equations that underpin the simulations undertaken throughout the thesis. This is necessary as THz engineers and scientists come from a broad range of backgrounds where different definitions are deemed standard. Chapter 2 looks at the use of plasmonic devices for THz. Initatilly, the proposed spoof plasmon structure is investigated as a benchmark for simulation comparison and, secondly, the use of semiconductor surfaces is studied to create frequency tuneable sensors with highly confined fields. Chapter 3 moves towards more conventional quasi-optical metal mesh filters for low cost manufacture, comprising a single ultra thin metallic layer on a thick substrate. The chapter concludes with the initial design and simulations of a THz stress sensor based on the metal mesh filter. Chapter 4 looks into the use of photonic crystal technology. Several state-of-the-art devices are demonstrated: resonators, switches and attenuators. These devices have the potential to provide the building blocks for a future monolithically integrated THz architecture.
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Liu, L. "Compressed sensing on terahertz imaging." Thesis, University of Liverpool, 2017. http://livrepository.liverpool.ac.uk/3008850/.
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Paulillo, Bruno. "Circuit-tunable subwavelength terahertz devices." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS130/document.
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La demande croissante en composants optoélectroniques de taille réduite, rapides, de faible puissance et à faible coût oriente la recherche vers des sources et détecteurs de radiation ayant une dimension inférieure à la longueur d'onde émise/détectée. Cette dernière est entravée par la limite de diffraction qui fixe la dimension minimale des dispositifs optiques à la moitié de la longueur d'onde de fonctionnement. A l'inverse, les dispositifs électroniques, tels que les antennes et les oscillateurs, ne sont pas limitée en taille et leur fréquence peut être accordée par des composants discrets. Par conséquent, unifier les mondes de la photonique et de l'électronique permettrait de concevoir de nouveaux dispositifs optoélectroniques sans limitation de taille imposée par la longueur d'onde et ayant des fonctionnalités empruntées aux circuits électroniques. La région spectrale idéale pour développer ce paradigme est la gamme térahertz (THz), à mi-chemin entre les domaines de l'électronique et de l'optique. Dans la première partie de ces travaux, nous présentons de nouveaux micro-résonateurs sub-longueur d’onde en 3D qui fonctionnent comme des circuits LC microscopiques et où la fréquence de résonance peut être accordée en agissant séparément sur la région capacitive et/ou inductive. Dans la deuxième partie, nous illustrons la puissance de cette approche en réalisant de nouveaux méta-dispositifs THz passifs (polaritoniques, commutables optiquement, optiquement actifs) basés sur des composants discrets. La dernière partie de cette thèse est consacrée aux méta-dispositifs actifs. Des photodétecteurs THz à puits quantiques ayant une dimension ≈λ eff /10, en configuration objet unique et réseau sont démontrées, grâce à un schéma de contact efficace et originale pour extraire (injecter) un courant depuis (dans) le cœur semi-conducteur intégré dans chaque résonateur. Enfin, une étude de faisabilité d'un laser sub-longueur d’onde aux fréquences THz est présentéeThe need for small, fast, low-power and low-cost optoelectronic components is driving the research towards radiation sources and detectors having a dimension that is smaller than the emitted/detected wavelength. This is hampered by the optical diffraction limit which constrains the minimum dimension of optical devices at half the operating wavelength. Conversely, electronic devices, such as antennas and oscillating circuits, are not diffraction-limited in size and can be frequency tuned with lumped components. Hence, blending the worlds of photonics and electronics has the potential to enable novel optoelectronic devices with no lower size limit imposed by the wavelength, and with novel functionalities borrowed from electronic circuits. The ideal spectral region to develop this paradigm is the terahertz (THz) range, halfway between the electronics and optics realms. In the first part of this work, we present novel subwavelength 3D micro-resonators that behave as microscopic LC circuits, where the resonant frequency can be tuned acting separately on the capacitive and/or inductive regions. In the second part we illustrate the power of this concept by implementing novel lumped-elements-based passive THz meta-devices (polaritonic, optically switchable, optically active). The last part of this thesis is devoted to active meta-devices. Single-pixel and arrays of THz quantum well photodetectors featuring a ≈λeff/10 dimension are demonstrated, thanks also to an effective and original contact scheme to extract (inject) current from (into) the semiconductor core embedded by each resonator. Finally, a feasibility study of a subwavelength laser at THz frequencies is reported
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Duhant, Alexandre. "Contrôle non destructif par reconstruction en tomographie térahertz." Thesis, Montpellier, 2019. http://www.theses.fr/2019MONTS006/document.
