Le travail de cette thèse porte sur l’identification et le contrôle des propriétés thermiques et acoustiques de nanostructures à fort potentiel thermoélectrique appelés « Super-réseaux ». Le manuscrit comporte trois parties : La première partie est consacrée à la description théorique des phénomènes de transport thermique par diffusion dans les solides isolants et semi-conducteurs. Nous abordons tout d’abord le point de vue atomique, puis macroscopique en utilisant la méthode des quadripôles thermiques. La fin du chapitre est consacrée aux propriétés acoustiques et thermiques des super-réseaux. La deuxième partie présente et compare les méthodes de Thermoreflectance laser synchrone et asynchrone utilisées pour extraire les propriétés thermiques de couches minces et de super-réseaux. Nous montrons que dans le cas synchrone, les signaux sont soumis à des artefacts modifiant leur allure et rendant difficile l’identification des propriétés thermiques. Dans le cas asynchrone, la suppression de tous les éléments mobiles permet d’obtenir un signal sans artéfact. Nous traitons ensuite des fonctions de sensibilité au modèle développé puis nous validons la méthode d’identification en estimant la conductivité thermique d’un film mince de SiO2. La troisième partie présente les résultats des identifications de la conductivité thermique de différents super-réseaux de SiGe. Nous montrons que les résistances d’interface jouent un rôle majeur dans l’explication de la réduction de la conductivité thermique. Nous étudions également des super-réseaux contenant des îlots de Ge, nous montrons que de telles structures permettent d’obtenir non seulement des conductivités proches de celles des matériaux amorphes, mais le comportement linéaire de la conductivité en fonction de la période montre qu’il est possible de contrôler cette dernière. Enfin, nous utilisons la Thermoreflectance pour réaliser une étude de spectroscopie THz de phonons cohérents dans les super-réseaux et nous mettons en évidence la sélectivité spectrale des ces nanostructures. / The work presented in this thesis deals with identification and control of the thermal and acoustic properties of high thermoelectric potential nanostructures called “superlattices”. This thesis is divided in three parts: The first part gives a theoretical description of thermal diffusion in insulating and semiconducting materials. We first broach the atomic description then the macroscopic view using the Thermal Quadrupole model. The end of this chapter deals with acoustic and thermal properties specific to superlattices. The second part describes and compares synchronous and asynchronous thermoreflectance techniques used to extract thermal properties of thin films and superlattices. We find that for the synchronous case signals are subject to artifacts which confound parameter estimations. For the asynchronous case, we find that lack of a mechanical translation stage removes these artifacts. We then investigate the sensitivity functions, and finally validate our identification method by estimation of the thermal conductivity of a SiO2 thin film. The third part presents the results of thermal parameter identification in SiGe superlattices. We show that thermal interfaces play a major role to in the overall thermal conductivity. We also study superlattices with Ge nanodots and show that for such structures we are able to obtain thermal conductivity values near the amorphous values. Moreover, the linear behavior of the thermal conductivity with period thickness shows that it is possible to control this value. Finally, we use Thermoreflectance to perform THz coherent phonon spectroscopy of superlattices, revealing the spectral selectivity of these nanostructures.

Cílem diplomové práce je navrhnout konstrukční řešení držáků vzorků pro vysokofrekvenční elektron paramagnetickou resonanci. Předmětem návrhu je vytvořit jednoduchý zamykací systém pro spojování mikrovlnného vlnovodu a držáku vzorku. Dále navrhnout systém s řešením držáku pro více vzorků. Toto unikátní provedení držáku povede k několikanásobné úspoře celkového času měření vzorků. Poslední návrh spočívá v optimalizaci držáku vzorku s možností naklápění osy, kterou lze díky přímému napojení na piezoelektrický rotátor pootáčet s přesností na miliradiány. Oba typy držáku vzorku jsou navrženy s ohledem na automatizaci měření.

