This literature analysis examines terahertz (THz) imaging as a non-invasive tool for the imaging of friction ridge detail from the first painted layer (imprimatura) in multilayered painted works of art. The paintings of interest are those created utilizing techniques developed during the Renaissance and still in use today. The goal of analysis serves to answer two questions. First, can THz radiation penetrate paint layers covering the imprimatura to reveal friction ridge information? Secondly, can the this technology recover friction ridge detail such that the fine details are sufficiently resolved to provide images suitable for comparison and identification purposes. If a comparison standard exists, recovered friction ridge detail from this layer can be used to establish linkages to an artist or between works of art. Further, it can be added to other scientific methods currently employed to assist with the authentication efforts of unattributed paintings. Flanked by the microwave and far-infrared edges, THz straddles the electronic and optic perspectives of the electromagnetic spectrum. As a consequence, this range is imparted with unique and useful properties. Able to penetrate and image through many opaque materials, its non-ionizing radiation is an ideal non-destructive technique that provides visual information from a painting’s sub-strata. Imaging is possible where refractive index differences exist between different paint layers. Though it is impossible, at present, to determine when a fingerprint was deposited, one can infer approximately when a print was created if it is recovered from the imprimatura layer of a painting, and can be subsequently attributed to a known source. Fingerprints are unique, a person is only able to deposit prints while their physical body is intact and thus, in some cases, the multiple layer process some artists use in their work may be used to the examiner’s advantage. Impressions of friction ridge detail have been recorded on receiving surfaces from human hands throughout time (and have also been discovered in works of art). Yet, the potential to associate those recorded impressions to a specific individual was only realized just over one hundred years ago. Much like the use of friction ridge skin, the relatively recently discovered THz range is now better understood; its tremendous potential unlocked by growing research and technology designed to exploit its unique properties.

A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 mu m with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 mu m to 2.191 mu m with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 mu m) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 mu W and DFG conversion efficiency of 4.32x10(-7). The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 mu W and 5.86x10(-7) respectively. This provides a potential practical palm-top tunable THz sources for portable applications.

Terahertz (THz) technology is still currently a rapidly developing area of research with applications already demonstrated in the fields of biology, medicine, security, chemical/materials inspection and astrophysics to name a few. The diversity of applications which require the generation and measurement of THz or sub-millimeter (sub-mm) electromagnetic (EM) signals is the result of the vast number of chemical elements and compounds which exhibit molecular transitions and vibrational behavior that occur at frequency ranges corresponding to the so-called 'THz gap', roughly defined as 0.05-10 THz. The THz gap was named as such because of the relative difficulty in generating and analysing EM waves in this frequency band. This was due to the inherent challenges in generating either electrical signals with response periods below 1 picosecond (ps), or optical signals with wavelengths in the far-infrared (FIR) range. High absorption of THz signals in atmosphere via absorption by molecules such as H2O also impeded early developments and is a key issue in THz systems even today. There is now a wide variety of THz system solutions, each of which exhibits a different set of operational advantages and limitations. Arguably, the most well-established THz technique to date is based on the use of photoconductive antennas (PCAs) driven by ultrafast pulsed or dual-wavelength laser systems. This technique is the basis for the work presented in this thesis, which is an investigation into the potential utilisation of quantum dot (QD)-based semiconductor materials and devices in THz systems. This thesis discusses the work carried out in the development of a novel class of PCA devices which were postulated to enable efficient optical-to-THz signal conversion, whilst also overcoming several major limitations normally exhibited by PCA devices such as limited optical wavelength pumping range and thermal breakdown. To summarise briefly, these issues were addressed by considering: the additional pump absorption energy ranges enabled by the inclusion of multiple bandgap-engineered semiconductor materials and quantum-confined structures; the higher thermal conductivity and hence pump tolerance exhibited by relatively high-quality (low defect) absorption layers; and by simultaneously harnessing the ultrafast charge carrier modulation exhibited by the integrated QDs. Additionally, some work was carried out using QD-based lasers as pump sources, with the initial intention to explore the feasibility of a fully QD-based THz transceiver system and draw some conclusions as to the future potential for ultra-compact or even lab-on-chip THz systems, for example.

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.

No description available.

