Ce travail a été consacré à l'étude de composants optoélectroniques ultra-rapides pour le traitement de signaux RF jusqu'au domaine THz, ainsi qu'à l'étude de composants pour les faisceaux THz. Tout d'abord, le travail a porté sur des photo-commutateurs optoélectroniques fabriqués avec des semi-conducteurs ultrarapides. Le dispositif, éclairé par le battement de 2 faisceaux optiques et polarisé par une tension RF, réalise le mélange de ces fréquences. Les propriétés du dispositif (bande passante, efficacité, génération de fréquences…) ont été modélisées et les simulations ont été validées par des mesures expérimentales. Quand le signal RF est modulé par un signal « information », cette information peut être directement extraite en égalant fréquences RF et de battement optique. Le signal démodulé est très pur : ainsi nous avons mesuré une largeur spectrale à -3 dB de 11 Hz. D'autres matériaux pour la génération THz et la photo-commutation ultrarapide furent aussi étudiés, comme des boîtes quantiques en InAs. Enfin, nous avons réalisé une étude par spectroscopie THz dans le domaine temporel des propriétés de dispositifs métalliques sous-longueur d'onde pour la manipulation de faisceaux THz, comme des réseaux de trous dans une plaque métallique ou de filtres à grille, ainsi que de films nanométriques de graphène ou de nanotubes de carbone. / This PhD work was devoted to the study of ultrafast optoelectronic components for processing RF signals up to the THz range, and of related THz devices. First, we used a photoconductive switch, made of low-temperature grown GaAs, excited by the optical beating of two CW laser beams and biased by a RF signal. The switch serves as a frequency mixer, whose properties (bandwidth, efficiency, sideband generation…) were modeled and the simulation results were experimentally validated. When the RF signal is modulated by information, this information can be directly extracted by setting the beating frequency equal to the RF one. The demodulated signal exhibits a high spectral purity, 11 Hz bandwidth at -3 dB. Other materials for THz generation and fast photo-switching were also studied, like InAs quantum dots. Finally, we performed a THz time-domain spectroscopy study of metallic sub-wavelength devices for THz beam processing, like hole arrays and metallic mesh filters, as well as nanometric thin films of graphene and carbon nanotubes.

This thesis presents the research made on signal and data processing for THz imaging, with emphasis in noise analysis and tomography in amplitude contrast using a THz time-domain spectrometry system. A THz computerized tomography system was built, tested and characterized. The system is controlled from a personal computer using a program developed ad hoc. Detail is given on the operating principles of the system’s numerous optical and THz components, the design of a computer-based fast lock-in amplifier, the proposal of a local apodization method for reducing spurious oscillations in a THz spectrum, and the use of a parabolic interpolation of integrated signals as a method for estimating THz pulse delay. It is shown that our system can achieve a signal-to-noise ratio of 60 dB in spectrometry tests and 47 dB in tomography tests. Styrofoam phantoms of different shapes and up to 50x60 mm is size are used for analysis. Tomographic images are reconstructed at different frequencies from 0.2 THz to 2.5 THz, showing that volume scattering and edge contrast increase with wavelength. Evidence is given that refractive losses and surface scattering are responsible of high edge contrast in THz tomography images reconstructed in amplitude contrast. A modified Rayleigh roughness factor is proposed to model surface transmission scattering. It is also shown that volume scattering can be modelled by the material’s attenuation coefficient. The use of 4 mm apertures as spatial filters is compared against full beam imaging, and the limitations of Raleigh range are also addressed. It was estimated that for some frequencies between 0.5 THz and 1 THz the Rayleigh range is enough for the tested phantoms. Results on the influence of attenuation and scattering at different THz frequencies can be applied to the development of THz CW imaging systems and as a point of departure for the development of more complex scattering models.

