Le sujet de la thèse a porté sur la mise au point, la caractérisation et l'utilisation de matériaux semi-conducteurs, au sein desquels les porteurs libres ont un temps de vie extrêmement brefs (picoseconde ou sub-picoseconde), pour réaliser des antennes photoconductrices émettrices ou détectrices de rayonnement électromagnétique térahertz (THz). Contrairement au semi-conducteur LTG-GaAs (low temperature grown GaAs) à la technologie bien dominée et aux performances exceptionnelles lorsque photo-excité par des impulsions lasers de longueurs d'onde typiquement inférieures à 0,8 µm, le travail portait ici sur des matériaux permettant l'emploi de lasers dont les longueurs d'onde sont celles des télécommunications optiques, à savoir aux alentours de 1,5 µm. L'intérêt est de bénéficier de la technologie mature de ces lasers, et du coût relativement modique des composants pour les télécommunications optiques. Pour réaliser des antennes THz performantes et efficaces, le matériau semi-conducteur doit présenter plusieurs qualités : vie des porteurs libres très courte, grande mobilité des porteurs, haute résistivité hors éclairement, et bonne structure cristallographique pour éviter les claquages électriques. Pour obtenir une courte durée de vie, on introduit un grand nombre de pièges dans le semi-conducteur, qui capturent efficacement les électrons libres. Pour les matériaux de type InGaAs employés à 1,5 µm, le problème est que le niveau en énergie de ces pièges, par exemple pour les matériaux épitaxiés à basse température, est très proche de la bande de conduction du semi-conducteur. Cela est équivalent à un dopage n du matériau, ce qui en diminue fortement sa résistivité hors éclairement. Plusieurs solutions ont été apportées par différents laboratoires : compensation par dopage p pour les matériaux épitaxiés à basse température, bombardement ionique, implantation ionique, ou même structures à couches alternées où la photo-génération et la recombinaison des porteurs libres se produisent à des endroits différents. Le but du travail de thèse était de fabriquer des matériaux préparés suivant ces différentes techniques, de les caractériser et de comparer leurs performances pour l'optoélectronique THz. Les semi-conducteurs à étudier étaient de type InGaAs comme déjà publiés par la concurrence, l'originalité de thèse portant sur la comparaison de ces différents matériaux et si possible leur optimisation,. Au cours de ce travail de thèse, de nombreuses couches d'InGaAs ont été épitaxiées, en faisant varier les paramètres de dépôt, et des antennes THz ont été fabriquées. Les couches ont été caractérisées du point de vue cristallographique, ainsi que pour la conductivité électrique DC (mesures 4 pointes, mobilité Hall...), les propriétés d'absorption optique (spectroscopie visible et IR), la durée de vie des porteurs par mesure optique pompe-sonde. Pour les couches épitaxiées à basse température, l'influence d'un recuit thermique ainsi que du dopage en béryllium ont été étudiés. Dans le cas de couches bombardées ou implantées, plusieurs ions ont été utilisés, le brome, le fer et l'hydrogène. Les relations entre la cartographie des défauts structuraux et/ou des ions implantés et les propriétés électriques et de dynamique des porteurs ont été examinées en détail. Ces études permettent de comprendre le type de défauts qui piègent les porteurs dans ces matériaux, ainsi que leur formation lors du processus de fabrication et de traitement des couches. Finalement les meilleures couches fabriquées présentent des performances comparables à celles publiées par ailleurs. Les derniers travaux de thèse ont permis d'obtenir les premiers signaux de rayonnement THz générés par une antenne fabriquée avec l'InGaAs optimisé.

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

Terahertz radiation or T-rays or THz radiation refers to the region of the electromagnetic spectrum between approximately 100 GHz and 30 THz. This spectral region is often referred to as the THz gap as these frequencies fall between electronic (measurement of field with antennas) and optical (measurement of power with optical detectors) means of generation. THz measurements may yield useful information about the structural and chemical nature of the material inspected. Examples include detection of voids in materials and protein binding in biomolecules. This report provides an overview of THz measurements of defects in fiber composites. We find that it efficiently detects defects such as voids and delamination in glass fiber composites better than ultrasound, which was widely used for defect characterization in glass fiber earlier. Comparison of the existing methods with THz is presented in the report for characterization of defects.