The emergence of 6G and beyond networks is set to enable a range of novel services such as personalized highly immersive experiences, holographic teleportation, and human-like intelligent robotic applications. Such applications require a set of stringent sensing, communication, control, and intelligence requirements that mandate a leap in the design, analysis, and optimization of today's wireless networks. First, from a wireless communication standpoint, future 6G applications necessitate extreme requirements in terms of bidirectional data rates, near-zero latency, synchronization, and jitter. Concurrently, such services also need a sensing functionality to track, localize, and sense their environment. Owing to its abundant bandwidth, one may naturally resort to terahertz (THz) frequency bands (0.1 − 10 THz) so as to provide significant wireless capacity gains and enable high-resolution environment sensing. Nonetheless, operating a wireless system at the THz band is constrained by a very uncertain channel which brings forth novel challenges. In essence, these channel limitations lead to unreliable intermittent links ergo the short communication range and the high susceptibility to blockage and molecular absorption. Second, given that emerging wireless services are "intelligence-centric", today's communication links must be transformed from a mere bit-pipe into a brain-like reasoning system. Towards this end, one can exploit the concept of semantic communications, a revolutionary paradigm that promises to transform radio nodes into intelligent agents that can extract the underlying meaning (semantics) or significance in a data stream. However, to date, there has been a lack in holistic, fundamental, and scalable frameworks for building next-generation semantic communication networks based on rigorous and well-defined technical foundations. Henceforth, to panoramically develop the fully-fledged theoretical foundations of future 6G applications and guarantee affluent corresponding experiences, this dissertation thoroughly investigates two thrusts. The first thrust focuses on developing the analytical foundations of THz systems with a focus on network design, performance analysis, and system optimization. First, a novel and holistic vision that articulates the unique role of THz in 6G systems is proposed. This vision exposes the solutions and milestones necessary to unleash THz's true potential in next-generation wireless systems. Then, given that extended reality (XR) will be a staple application of 6G systems, a novel risk and tail-based performance analysis is proposed to evaluate the instantaneous performance of THz bands for specific ultimate virtual reality (VR) services. Here, the results showcase that abundant bandwidth and the molecular absorption effect have only a secondary effect on the reliability compared to the availability of line-of-sight. More importantly, the results highlight that average metrics overlook extreme events and tend to provide false positive performance guarantees. To address the identified challenges of THz systems, a risk-oriented learning-based design that exploits reconfigurable intelligent surfaces (RISs) is proposed so as to optimize the instantaneous reliability. Furthermore, the analytical results are extended to investigate the uplink freshness of augmented reality (AR) services. Here, a novel ruin-based performance is conducted that scrutinizes the peak age of information (PAoI) during extreme events. Next, a novel joint sensing, communication, and artificial intelligence (AI) framework is developed to turn every THz communication link failure into a sensing opportunity, with application to digital world experiences with XR. This framework enables the use of the same waveform, spectrum, and hardware for both sensing and communication functionalities. Furthermore, this sensing input is intelligently processed via a novel joint imputation and forecasting system that is designed via non-autoregressive and transformed-based generative AI tools. This joint system enables fine-graining the sensing input to smaller time slots, predicting missing values, and fore- casting sensing and environmental information about future XR user behavior. Then, a novel joint quality of personal experience (QoPE)-centric and sensing-driven optimization is formulated and solved via deep hysteretic multi-agent reinforcement learning tools. Essentially, this dissertation establishes a solid foundation for the future deployment of THz frequencies in next-generation wireless networks through the proposal of a comprehensive set of principles that draw on the theories of tail and risk, joint sensing and communication designs, and novel AI frameworks. By adopting a multi-faceted approach, this work contributes significantly to the understanding and practical implementation of THz technology, paving the way for its integration into a wide range of applications that demand high reliability, resilience, and an immersive user experience. In the second thrust of this dissertation, the very first theoretical foundations of semantic communication and AI-native wireless networks are developed. In particular, a rigorous and holistic vision of an end-to-end semantic communication network that is founded on novel concepts from AI, causal reasoning, transfer learning, and minimum description length theory is proposed. Within this framework, the dissertation demonstrates that moving from data-driven intelligence towards reasoning-driven intelligence requires identifying association (statistical) and causal logic. Additionally, to evaluate the performance of semantic communication networks, novel key performance indicators metrics that include new "reasoning capacity" measures that could go beyond Shannon's bound to capture the imminent convergence of computing and communication resources. Then, a novel contrastive learning framework is proposed so as to disentangle learnable and memoizable patterns in source data and make the data "semantic-ready". Through the development of a rigorous end-to-end semantic communication network founded on novel concepts from communication theory and AI, along with the proposal of novel performance metrics, this dissertation lays a solid foundation for the advancement of reasoning-driven intelligence in the field of wireless communication and paves the way for a wide range of future applications. Ultimately, the various analytical foundations presented in this dissertation will provide key guidelines that guarantee seamless experiences in future 6G applications, enable a successful deployment of THz wireless systems as a versatile band for integrated communication and sensing, and build future AI-native semantic communication networks. / Doctor of Philosophy / To date, the evolution of wireless networks has been driven by a chase for data rates, i.e., higher download or upload speeds. Nonetheless, future 6G applications (the generation succeeding today's fifth generation 5G), such as the metaverse, extended reality (encompassing augmented, mixed, and virtual reality), and fully autonomous robots and vehicles, necessitate a major leap in the design and functionality of a wireless network. Firstly, wireless networks must be able to perform functionalities that go beyond communications, encompassing control, sensing, and localization. Such functionalities enable a wide range of tasks such as remotely controlling a device, or tracking a mobile equipment with high precision. Secondly, wireless networks must be able to deliver experiences (e.g. provide the user a sense of immersion in a virtual world), in contrast to a mere service. To do so, extreme requirements in terms of data rate, latency, reliability, and sensing resolution must be met. Thirdly, intelligence must be native to wireless networks, which means that they must possess cognitive and reasoning abilities that enable them to think, act, and communicate like human beings. In this dissertation, the three aforementioned key enablers of future 6G experiences are examined. Essentially, one of the focuses of this dissertation is the design, analysis, and optimization of wireless networks operating at the so-called terahertz (THz) frequency band. The THz band is a quasi-optical (close to the visible light spectrum) frequency band that can enable wireless networks to potentially provide the extreme speeds needed (in terms of communications) and the high-resolution sensing. However, such frequency bands tend to be very susceptible to obstacles, humidity, and many other weather conditions. Therefore, this dissertation investigates the potential of such bands in meeting the demands of future 6G applications. Furthermore, novel solutions, enablers, and optimization frameworks are investigated to facilitate the successful deployment of this frequency band. To provide wireless networks with their reasoning ability, this dissertation comprehensively investigates the concept of semantic communications. In contrast to today's traditional communication frameworks that convert our data to binary bits (ones and zeros), semantic communication's goal is to enable networks to communicate meaning (semantics). To successfully engineer and deploy such networks, this dissertation proposes a novel suite of communication theoretic tools and key performance indicators. Subsequently, this dissertation proposes and analyzes a set of novel artificial intelligence (AI) tools that enable wireless networks to be equipped with the aforementioned cognitive and reasoning abilities. The outcomes of this dissertation have the potential to transform the way we interact with technology by catalyzing the deployment of holographic societies, revolutionizing the healthcare via remote augmented surgery, and facilitating the deployment of autonomous vehicles for a safer and more efficient transportation system. Additionally, the advancements in wireless networks and artificial intelligence proposed in this dissertation could also have a significant impact on various other industries, such as manufacturing, education, and defense, by enabling more efficient and intelligent systems. Ultimately, the societal impact of this research is far-reaching and could contribute to creating a more connected and advanced world.