Orientador: Newton Cesário Frateschi / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T11:19:53Z (GMT). No. of bitstreams: 1 Jarschel_PauloFelipe_M.pdf: 3096320 bytes, checksum: f09830dd682d577e43058480d256c5a1 (MD5) Previous issue date: 2009 / Resumo: A radiação eletromagnética em Terahertz (THz) situa-se na faixa de 1012 Hz, com comprimentos de onda associados variando de 30 µm até 3 mm. É possível então usar esta radiação para investigar propriedades físicas de materiais que requerem uma definição desta ordem. Além de aplicações na espectroscopia, estes comprimentos de onda são capazes de penetrar em papel, tecidos, pele e até neblina atmosférica, gerando um grande interesse em sistemas de segurança, medicina e aviação 1. Neste trabalho será apresentado o desenvolvimento de emissores de Terahertz baseados em SI-GaAs (GaAs Semi-Isolante), incluindo uma discussão detalhada da teoria desta emissão, descrição dos processos de fabricação e resultados obtidos de várias amostras. Esta antena consiste em eletrodos interdigitados depositados no topo de um substrato, de forma similar a fotodetectores MSM 2. A principal diferença entre estes dois dispositivos é que no emissor é feita uma segunda metalização,"opaca " e acima da anterior, separada por uma camada isolante de Si3N4. O princípio físico básico envolvido neste dispositivo é a emissão de radiação por cargas aceleradas. A idéia é que pares elétron-buraco sejam gerados por um laser de femtossegundo incidente na amostra, que se movem rapidamente entre os eletrodos, devido à tensão aplicada. Para obter a máxima eficiência de absorção, a camada isolante também serve como anti-refletora para o laser. A segunda metalização possui um papel essencial, pois ela garante que todos os portadores são acelerados no mesmo sentido, possibilitando então a interferência construtiva no campo distante (Far-Field) 3. Considerando a grande dificuldade de obtenção de lasers com pulsos de femtossegundos no período deste trabalho, utilizamos nosso dispositivo para a geração de ondas na faixa de MHz a partir de um laser pulsado eletronicamente, para verificação do princípio. Muito boa concordância entre nossa simulação e as medidas foi obtida. No entanto, deve-se observar que as propriedades da onda gerada neste caso são mais dependentes do pulso óptico em si do que da velocidade do dispositivo. De toda forma, o resultado mostra que o princípio de geração de ondas de rádio a partir de pulsos ópticos foi demonstrado com sucesso com nosso dispositivo. / Abstract: Terahertz electromagnetic radiation (THz) is situated around 1012 Hz in the electromagnetic wave spectrum, with associated wavelengths varying from 30 µm to 3 mm. It is possible then to use this radiation to investigate physical properties of materials that requires a definition of this order. Besides spectroscopy applications, these wavelengths are capable of penetrating deep into paper, skin, clothes, and even atmospheric fog, generating a great interest in using it for security systems, medicine and aviation¹. In this work, the development of Terahertz emitters based on photoconductive SI-GaAs (Semi-Insulator GaAs) will be presented, including a detailed discussion on the theory of this emission, description of the fabrication processes and results obtained from various samples. This antenna consists on interdigitated finger electrodes deposited on the top of a substrate, similar to MSM Photodetectors². The main difference between these two devices is that on our emitter,a second "opaque " metallization is done on top of the previous,separated by a Si3N4 dielectric layer. The basic physical concept involved in this device is radiation emission from accelerated charges. The idea is to generate electron-hole pairs by an incident femtosecond laser, which rapidly move between the electrodes, due to the bias voltage applied. To have maximum absorption efficiency, the insulating layer also serves as an anti-reflective coating for the pump laser. The second metallization plays an essential role, for it ensures that all carriers are accelerated in the same direction, thus allowing constructive interference on the far-field³. Giving the unavailability of a femtosecond laser for this work, we have used our device to generate radio waves in the MHz range using an electronically driven laser source. Very good agreement between our simulation and the results was obtained. One should note that in this case the emitted radiation is more dependent upon the optical pulse shape than the device speed itself. Nevertheless, our results show we have successfully demonstrated the generation of radio waves from optical pulses using our device. / Mestrado / Física da Matéria Condensada / Mestre em Física

ITC/USA 2009 Conference Proceedings / The Forty-Fifth Annual International Telemetering Conference and Technical Exhibition / October 26-29, 2009 / Riviera Hotel & Convention Center, Las Vegas, Nevada / The field of Terahertz (THz) radiation, electromagnetic energy, between 0.3 to 3 THz, has seen intense interest recently, because it combines some of the best properties of IR along with those of RF. For example, THz radiation can penetrate fabrics with less attenuation than IR, while its short wavelength maintains comparable imaging capabilities. We discuss major challenges in the field: designing systems and applications which fully exploit the unique properties of THz radiation. To illustrate, we present our reflective, radar-inspired THz imaging system and results, centered on biomedical burn imaging and skin hydration, and discuss challenges and ongoing research.

