Time-domain terahertz spectroscopy is now a well-established technique. Of the many methods available for a terahertz source for terahertz spectroscopy, the most widely used may be the GaAs-based photoconductive antenna, as it provides relatively high power at terahertz frequencies, commercially available up to 150 µW, and a wide-bandwidth, approximately 70 GHz to 3.5 THz. One of the limitations for developing more accurate and sensitive terahertz interrogation techniques is the lack of higher power sources. Because of our research interests in terahertz spectroscopy, we investigated detailed design and fabrication parameters involved in the photoconductive antenna, which exploits the surface plasma oscillation to produce a wideband pulse. The investigation enabled us to develop a new photoconductive antenna that is capable of generating a high power terahertz beam, at least twenty times stronger than those currently available. Throughout this research, it was discovered that antenna electrodes with particular geometries could produce superradiance, also known as the Dicke effect. Chaotic electrodes with a predisposition to lead charge-carriers into chaotic trajectories, e.g. rippled geometry, were exploited to reduce undesirable heat effects by driving thermal-electrons away from the terahertz generation site, i.e. the location of the surface plasma, while concentrating the removed charge-carriers in separate locations slightly away from the surface plasma. Then, spontaneous emission of coherent terahertz radiation may occur when the terahertz pulse generated by the surface plasma stimulates the concentrated carriers. This spontaneous emission enhanced the total coherent terahertz beam strength, as it occurs almost simultaneously with the primary terahertz beam. In principle, the spontaneous emission power increases as N^2, with the number N of dipole moments resulted from the concentrated charge carriers. Hence, if the design parameters are optimized, it may be possible to increase the strength of coherent terahertz beam by more than one order of magnitude with a photoconductive antenna containing rippled electrodes. However, as the parameters are yet to be optimized, we have only demonstrated 10-20 % enhancement with our current photoconductive antennas. Photoconductive antennas were fabricated via photolithography and characterized by time-domain terahertz spectroscopy and pyroelectric detection. In addition to chaotic electrodes, a variety of other parameters were characterized, including GaAs substrate thickness, GaAs crystal lattice orientation, trench depth for electrodes, metal-semiconductor contact, and bias voltage across electrodes. Nearly all parameters were found to play a crucial role influencing terahertz beam emission and carrier dynamics. By exploiting wave chaos and other antenna parameters, we developed a new photoconductive antenna capable of continuous operation with terahertz power twenty times larger than that of the conventional photoconductive antennas, improving from 150 µW to 3 mW. With further optimizations of the parameters, we expect more dramatic improvement of the photoconductive antenna in the near future. / Physics

Construímos um arranjo experimental para o estudo de ressonadores acústicos, que tem sido considerados como análogos clássicos de bilhares quânticos. O equipamento mantém estabilidade durante vários dias, o que é uma condição necessária para a obtenção de espectros de autofreqüências com a resolução requerida para a caracterização precisa destes sistemas. Caracterizamos 7 amostras, que são placas de alumínio com espessura < 2 mm e que possuem as seguintes geometrias: dois estádios de Sinai, com e sem dessimetrização planar; três triângulos sendo um equilátero, um retângulo e outro escaleno, este com todos os ângulos agudos e irracionais em unidades de ; além de duas amostras circulares, com e sem dessimetrização planar. Observamos que três amostras apresentam estatísticas GOE, uma 2GOE, uma semi-Poisson, uma Poisson com perda de níveis, e outra aparentemente intermediária entre a GOE e a 2GOE, que nao foi possível classificar. A qualidade dos dados também permitiu a obtenção das energias dos espectros, onde obtivemos resultados coerentes com a classificação a / We have built an experimental apparatus to study acoustic resonators which have been considered as classical analogs of quantum billiards. The equipment was able to keep the stability during several days, which is a requirement to the precise eigenfrequency measurements allowing a characterization of the systems. We have characterized 7 samples made of aluminum plates with thickness smaller than 2 mm having the following geometries: two Sinai\'s stadiums (with and without planar symmetry), an equilateral triangle, a rectangle triangle, and a scalene triangle with three acute and irrational angles, and two circular shaped samples, with and without planar symmetry. We observed that three of the samples followed the GOE statistics (the asymmetrical Sinai stadium, the rectangle triangle and the scalene one). The asymmetrical Sinai stadium was described by 2GOE statistics, the equilateral triangle by the semi-Poisson, the symmetrical circle by a Poisson with missing levels and the asymmetrical circle has statistics apparently between 1GOE and 2GOE which was not possible to classify. The high quality of data allowed us to calculate the spectra energies and we found these results compatible with the previous one.