In this thesis, intrinsic Josephson junctions, naturally formed in the strongly anisotropic high-temperature superconductor Bi2Sr2CaCu2O8 (Bi-2212), are studied experimentally. For this purpose, small mesa structures are fabricated on the surface of single crystals using micro- and nano-fabrication tools, focused ion beam is used to reduce the area of the mesa-structures down to ≈ 1 × 1 μm2. The properties of charge transport across copper-oxide layers inside the mesas are studied by intrinsic tunneling spectroscopy. Temperature, bias and magnetic field dependences of current-voltage characteristics are examined. In the main part of the thesis, the behavior of intrinsic Josephson junctions in magnetic fields B parallel to the copper-oxide planes is studied. Parallel magnetic fields penetrate the junctions in the form of Josephson vortices (fluxons). At high magnetic fields, fluxons are arranged in a regular lattice and are accelerated by a sufficient high transport current. As the fluxon lattice is moving through the mesa, it emits electromagnetic waves in the important THz frequency range. Properties of Bi-2212 mesas in this flux-flow regime are studied in this thesis. The following new observations were made during the course of this work: a crossover from thermal activation above Tc to quantum tunneling below Tc is seen in the interlayer transport-mechanism, the Fraunhofer pattern of Ic(B) is observed clearly in Bi-2212, superluminal electromagnetic cavity resonances and phonon-polaritons are observed in Bi-2212. It is argued that the employed technique for miniaturization of mesas and the obtained results can be useful for a better understanding of fundamental properties of high-temperature superconductors and for the realizations of coherent flux-flow oscillators and coherent phonon-polariton generators in the important THz frequency range. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript.

This thesis is concerned with terahertz (THz) technology from both optical and electronic approaches. On the optical front, the investigation of optimised photoconductive switches included the characterisation, fabrication and testing of devices which can generate and detect THz radiation over the frequency range from DC to ~ 2.5 THz. These devices incorporated semiconductor photoconductors grown under low temperature (LT) Molecular Beam Epitaxy (MBE) conditions and using distributed Bragg reflectors (DBRs). The material properties were studied via numerous characterisation techniques which included Hall Effect and mid infrared reflections. Antenna structures were fabricated on the surface of the active layers and pulsed/continuous wave (CW) signal absorbed by these structures (under bias) generates photocurrent. With the help of the DBRs at certain wavelengths (800 nm and 1550 nm), the absorption coefficient at the corresponding illumination wavelength increased thus leading to significant increase of the THz output power while the materials kept the desirable photoconductive material properties such as high dark resistivity and high electron mobility. The inclusion of DBRs resulted in more than doubling of the THz peak signals across the entire operating frequency range and significant improvements in the relative THz power. For the THz electronic approach, a new type of InP-based Asymmetric Spacer Tunnel Diode (ASPAT), which can be used for high frequency detector, was studied. The asymmetric DC characteristics for this novel tunnel diode showed direct compatibility with high frequency zero-bias detector applications. The devices also showed an extreme thermal stability (less than 7.8% current change from 77 K to 400 K) as the main carrier transport mechanism of the ASPAT was tunnelling. Physical models for this ASPAT diode were developed for both DC (direct current) and AC (alternating current) simulations using the TCAD software tool SILVACO. The simulated DC results showed almost perfect matches with measurements across the entire temperature range from 77 K to 400 K. From RF (radio frequency) measurements, the intrinsic diode parameters were extracted and compared with measured data. The simulated zero biased detector circuits operating at 100 GHz and 240 GHz using the new InGaAs-AlAs ASPAT diode (4*4 micrometer square) showed comparable voltage sensitivities to state of the art Schottky barrier diodes (SBDs) detectors but with the added advantage of excellent thermal stability.