Disertační práce se zabývá návrhem a optimalizací kruhově polarizované anténa pro oblast terahertzových kmitočtů. V práci se věnuji zjednodušené teorii terahertzového zdroje a návrhu vhodné antény pro tento zdroj. Návrh je zaměřen na dosažení kruhové polarizace z lineárně polarizovaných antén. Abych potlačil šíření povrchové vlny na elektricky tlustém dielektrickém substrátu, věnuji se návrhu a optimalizaci specifických periodických struktur. Návrh těchto struktur je poměrně komplikovaný, protože neexistuje přímočarý vztah mezi vlastnostmi struktur s elektromagnetickým zádržným pásmem (EBG) a geometrií buňky. Abych vhodně koncentroval vyzařovanou energii do úzkého svazku, věnuji se návrhu a optimalizaci částečně odrazného plochy (PRS), které působí jako planární čočka pro terahertzovou anténu.

Imaging with THz radiation at nanoscale resolution is highly desirable for specific material investigations that cannot be obtained in other parts of the electromagnetic spectrum. Nevertheless, classical free-space focusing of THz waves is limited to a >100 μm spatial resolution, due to the diffraction limit. However, the scattering- type scanning near-field optical microscopy (s-SNOM) promises to break this diffraction barrier. In this work, the realization of s-SNOM and spectroscopy for the THz spectral region from 30–300 μm (1–10 THz) is presented. This has been accomplished by using two inherently different radiation sources at distinct experimental setups: A femtosecond laser driven photoconductive antenna, emitting pulsed broadband THz radiation from 0.2–2 THz and a free-electron laser (FEL) as narrow-band high-intensity source, tunable from 1.3–10 THz. With the photoconductive antenna system, it was demonstrated for the first time that near-field spectroscopy using broadband THz-pulses, is achievable. Hereby, Terahertz time-domain spectroscopy with a mechanical delay stage (THz-TDS) was realized to obtain spectroscopic s-SNOM information, with an additional asynchronous optical sampling (ASOPS) option for rapid far-field measurements. The near-field spectral capabilities of the microscope are demonstrated with measurements on gold and on variably doped silicon samples. Here it was shown that the spectral response follows the theoretical prediction according to the Drude and the dipole model. While the broadband THz-TDS based s-SNOM in principle allows for the parallel recording of the full spectral response, the weak average power of the THz source ultimately limits the technique to optically investigate selected sample locations only. Therefore, for true THz near-field imaging, a FEL as a high-intensity narrow- band but highly-tunable THz source in combination with the s-SNOM technique, has been explored. Here, the characteristic near-field signatures at wavelengths from 35–230 μm are shown. Moreover, the realization of material sensitive THz near-field imaging is demonstrated by optically resolving, a structured gold rod with a reso- lution of up to 60 nm at 98 μm wavelength. Not only can the gold be distinguished from the silica substrate but moreover parts of the structure have been identified to be residual resin from the fabrication process. Furthermore, in order to explore the resolution capabilities of the technique, the near-fields of patterned gold nano- structures (Fischer pattern) were imaged with a 50 nm resolution at wavelengths up to 230 μm (1.2 THz). Finally, the imaging of a topography-independent optical material contrast of embedded organic structures, at exemplary 150 μm wavelength is shown, thereby demonstrating that the recorded near-field signal alone allows us to identify materials on the nanometer scale. The ability to measure spectroscopic images by THz-s-SNOM, will be of benefit to fundamental research into nanoscale composites, nano-structured conductivity phenomena and metamaterials, and furthermore will enable applications in the chemical and electronics industries. / Die Bildgebung mit THz Strahlung im Nanobereich ist höchst wünschenswert für genaue Materialuntersuchungen, welche nicht in anderen Spektralbereichen durchgeführt werden kann. Aufgrund des Beugungslimits ist kann jedoch mit klassischen Methoden keine bessere Auflösung als etwa 100 μm für THz-Strahlung erreicht werden. Die Methode der Streulicht-Nahfeldmikroskopie (s-SNOM) verspricht jedoch dieses Beugungslimit zu durchbrechen. In der vorliegenden Arbeit wird die Realisierung der Nahfeld-Mikroskopie und Spektroskopie im THz-Spektralbereich von 30–1500 μm (0.2–10 THz) präsentiert. Dies wurde mittels zweier grundsätzlich unterschiedlichen Strahlungsquellen an separaten Experimentaufbauten erreicht: Einer photoleitenden Antenne welche gepulste breitbandige THz-Strahlung von 0.2–2 THz emittiert, sowie einem Freie- Elektronen Laser (FEL) als schmalbandige hochleistungs Quelle, durchstimmbar von 1.3–10 THz. Mit dem photoleitenden Antennensystem konnte zum ersten mal demonstriert werden, dass mit breitbandigen THz-Pulsen Nahfeldspektroskopie möglich ist. Dazu wurde die übliche THz-Time-Domain-Spektroskopie (THz-TDS) zur Erhaltung der spektroskopischen s-SNOM Informationen, sowie asynchrones optisches Abtasten (ASOPS) für schnelle Fernfeld Spektroskopie eingesetzt. Die nahfeldspektroskopischen Fähigkeiten des Mikroskops wurden anhand von Messungen an Gold sowie unterschiedlich dotierten Siliziumproben demonstriert. Dabei konnte gezeigt werden, dass die spektrale Antwort den theoretischen Voraussagen des Drude- sowie Dipol Modells folgt. Während das breitband THz-TDS basierte s-SNOM spektroskopische Nahfelduntersuchungen zulässt, limitiert jedoch die schwache Ausgangsleistung der THz-quelle diese Technik insofern, dass praktisch nur Punktspektroskopie an ausgesuchten Probenstellen möglich ist. Für echte nanoskopische Nahfeldbildgebung wurde daher ein FEL als durchstimmbare hochleistungs THz-Quelle in Kombination mit der s-SNOM-Technik erforscht. Hierzu wurden die charakteristischen Nahfeld-Signaturen bei Wellenlängen von 35–230 μm untersucht, gefolgt von die Verwirklichung materialsensitiver THz Nahfeldbildgebung gezeigt an Goldstreifen mit bis zu 60 nm Auflösung. Dabei kann nicht nur das Gold von dem Glassubstrat unterschieden werden, sondern auch Ablagerungen als Überreste des Fabrikationsprozesses identifiziert werden. Um die Grenzen der Auflösungsmöglichkeiten dieser Technik zu sondieren, wurden weiterhin die Nahfelder von gemusterten Gold-Nanostrukturen (Fischer-Pattern) bei Wellenlängen bis zu 230 μm (1.2 THz) abgebildet. Hierbei wurde eine Auflösung von 50 nm festgestellt. Schliesslich konnte der topographieunabhängige Materialkontrast von eingebetteten organischen Strukturen, exemplarisch bei 150 μm Wellenlänge, gezeigt werden. Die Fähigkeit, spektroskopische Aufnahmen mittels der THZ-s-SNOM Technik zu erzeugen, wird der Grundlagenforschung und in der Nanotechnologie zu Gute kommen, und weiterhin Anwendungen in der Chemischen- und Halbleiterindustrie ermöglichen.