<p>This thesis focuses on chaotic stationary waves, both quantum mechanical and classical. In particular we study different statistical properties regarding thesewaves, such as energy transport, intensity (or density) and stress tensor components. Also, the methods used to model these waves are investigated, and somelimitations and specialities are pointed out.</p>

This thesis focuses on chaotic stationary waves, both quantum mechanical and classical. In particular we study different statistical properties regarding thesewaves, such as energy transport, intensity (or density) and stress tensor components. Also, the methods used to model these waves are investigated, and somelimitations and specialities are pointed out.

Construímos um arranjo experimental para o estudo de ressonadores acústicos, que tem sido considerados como análogos clássicos de bilhares quânticos. O equipamento mantém estabilidade durante vários dias, o que é uma condição necessária para a obtenção de espectros de autofreqüências com a resolução requerida para a caracterização precisa destes sistemas. Caracterizamos 7 amostras, que são placas de alumínio com espessura < 2 mm e que possuem as seguintes geometrias: dois estádios de Sinai, com e sem dessimetrização planar; três triângulos sendo um equilátero, um retângulo e outro escaleno, este com todos os ângulos agudos e irracionais em unidades de ; além de duas amostras circulares, com e sem dessimetrização planar. Observamos que três amostras apresentam estatísticas GOE, uma 2GOE, uma semi-Poisson, uma Poisson com perda de níveis, e outra aparentemente intermediária entre a GOE e a 2GOE, que nao foi possível classificar. A qualidade dos dados também permitiu a obtenção das energias dos espectros, onde obtivemos resultados coerentes com a classificação a / We have built an experimental apparatus to study acoustic resonators which have been considered as classical analogs of quantum billiards. The equipment was able to keep the stability during several days, which is a requirement to the precise eigenfrequency measurements allowing a characterization of the systems. We have characterized 7 samples made of aluminum plates with thickness smaller than 2 mm having the following geometries: two Sinai\'s stadiums (with and without planar symmetry), an equilateral triangle, a rectangle triangle, and a scalene triangle with three acute and irrational angles, and two circular shaped samples, with and without planar symmetry. We observed that three of the samples followed the GOE statistics (the asymmetrical Sinai stadium, the rectangle triangle and the scalene one). The asymmetrical Sinai stadium was described by 2GOE statistics, the equilateral triangle by the semi-Poisson, the symmetrical circle by a Poisson with missing levels and the asymmetrical circle has statistics apparently between 1GOE and 2GOE which was not possible to classify. The high quality of data allowed us to calculate the spectra energies and we found these results compatible with the previous one.

Les chambres réverbérantes à brassage de modes (CRBM) utilisées dans l'industrie pour tester l'immunité ou la susceptibilité des systèmes électroniques embarqués (avion, automobile , smartphone,...) vis-à-vis des ondes électromagnétiques (EM) présentes dans leur environnement. Les CRBM doivent toutes répondre à un certain nombre de critères statistiques fixés par une norme internationale. Le critère principale étant l'obtention d'un champ statistiquement uniforme et isotrope autour de l'objet sous test. Afin améliorer et de mieux maîtriser les propriétés statistiques de ces systèmes pour des fréquences proches de leur fréquence minimale d'utilisation, nous proposons de les rendre chaotiques afin de profiter des propriétés statistiques universelles des résonances des cavités chaotiques. Nous commencerons par montrer comment rendre chaotique, par des modifications simples, des chambres réverbérantes conventionnelles, et comment étendre les prédictions de la théorie des matrices aléatoire appliquée (TMA) à l'hamiltonien effectif, permettant de décrire les systèmes chaotiques ouverts, au cas de systèmes décrits par des champs vectoriels. Ensuite, nous comparerons, au moyen de simulations et d’expériences, les distributions d'intensité et les fluctuations des maxima du champ EM dans une CRBM conventionnelle et dans une CR chaotique au voisinage de la fréquence minimale d’utilisation. Ce travail illustre que les propriétés statistiques spectrales et spatiales universelles des CR chaotiques permettent de mieux répondre aux critères exigés par la norme internationale pour réaliser des tests de compatibilité électromagnétiques. / Mode-stirred reverberation chambers (RC) are used in the industry to test the immunity or the susceptibility of on-board electronic systems (plane, automobile, smartphone) towards the electromagnetic waves present in their environment. Mode-stirred RCs have to comply with a number of statistical criteria fixed by international standards. The chief criterion relies on a statistically uniform and isotropic field around the object under test. In order to improve and master the statistical properties of these systems for frequencies close to their lowest useable frequency, we suggest making them chaotic to take advantage of universal statistical properties of the resonances of chaotic cavities. We first show how to make chaotic RCs by simple modifications of a conventional RC and how to extend the predictions of the random matrix theory applied to the effective hamiltonien describing the open chaotic systems, to the case of vectorial fields. Then, we compare, by means of simulations and experiments, the distributions of intensity and the fluctuations of the maxima of the field in a conventional RC and in a chaotic RC close to the lowest useable frequency. This work illustrates that the universal spectral and spatial statistical properties of chaotic RCs allow to better comply with the criteria required by the international standards.