In this thesis the control of ultrafast (10⁻¹³ s) optical pulses used for metrological applications has been investigated. Two different measurement set-ups have been considered, both based around the `pump-probe' technique, where an optical pulse is divided into two parts, one to `pump' or excite a physical system of interest, the other to `probe' or measure the outcome. In both cases the measurement uses electro-optic sampling (EOS), where an electric field is measured by detecting changes in the optical probe pulse polarisation after interaction with the field. In the first study, a method for wavelength metrology in the terahertz (THz) region has been demonstrated by producing an optical pulse shaper and genetic algorithm to control pump pulses and so indirectly influence the THz spectra they generate. In the second study an OPO (optical parametric oscillator) has been developed to provide ultrafast optical pulses for the generation of < 100 fs electrical pulses for metrology using quantum interference control (QUIC). QUIC electrical signals have been demonstrated successfully by charge accumulation measurements and the QUIC electrical pulse temporally measured using EOS, though the low signal levels due to power restrictions mean the QUIC electrical pulse is unsuitable for metrology at this time. Finally, a portable optical pulse measurement device based around frequency-resolved optical gating (FROG) has been designed, built and tested. This has been shown to be capable of retrieving amplitude and phase information in both the temporal and spectral domains for optical pulses as short as 20 fs duration. The ability to characterise shaped pulses also has been demonstrated successfully, with the requirements for full automation identified.

Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO₂-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH₃NH₃PbI₃ and several microns for CH₃NH₃PbI_3-xCl_x. These results are shown to hold for both, solution processed and vapour-deposited CH₃NH₃PbI_3-xCl_x and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH₃NH₃PbI_3-xCl_x, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.

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. / The subject of the thesis focused on the development, characterization and use of semiconductor materials, in which the free carriers have a very short lifetime (picosecond or sub-picosecond) to produce photoconductive antennas emitting and detecting electromagnetic terahertz (THz) radiation. Unlike semiconductor LTG-GaAs (low temperature grown GaAs) which is a well-dominated technology and present exceptional performances when photoexcited by typically less than 0.8 micron wavelength laser pulses, the work focused on here materials for the use of lasers whose wavelengths are those of the optical communication, namely around 1.5 microns. The interest is to benefit from the mature technology of these lasers, and relatively low cost components for optical telecommunications. To achieve effective and efficient THz antennas, the semiconductor material must have several qualities : lifetime of free carriers very short, high carrier mobility, high resistivity outside lighting, and good crystallographic structure to prevent electrical breakdown. For a short lifetime, a large number of traps are introduced into the semiconductor, which effectively capture the free electrons. For InGaAs materials used at 1.5 microns, the problem is that the energy level of the traps, for example, the epitaxial material at low temperature is very close to the conduction band of the semiconductor. This is equivalent to an n-doped material, what greatly reduces its resistivity outside illumination. Several solutions have been made by different laboratories : compensation for the p-doped epitaxial materials at low temperature, ion bombardment, ion implantation, or even alternating layer structures where photo-generation and recombination of free carriers occur in different places. The aim of the thesis was to produce materials prepared using these techniques to characterize and compare their performance to THz optoelectronics. The studied InGaAs-based semiconductors were as previously published by the competition, the originality of the thesis was on the comparison of these different materials and if possible their optimization. During this work, many of InGaAs layers were grown epitaxially by varying the deposition parameters, and THz antennas were fabricated. The layers were characterized from the crystallographic point of view, as well as the DC electrical conductivity (measures 4 points, Hall mobility ... ), the optical absorption properties (visible and IR spectroscopy ), the lifetime of carriers by optical pump-probe measurement. For low temperature epitaxial layers, the influences of thermal and doping beryllium annealing were studied. In the case of shelled or implanted layers, several ions were used, bromine, iron and hydrogen. The relationship between the mapping of structural defects of the implanted ions and electrical and carrier dynamics properties were discussed in detail. These studies allow us to understand the type of defects that trap carriers in these materials, as well as training in the process of manufacturing and processing layers. Finally the best layers are made comparable to those published elsewhere performance. The last study allowed to achieve the first signals of THz radiation generated by InGaAs-based optimized antenna.

We introduce an approach to implement full coherent control on nanometer length scales. It is based on spatiotemporal modulation of the surface plasmon polariton (SPP) fields at the thick edge of a nanowedge. The SPP wavepackets propagating toward the sharp edge of this nanowedge are compressed and adiabatically concentrated at a nanofocus, forming an ultrashort pulse of local fields. The profile of the focused waveform as a function of time and one spatial dimension is completely coherently controlled. We establish the principal limits for the nanoconcentration of the terahertz (THz) radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration. We predict that the adiabatic compression of THz radiation from the initial spot size of vacuum wavelength R λ 300 μm 0 0 ≈ ≈ to the unprecedented final size of R = 100 − 250 nm can be achieved, while the THz radiation intensity is increased by a factor of 10 to 250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses, will allow the observation of nonlinear THz effects and a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative. This should find a wide spectrum of applications in science, engineering, biomedical research and environmental monitoring. We also develop a theory of the spoof plasmons propagating at the interface between a dielectric and a real conductor. The deviation from a perfect conductor is introduced through a finite skin depth. The possibilities of guiding and focusing of spoof plasmons are considered. Geometrical parameters of the structure are found which provide a good guiding of such modes. Moreover, the limit on the concentration by means of planar spoof plasmons in case of non-ideal metal is established. These properties of spoof plasmons are of great interest for THz technology.