This thesis investigates the use of high-power lasers for synchrotron radiation sources with high brilliance, from the EUV to the hard X-ray spectral range. Hereby lasers accelerate electrons by laser-wakefield acceleration (LWFA), act as optical undulators, or both. Experimental evidence shows for the first time that LWFA electron bunches are shorter than the driving laser and have a length scale comparable to the plasma wavelength. Furthermore, a first proof of principle experiment demonstrates that LWFA electrons can be exploited to generate undulator radiation. Building upon these experimental findings, as well as extensive numerical simulations of Thomson scattering, the theoretical foundations of a novel interaction geometry for laser-matter interaction are developed. This new method is very general and when tailored towards relativistically moving targets not being limited by the focusability (Rayleigh length) of the laser, while it does not require a waveguide. In a theoretical investigation of Thomson scattering, the optical analogue of undulator radiation, the limits of Thomson sources in scaling towards higher peak brilliances are highlighted. This leads to a novel method for generating brilliant, highly tunable X-ray sources, which is highly energy efficient by circumventing the laser Rayleigh limit through a novel traveling-wave Thomson scattering (TWTS) geometry. This new method suggests increases in X-ray photon yields of 2-3 orders of magnitudes using existing lasers and a way towards efficient, optical undulators to drive a free-electron laser. The results presented here extend far beyond the scope of this work. The possibility to use lasers as particle accelerators, as well as optical undulators, leads to very compact and energy efficient synchrotron sources. The resulting monoenergetic radiation of high brilliance in a range from extreme ultraviolet (EUV) to hard X-ray radiation is of fundamental importance for basic research, medical applications, material and life sciences and is going to significantly contribute to a new generation of radiation sources and free-electron lasers (FELs).