The pharmaceutical process of dry granulation using roller-compaction (DG/RC) is effectively a non-batch based procedure orientated to deliver a continuous stream of material free of a pre-defined batch-size with reduced plant equipment/scale-up R&D resources and an enhanced work-throughput, particularly suitable for moisture sensitive formulation. The desirable accreditations of DG/RC are many; yet by the nature of a more flexible approach than (i.e. wet-granulation), it must be highly monitored and controlled to accomplish higher-throughput rates and reduced ‘static’ material testing stages. To monitor rapidly and in-line with production, pre-granulated ribbons of RC (which highly correlates to the post milled granulates), terahertz time-domain spectroscopy (TDS) is used to elucidate the key physical attributes of post-compression density and thickness uniformity, key to end-product consistency. Invariably a great number of conditions apply to DG/RC (viz: System design, material characteristics, environmental and unit configuration), although widely regarded as the key processing parameters (PP’s) are roll-pressure and roll-gap [1-4]. The target of the study is to derive a strategy to position TDS as PAT to DG/RC. Two terahertz time-domain TD methods of a conventional transmission setup and reflection (TPI) THz analysis are used on standards of glass slides for verifying the interpretational foundations of the TD methods. Achieving RI/thickness error-discrepancies +2.2 to -0.4% c.f. literature ([150]) values provides foundations to test the solid-fraction ratios of pharma tablets with regard to RI’s being surrogate values to SF/path-length (R2 = 1). Combining transmission principles to the portion of reflected EMR removes the pre-requisite for RI or path-length knowledge, giving +1.5 to +2.4% RI agreement (vs. frequency-domain attained results) thus enabling thickness estimations to be above 95% against physical micrometre judgement in all models. Augmentation of the TD methods, refined in Experimental chapter 2 ,then chiefly focuses on TPI as the principle THz-TD method (as the most ideal tool for PAT) for adopting the RI measures for ribbon uniformity analysis in Experimental chapter 4 in an off-line environment again resulting in RI and thicknesses < 5 % error of known parameters of thickness and further use of RI as a proxy porosity equivalent to gas pycnometry. Elucidated in the work are the limitations encountered with tablets and RC’s, data interpretation of industrial considerations. Experimental chapter 3 diverges from RI to differentiate thickness in-order to assess the FD transmission for non-destructive mechanical assessment. This demonstrates a clear relationship between compaction force and the surrogate value for density, following a linear trend below a certain threshold of force. The ‘threshold’ value is observed for less massive tablets, and concluded is that the mechanistic interplay and permanent (plastic) consolidation is greater in instances where compaction-force increases proportionally with target-fill weights, and thus the various behaviour of MCC to stress.

No description available.

This thesis presents a multi-phase injection locking (IL) technique and its application in the locking range extension in multi-phase injection locking oscillators (ILOs) for Terahertz (THz) signal generation. The proposed technique can significantly increase the frequency locking range of a multi-phase injection locking oscillator compared to the conventional single-phase injection locking scheme. Based on the multi-phase IL technique and sub-harmonic ILOs, an “active frequency multiplier chain” architecture and a multi-ring system layout topology are also proposed to achieve scalable THz signal generation. As proof of concept, a cascaded 3-stage 3-phase 2nd-order sub-harmonic ILO chain is implemented in the IBM 9HP SiGe BiCMOS process. The design achieves a maximum output power of -16.6dBm at 498GHz, a phase noise of -87dBc/ Hz at 1MHz offset, and a total 5.1% frequency tuning range from 485.1GHz to 510.7GHz, which is the largest frequency tuning range among all the reported silicon-based THz oscillator sources in the 0.5THz band.