Orientador: Jose Alexandre Diniz / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-11T06:36:37Z (GMT). No. of bitstreams: 1 Melo_ArlineMaria_D.pdf: 17701586 bytes, checksum: b5160ad894463745fdeea0dae1f71065 (MD5) Previous issue date: 2007 / Resumo: Este trabalho apresenta importantes informações sobre tecnologia THz, atualmente considerada um desafio tecnológico e estado da arte em diversas áreas de desenvolvimento. Com o objetivo de formar um sistema imageador THz, ainda indisponível comercialmente, foram dispensados esforços em diferentes frentes. Câmeras para o infravermelho médio (8 ¿ 14 µm) foram adquiridas, caracterizadas e testadas, com resultados relevantes e inéditos em Física Solar, aplicação considerada para este trabalho. Filtros de malha ressonante para diversas bandas na região THz (centrados em 0,4, 0,67, 0,85, 2, 4 e 8,5 THz) foram projetados, fabricados e testados exibindo excepcional resposta. E finalmente um primeiro protótipo imageador foi montado acoplando os filtros de malha ressonante a câmera IV sem sua óptica original de germânio. As primeiras imagens THz de uma resistência aquecida é apresentada, provando a funcionalidade do conceito original. Os desenvolvimentos e estudos aqui apresentados estarão contribuindo para dois experimentos espaciais recentemente propostos, DESIR ( Detection of Solar eruptive Infrared Radiation) e SIRA (Submillimeter-wave to InfraRed solar Activity emissions), prevendo a utilização de sistemas THz em observações de transientes em plasmas ativos solares em diferentes freqüências, a partir do solo e no espaço / Abstract: This work presents important information on THz technology, currently considered a technological challenge and state of the art in different areas of development. To form a THz imaging system, still unavailable commercially, efforts in different fronts had been excused. Cameras for medium IR (8 - 14 µm) had been acquired, characterized and tested, with excellent and unknown results in Solar Physics, application considered for this work. Filters of resonant metal mesh for diverse bands in the THz region (centered in 0,4, 0,67, 0,85, 2, 4 and 8,5 THz) had been projected, manufactured and tested showing excellent results. Finally a first imaging system was mounted connecting the filters of resonant mesh to the IR camera without its original optics of germanium. The first THz images of a warm resistance are presented, proving the functionality of the original concept. The developments and studies presented here will contribute for two space experiments recently considered, DESIR ( Solar Detection of eruptive Infrared Radiation) and SIRA ( Submillimeter-wave you the solar InfraRed Activity emissions), requiring the use of THz systems for observations of transient in solar active plasmas in different frequencies, from the ground and in the space / Doutorado / Eletrônica, Microeletrônica e Optoeletrônica / Doutor em Engenharia Elétrica