In dieser Arbeit werden niederfrequente optische Messungen an vier neuartigen Supraleitern vorgestellt. Im Bereich von 100 GHz bis 3 THz zeigen die vier untersuchten Systeme – LuNi2B2C, Ba(Fe0,9Co0,1)2As2, T’-Pr2CuO4 und Si:Ga – ein sehr unterschiedliches Verhalten. Die beiden erst genannten Supraleiter sind Mehrbandsupraleiter, bei denen die Cooper-Paarkopplung unterschiedlich für verschiedene Fermiflächen ist. T’-Pr2CuO4 ist ein undotierter Kupratsupraleiter, der nach bisheriger Lehrmeinung nicht existieren dürfte. Mit THz-Spektroskopie konnte erstmals die Bildung einer Meißner-Phase in T’-Pr2CuO4 mit optischen Methoden beobachtet werden. Eine gewisse Sonderstellung nimmt Si:Ga als amorpher Supraleiter ein. Si:Ga wird durch Ionenimplantation von Gallium in einen Siliziumwafer hergestellt. Es besteht die Hoffnung, mit Si:Ga halb- und supraleitende Logikblöcke in großem Maßstab auf einem Chip vereinen zu können, da die Ionenimplantation mit den Produktionsprozessen der Halbleiterindustrie kompatibel ist.

Das Thema der vorgelegten Dissertation ist der quasi-isochrone Betrieb der Metrology Light Source zur Erzeugung kurzer Elektronenpakete mit der damit verbundenen Emission von kohärenter Sychrotronstrahlung. Die Metrology Light Source wurde schon in der Planungsphase auf den quasi-isochronen Betrieb ausgelegt. Es stehen Quadrupol-, Sextupol- und Oktupolmagnete zur Verfügung, um die drei führenden Ordnungen des sogenannten momentum compaction factors zu kontrollieren. Der Schwerpunkte der Arbeit ist nichtlineare, longitudinale Strahldynamik, insbesondere die sogenannten "alpha-buckets". Der Vergleich zwischen analytischen Ansätzen, numerischen Simulation und experimentellen Daten wird vorgestellt und diskutiert. Desweiteren wurde die Stromlimitierung durch die Bursting-Instabilität an der Metrology Light Source untersucht. Der Großteil der Messungen ist dabei an der Metrology Light Source durchgeführt worden mit komplementären Messungen am Elektronenspeicherring BESSY II. / The subject of this thesis is the operation of an electron storage ring at a low momentum compaction to generate short electron bunches as a source for coherent synchrotron radiation. For this purpose the Metrology Light Source is ideally suited, as it is the first light source designed with the ability to adjust the three leading orders of the momentum compaction factor by quadrupole, sextupole and octupole magnets. Therefore, new opportunities to shape the longitudinal phase space arise. Focus will be put on beam dynamics dominated by nonlinear momentum compaction, in particular the generation of a new bucket type "alpha-buckets" and possible applications. Relation of analytical theory, numerical simulations and experimental data will be presented and discussed. In addition, the current limitation due to the bursting instability at the Metrology Light Source bunches will be investigated. The majority of measurements were conducted at the Metrology Light Source complemented by measurements at the BESSY II storage ring.

Until recent years, terahertz (THz) waves were an undiscovered, or most importantly, an unexploited area of electromagnetic spectrum. This was due to difficulties in generation and detection of THz waves. Recent advances in hardware technology have started to open up the field to new applications such as THz imaging. This non-destructive and non-contact imaging technique can penetrate through diverse materials such that internal structures, in some cases invisible to other imaging modalities, can be visualized. Today, there are variety of techniques available to generate and detect THz waves in both pulsed and continuous fashion in two different geometries; transition, and reflection modes. In this thesis continuous wave THz imaging was employed for higher spatial resolution. However, with any new technology comes its challenges; automated processing of THz images can be quite cumbersome. Low contrast and the presence of a widely unknown type of noise make the analysis of these images difficult. In this work, there is an attempt to detect defects in composite material via segmentation by using a Terahertz imaging system. According to our knowledge, this is the first time that this type of materials are being tested under Terahertz cameras to detect manufacturing defects in aerospace industry. In addition, segmentation accuracy of THz images have been investigated by using a phantom. Beyond the defect detection for composite materials, this can establish some general knowledge about Terahertz imaging, its capabilities and limitations. To be able to segment the THz images successfully, pre-processing techniques are inevitable. In this thesis, a variety of different image processing techniques, self-developed or available from literature, have been employed for image enhancement. These methods range from filtering to contrast adjustment to fusion of phase and amplitude images by using fuzzy set theory, to just name a few. The result of pre-procssing and segmentation methods demonstrates promising outcome for future work in this field.