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

Terahertz (THz) frequencies comprise the portion of the electromagnetic spectrum more energetic than microwaves, but less energetic than infrared light. The THz band presents many opportunities for condensed matter physics and optics engineering. From the physics perspective, advances in the generation and detection of THz radiation have opened the door for spectroscopic studies of a range of solid-state phenomena that manifest at THz frequencies. From an engineering perspective, THz frequencies are an under-used spectral region, ripe for the development of new devices. In both cases, the challenge for researchers is to overcome a lack of sources, detectors, and optics for THz light, termed the THz Gap. Metamaterials (MMs), composite structures with engineered index of refraction, n, and impedance, Z, provide one path towards realizing THz optics. MMs are an ideal platform for the design of local EM field distributions, and far-field optical properties. This is especially true at THz frequencies, where fabrication of inclusions is easily accomplished with photolithography. Historically, MM designs have been based around static configurations of resonant inclusions that work only in a narrow frequency band, limiting applications. Broadband and tunable MMs are needed to overcome this limit. This dissertation focuses on creating tunable and controllable MM structures through the manipulation of electromagnetic interactions between MM inclusions. We introduce three novel MM systems. Each system is studied computationally with CST-Studio, and experimentally via THz spectroscopy. First, we look at the tunable transmission spectrum of two coupled split ring resonators (SRRs) with different resonant frequencies. We show that introducing a lateral displacement between the two component resonators lowers the electromagnetic coupling between the SRRs, activating a new resonance. Second, we study an SRR array, coupled to a non-resonant closed ring array. We show that lowering the interaction strength through lateral displacement changes the MM oscillator strength by ~ 40% and electric field enhancement by a factor of 4. Finally, we show that interactions between a superconducting SRR array and a conducting ground plane result in a temperature and field strength dependent MM absorption. The peak absorption changes by ~ 40% with increasing electric field and by ~ 66% with increasing temperature.

One of the major challenges for the semiconductor industry is to continue with the miniaturization of the device features, increasing the integration densities with higher operation frequency. Silicon the material of choice so far, has been arriving at its physical limits which has led the condensed matter researchers to look for alternative new material which can set the foundation for the next generation computing paradigms or lead to applications in spintronics. There has been a rising interest in so-called Dirac materials, characterized by a linear dispersion relation, giving rise to exotic physical phenomena such as high carrier velocities ~ 106 m/s and dissipationless charge transport. In this thesis, we have studied two classes of Dirac materials - graphene and topological insulators (TIs) namely, bismuth selenide (Bi2Se3) and antimony telluride (Sb2Te3). Specifically, we investigate the optical behavior of Dirac materials using terahertz time domain spectroscopy (THz-TDS) contact-free optical technique, used to probe the low-energy excitations in strongly correlated electron gases. Chapter 1 provides a broad introduction to the field of topological insulators and graphene the various optical and electronic methods, which have been employed to explore their response. In particular the focus in on detecting and isolating the response from the topological surface state (TSS) in TIs, which are “robust”, as it is protected against backscattering by spin− momentum locking and time reversal symmetry. Various literature reports describing the current understanding of the TI field are then discussed. This sets the context for understanding the approach undertaken in the rest of the thesis, towards investigating these materials. In Chapter 2 we discuss the intrinsic plasmonic response in chemical vapour deposited (CVD) graphene and its relation to the domain size of graphene. A novel ion gel based top gate is implemented with the possibility of tuning the plasmonic resonances by ~ 70 GHz. We further employ THz-TDS to map the conductivity of graphene film on different substrates such as germanium and sapphire. In chapter 3, we investigate Bi2Se3, a representative TI using THz spectroscopy and magnetotransport measurements. The temperature-dependent optical behavior of a 23-quintuple-thick film of Bi2Se3, is used to deconvolve the surface state response from the bulk resulting in an optical mobility exceeding 2000 cm^2/V·s at 4 K, indicative of a surface-dominated response. Further, a scattering lifetime of ∼0.18 ps and a carrier density of 6 × 10^12 cm^−2 at 4 K is obtained using the THz measurements. The electrical transport measurements reveal weak antilocalization behavior in the Bi2Se3 sample, consistent with the presence of a topological surface state. Chapter 4, discusses the phase transition in a rather less considered TI, Sb2Te3, using THz-TDS. We track through a series of topological phase transitions from 3D-TI to 2D hybrid topological insulator and then a 2D trivial insulator, as function of Sb2Te3 film thickness. Reducing the film thickness further resulted in a reduced mobility suggesting that the formation of a spin-conserving scattering channel characteristic of hybridized topological insulator phase. Finally, the Chapter 5, concludes with a summary of the thesis and presents future opportunities for further research arising from this work.