In this thesis we present a study about strong light-matter interaction in a broad single GaAs/AlGaAs quantum well representing a 3-level system. In particular we investigate the AC-Stark effect, where we observe in THz absorption spectra an Autler-Townes splitting as well as a Mollow-triplet. Compared to previous work, we showed for the first time an all-THz pump-probe experiment in the THz regime below the Reststrahlenband. Furthermore, we observe a strong frequency shift in the absorption energy of the first intersubband transition depending on the charge carrier density in the quantum well. The Autler-Townes splitting as well as the absorption frequency shift can be potentially exploited for THz-modulation applications. Beyond nonlinear optics many interesting effects occur in the strong light-matter interaction regime such as Rabi oscillations, coherent population trapping, lasing without inversion, electromagnetically induced transparency (EIT) and the AC-Stark effect. Our quantum well represents a 3-level system in which we investigate a splitting behaviour in the absorption spectrum of the first and second intersubband transition. Especially a splitting for the first intersubband transition is predicted also for electromagnetically induced transparency, while the second intersubband transition is pumped with a strong varying electric field. Naturally, a fundamental question is, how to distinguish EIT and an Autler-Townes duplet since both result in a spectrally transparent window. The method of choice for investigations combines narrowband pulses in the THz range provided by a free-electron laser and broadband THz pulses generated in a GaP crystl within a THz time-domain spectroscopy setup. In this unique configuration we perform time-resolved pump and probe spectroscopy experiments by pumping resonantly the second intersubband transition at 3.4 THz to induce a splitting of the second and third subband. Broadband THz pulses then probe an absorption splitting of about 0.2 THz related to the first intersubband transition at ≈ 2.3 THz as well as a splitting of the second intersubband transition (Mollow triplet). Analyzing experiments and using a theoretical criteria to distinguish EIT and Autler-Townes splitting, we conclude to observe an Autler-Townes doublet instead of an EIT effect. / In dieser Arbeit berichten wir über die starke Licht-Materie Wechselwirkung in 3-Niveau system anhand eines einzelnen, breiten GaAs/AlGaAs Quantentopfes. Insbesondere untersuchen wir den AC-Stark Effekt und beobachten eine Aufspaltung des Absorptionsspektrums durch das Autler-Townes Dublett und das Mollow Triplett. Im direkten Vergleich mit vorangegangenen Arbeiten zeigen wir zum ersten Mal ein reines THz Anrege-Abfrage Experiment mit Frequenzen unterhalb des Reststrahlenbandes. Weiterhin beobachten wir eine starke Frequenzverschiebung der Absorptionsenergie des ersten Intersubbandübergangs in Abhängigkeit von der Ladungsträgerdichte im Quantentopf. Sowohl das Autler-Townes Dublett als auch die Verschiebung der Absorptionsfrequenz ermöglichen potentielle Anwendung im Bereich der THz-Modulation. Im Bereich der starken Licht-Materie Wechselwirkung sind viele interessante Effekte beobachtbar wie Rabi Oszillationen, coherent population trapping, Lasern ohne Inversion, elektromagnetisch induzierte Transparenz (EIT) und der AC-Stark Effekt. Unser Quantentopf stellt ein 3-Niveau System dar, in welchem wir eine Aufspaltung der Absorption bezüglich des ersten und zweiten Intersubbandübergangs beobachten. Insbesondere für den ersten Intersubbandübergang ist auch eine Absorptionsaufspaltung durch den EIT Effekt vorhergesagt, während der zweite Intersubbandübergang durch ein starkes, elektrisches Wechselfeld angeregt wird. Es stellt sich dann die Frage, wodurch sich die Effekte EIT und Autler-Townes splitting unterscheiden, weil beide durch ein spektrales transparentes Fenster gekennzeichnet sind. Die von uns gewählte Methode verknüpft schmalbandige, starke elecktrische Wechselfelder im THz-Bereich eines freien Elektronen Lasers und breitbandigen THz-Pulsen, welche durch nichtlineare optische Effekte in einem THz Zeit-Bereichs Spektroskopie Aufbaus erzeugt werden. In dieser einzigartigen Konfiguration führen wir zeitaufgelöste Anrege-Abfrage Spektroskopie Experimente durch, in dem wir den zweiten Intersubbandübergang bei 3, 4 THz nahezu resonant anregen und das zweite und dritte Subband aufspalten. Mit breitbandigen THz Pulsen fragen wir dann die Absorptionsaufspaltung von ca. 0, 2 THz des ersten Intersubbandübergangs bei ≈ 2, 3 THz und des zweiten Intersubbandübergangs (Mollow-Triplett) ab. Nach Auswerten der Experimente und theoretischer Kriterien für die Unterscheidung zwischen EIT und Autler-Townes splitting schlussfolgern wir, ein Autler-Townes Dublett zu beobachten.

As the quest for higher data rates continues, future generations of wireless communications are expected to concur even higher frequency bands, particularly at terahertz (THz) frequencies. Even though the vast bandwidths at the THz band promise terabit-per-second (Tbps) data rates, current baseband technologies do not support such high rates. In particular, the complexities of Tbps channel code decoding and ultra-massive multiple-input multiple-output data detection are prohibitive. This work addresses the efficient data detection and channel-code decoding problem under THz-band channel conditions and Tbps baseband processing limitations. We propose ultra-massive multiple-input multiple-output THz channel models, then investigate the corresponding performance of several candidate data detection and coding schemes. We further investigate the complexity of different detectors and decoders, motivating parallelizability at both levels. We recommend which detector to combine best with which channel code decoder under specific THz channel characteristics.