This work describes infrared (IR) photon detector techniques based on novel semiconductor device concepts and detector designs. The aim of the investigation was to examine alternative IR detection concepts with a view to resolve some of the issues of existing IR detectors such as operating temperature and response range. Systems were fabricated to demonstrate the following IR detection concepts and determine detector parameters: (i) Near-infrared (NIR) detection based on dye-sensitization of nanostructured semiconductors, (ii) Displacement currents in semiconductor quantum dots (QDs) embedded dielectric media, (iii) Split-off band transitions in GaAs/AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors. A far-infrared detector based on GaSb homojunction interfacial workfunction internal photoemission (HIWIP) structure is also discussed. Device concepts, detector structures, and experimental results discussed in the text are summarized below. Dye-sensitized (DS) detector structures consisting of n-TiO2/Dye/p-CuSCN heterostructures with several IR-sensitive dyes showed response peaks at 808, 812, 858, 866, 876, and 1056 nm at room temperature. The peak specific detectivity (D*) was 9.5E+10 Jones at 812 nm at room temperature. Radiation induced carrier generation alters the electronic polarizability of QDs provided the quenching of excitation is suppressed by separation of the QDs. A device constructed to illustrate this concept by embedding PbS QDs in paraffin wax showed a peak D* of 3E+8 Jones at ~540 nm at ambient temperature. A typical HEIWIP/HIWIP detector structures consist of single (or multiple) period(s) of doped emitter(s) and undoped barrier(s) which are sandwiched between two highly doped contact layers. A p-GaAs/AlGaAs HEIWIP structure showed enhanced absorption in NIR range due to heavy/light-hole band to split-off band transitions and leading to the development of GaAs based uncooled sensors for IR detection in the 2 5 μm wavelength range with a peak D* of 6.8E+5 Jones. A HIWIP detector based on p-GaSb/GaSb showed a free carrier response threshold wavelength at 97 µm (~3 THz)with a peak D* of 5.7E+11 Jones at 36 μm and 4.9 K. In this detector, a bolometric type response in the 97 - 200 µm (3-1.5 THz) range was also observed.

Until recent years, terahertz (THz) waves were an undiscovered, or most importantly, an unexploited area of electromagnetic spectrum. This was due to difficulties in generation and detection of THz waves. Recent advances in hardware technology have started to open up the field to new applications such as THz imaging. This non-destructive and non-contact imaging technique can penetrate through diverse materials such that internal structures, in some cases invisible to other imaging modalities, can be visualized. Today, there are variety of techniques available to generate and detect THz waves in both pulsed and continuous fashion in two different geometries; transition, and reflection modes. In this thesis continuous wave THz imaging was employed for higher spatial resolution. However, with any new technology comes its challenges; automated processing of THz images can be quite cumbersome. Low contrast and the presence of a widely unknown type of noise make the analysis of these images difficult. In this work, there is an attempt to detect defects in composite material via segmentation by using a Terahertz imaging system. According to our knowledge, this is the first time that this type of materials are being tested under Terahertz cameras to detect manufacturing defects in aerospace industry. In addition, segmentation accuracy of THz images have been investigated by using a phantom. Beyond the defect detection for composite materials, this can establish some general knowledge about Terahertz imaging, its capabilities and limitations. To be able to segment the THz images successfully, pre-processing techniques are inevitable. In this thesis, a variety of different image processing techniques, self-developed or available from literature, have been employed for image enhancement. These methods range from filtering to contrast adjustment to fusion of phase and amplitude images by using fuzzy set theory, to just name a few. The result of pre-procssing and segmentation methods demonstrates promising outcome for future work in this field.