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La tomographie et ses algorithmes associés sont désormais bien connus dans le domaine des rayons X. En revanche tous ces outils s’appuient sur une modélisation qui diffère de celle qui pourrait être envisagée dans le domaine des ondes Térahertz (THz). On retrouve, dans l’état de l’art, des modèles de propagation de l’onde THz au sein d’un objet. Ces modèles génèrent une onde THz qui est soit éloigné d’une vérité terrain, soit d’une complexité algorithmique trop élevée pour être utilisée au sein d’une reconstruction tomographique dans des temps de calcul acceptables. Un des objectifs de ce travail de thèse est donc d’obtenir un modèle de propagation de l’onde THz permettant une meilleure modélisation du processus d’acquisition et pouvant être calculé dans des temps relativement courts. Lors de la mesure d’une projection d’un objet, le phénomène d’absorption n’est pas le seul phénomène responsable de l’atténuation de l’onde THz. Les phénomènes de réfraction et de réflexion sont aussi responsables d’une atténuation de l’onde THz mesurée. Lors d’une reconstruction tomographique THz, si ces phénomènes ne sont pas pris en compte, l’algorithme attribue cette atténuation au phénomène d’absorption. Cela se traduit par une reconstruction des coefficients d’absorption de l’objet éloignée de leur valeur réelle. Sous l’effet de ces phénomènes, le problème de la reconstruction tomographique THz est non linéaire. Cela empêche ainsi l’utilisation directe des méthodes de reconstruction classiques puisque ces méthodes impliquent que la relation liant un objet à ses projections soit linéaireTomography and its associated algorithms are now well known in the field of X-rays. On the other hand, all these tools are based on a modeling that differs from which could be envisaged in the field of Terahertz (THz) waves. We find, in the state of the art, models of propagation of the THz wave within an object. These models generate a THz wave that is either far from a ground truth, or of an algorithmic complexity that is too high to be used within a tomographic reconstruction in acceptable computing times. One of the objectives of this thesis work is therefore to obtain a propagation model of the THz wave allowing better modeling of the acquisition process and which can be calculated in relatively short times. When measuring the projection of an object, the absorption phenomenon is not the only phenomenon responsible for the attenuation of the THz wave. The phenomena of refraction and reflection are also responsible for attenuation of the measured THz wave. During a THz tomographic reconstruction, if these phenomena are not taken into account, the algorithm attributes this attenuation to the absorption phenomenon. This results in a reconstruction of the absorption coefficients of the object far from their real value. Under the effect of these phenomena, the problem of THz tomographic reconstruction is non-linear. This prevents the direct use of classical reconstruction methods since these methods imply that the relationship between an object and its projections is linear
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Holder, Gareth Michael. "Commissioning of an intense broad band source of terahertz radiation and terahertz exposure of biological molecules." Thesis, University of Liverpool, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.570087.
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This thesis discusses the commissioning of the most intense broadband source of terahertz (THz) radiation in Europe, at ALICE (Accelerators and Lasers In Combined Experiments), and the first to be equipped with a Tissue Culture Facility (TCF). The commissioning of the THz transport system is discussed in a chronological manner with experimental results compared with theoretical models to improve understanding of the ALICE accelerator and transp€H{'tl1e THz radiation to the TCF. This was first achieved in March 2011. The studies explain some of the complex issues that had to be resolved and the development of a new front end extraction system to maximize the transport efficiency of the beam line. The THz beam line at ALICE has the huge potential to test the long debated and very controversial Frohlich hypothesis concemmg the mechanism of biological organisation. ALICE's time structure results in high peak power, low average power THz radiation. One of the initial studies on cells performed at the ALICE THz beam line is discussed with a number of subsidiary studies performed to clarify the results. A study of THz exposure of Zebra fish embryos provides very interesting results, however at present, it is not possible to state whether the THz radiation has an effect on the growth of Zebra fish embryos due to a number of complications, which are all discussed. The results suggest a possible new hypothesis, where the effects of THz radiation are dependent on whether a biological system enters an arrested state. In this state, the embryo has no ability to fix any damages caused by the THz radiation. The final section of this thesis covers a study of conformational changes of a biological sugar, heparin. Heparin is a structural analogue for Heparan Sulfate which is the most abundant molecule found on a cell surface and plays a crucial role in all reactions and general structure of the extra cellular matrix (ECM). The Reflection Anisotropy Spectroscopy studies suggest that some of the cation forms of heparin dry into a film with an anisotropic direction. The THz radiation results, although not thoroughly understood suggest that THz can distinguish between heparin cation forms due to their differences in conformation. This technique has the long term iv -, potential to monitor conformational change and could contribute to the understanding their reaction with other ECM molecules. v