Spintronics is a spin-electronic field where the electron spinangular momentum, in conjunction with charge, is used to read andwrite information in magnetic sensors and logic circuits, e.g. hard disk drive (HDD), magnetic random access memory (MRAM) and broadband TeraHertz (THz) emitters. To realize the THz operations of the spin logic circuits THz manipulation of the magnetic state is pivotal. This THz manipulation of the magnetic state in anti-ferromagnetic magnetic materials can be realized by coupling the materials with THz antennas. On the other hand, these antennas enhance the THz amplitude of spin-electronic THz emitters when coupled with its output. Therefore, these THz antennas can not only be coupled with the input of magnetic logics to improve the efficiency of magnetic sate manipulation in logic devices but also with the output of the spintronic THz emitters to enhance the generated THz signal amplitude. In this project, we have examined four types of antennas: h-dipole, spiral, bow-tie, and a sub-THz antenna. All the antennas are placed on top of a MgO substrate material for simplicity. However, a bow-tie antenna is also fabricated on an antiferromagnetic substrate of TmFeO3 to check this antenna’s reliability to manipulate its magnetic state. We have studied the impact of antenna geometries on the generated electric field amplitude. We have optimized each antenna for maximum electric field norm profile, with an increase of 30% for the h-dipole and spiral antennas, and an increase of 100% for the bow-tie antenna. However, in this project we were not able to find any general conclusions about what geometrical parameters can further amplify the generated electric field. None of the antennas generated a large enough peak-to-peak electric field amplitude to manipulate the magnetic state of anti-ferromagnetic materials. However, they did successfully amplify the spintronic THz emitter output and could certainly be useful in that regard.

Dynamic Spectrum Access (DSA), also known as Dynamic Spectrum Management, is the method of utilizing a set of spectrum techniques in real time to provide the ability to share wireless channels between Primary (or licensed) users (PUs) and Secondary (or unlicensed) users (SUs). The system is so designed that under normal circumstances, the PUs always get priority, but DSA enables the SUs to use the licensed bands as long as they do not create any interference on the PUs. Hence, the goal of utilizing the spectrum more efficiently can be achieved. Though DSA has been researched extensively as a new concept, it is still under development and several challenges remain unsolved. DSA is recognized as a vital component in 5G-and-beyond network deployment scenarios. Although 5G networks can work in sub-6GHz bands, higher frequency bands (like 28 GHz and 60 GHz) are particularly of interest as they offer much larger bandwidth and regulatory agencies have been announcing licensing plans for these emerging bands. These higher frequency bands could enable extremely high-speed wireless communication by leveraging the gains of highly directional antennas. Smart devices used worldwide has already surpassed 22 billion and is only going to increase in the coming years. Channel allocation and high-speed communication will be the backbone to drive this enormous network of devices, and DSA and directional antenna communication mechanisms will be the key factors governing the future communication infrastructure. In this dissertation, we show how omnidirectional DSA techniques can be applied towards directional cases, i.e., replacing the omnidirectional antennas with directional antennas working in the millimeter wave (mmWave) bands. MmWave enables ultra-high speed transmission and reception, but with some caveats; these antennas should be deployed in line-of-sight (LOS) and a lot of transmission and reception properties depend on how the antennas are aligned, their steering angle, beamwidth and field-of-view (FOV). It is a challenge to take into consideration all of these factors and come up with a solution of ideal signal-to-interference-plus-noise-ratio (SINR) combination between a set of transmitters and receivers. This dissertation sets a guideline on how small cell mmWave transmitters and receivers can be deployed in a densely populated area by working in a coalition (such as by smartly allocating channels to coalitions with more users). Mobility and varying orientations of mmWave as part of dynamic coalitions present new challenges we undertake. Hence, an area where this research can be very apt is vehicular networks, leveraging the high-speed communication provided by mmWave networks. Since the nodes in this case, the vehicles, will be primarily in motion, our research can be applied especially, because we are investigating the antenna designs by considering their beamwidths, steering angles power budgeting.