L’objectif de ce travail de thèse est de développer des absorbants large bande et un détecteur thermique à base de matériaux carbonés issus de la pyrolyse de polymères, pour des applications dans le domaine terahertz. Deux types de polymères précurseurs ont été utilisés: un polymère organique (polyimide) et un hybride organique-inorganique (organopolysiloxane). La conversion thermique sous atmosphère inerte a été suivie par diverses techniques d’analyse : la thermogravimétrie couplée à la spectrométrie de masse, les spectroscopies Infrarouge et Raman. Les matériaux issus du polymère organique sont constitués principalement de carbone graphitique avec des rendements à la pyrolyse de 55% en poids. En revanche, la pyrolyse des organosiloxanes conduit à des matériaux céramiques de type composite dans lesquels des domaines de graphène sont répartis dans un réseau oxycarbure de silicium, avec des rendements de 85% en poids. L’absorption THz de 78%, sur une large bande 0.2-3 THz, a été obtenue sur l’échantillon d’épaisseur 0.95 mm, issu de la pyrolyse à 1500 °C du polymère organosilicé. Le principal avantage de l’utilisation de polymères réside dans la facilité de leur mise en forme pour obtenir des formes complexes aux dimensions souhaitées, grâce au procédé par coulage. Comme preuve de concept, la fabrication d’un micro-bolomètre, utilisant le matériau carboné céramique à structure pyramidale comme absorbant THz à large bande, a permis d’atteindre une sensibilité SV de 0.76V/W, une constante de temps de 180ms et un seuil de détection de puissance de 2 nW/Hz1/2, valeur avoisinant, voire meilleure, que celle des dispositifs actuellement disponibles sur le marché. / The aim of this work is the development of broadband absorbers and thermal detectors, based on carbon materials issued from pyrolysis of polymers and devoted to terahertz (THz) applications. Two types of polymer, organic (polyimide) and hybrid organic-inorganic polysiloxane, have been used. The progress of thermal conversion of these polymers in inert atmosphere was followed by thermogravimetric analysis coupled with mass spectrometry, and IR and Raman spectroscopies. The pyrolysis of polymers up to 1200 °C left black residues materials.Organic-derived materials are composed of mainly graphitic-carbon with a yield of about 55 wt%, while organosiloxane polymers result in a ceramic composite material, in which free carbon domains are embedded into an oxycarbide network, with a yield of 85 wt%.It has been found that the presence of sp2 carbon is necessary but not sufficient in and itself for the absorption of terahertz radiations. Carbon must be at least partially ordered into a few stacked-graphene layers. The THz absorbance of up to 78% over the broadband of 0.2-3 THz was obtained for a 0.95 mm thick sample resulting from pyrolysis of organosiloxane polymer at 1500 °C. The main advantage of using polymers lies in their ability to form complex small parts in a near net-shape by the casting process.Finally, using net-shaped pyramidal ceramic structures as absorbers, a proof of concept was achieved with the fabrication of a broadband terahertz micro-bolometer, with a responsivity of 0.76 V/W, time constant of 180 ms, and noise equivalent power of 2 nW/Hz1/2, thus putting it in fair competition with commercial thermal detectors.

Terahertz radiation refers to a specific part of the electromagnetic spectrum, flanked by microwave and infrared radiation at the higher and lower frequency end respectively. This thesis is about the development of applications using a new source of pulsed, coherent light for the physical characterisation of solids in the pharmaceutical setting. Terahertz radiation has excellent potential in the advancement of science but is, as yet, largely unexplored. Recent developments in semiconductor physics have made it possible to provide light at terahertz frequencies (a frequency of 1 THz equals a wavelength of 0.3 mm) in a relatively easy way. Light located in this range of the electromagnetic spectrum was very difficult to generate previously. It has unique properties in that it is possible to extract the full spectroscopic fingerprint of the materials by looking at the frequency response of the terahertz pulse. Here, vibrations of the whole molecule are probed rather than just the vibrations of single functional moieties within a molecules as is the case in infrared spectroscopy, which makes terahertz spectroscopy a very powerful tool for the analysis of the complex solid-state materials properties. In addition to structural information it easily penetrates through most plastics and polymeric materials used as excipients for pharmaceutical tablets. It is therefore especially useful for non-destructive imaging applications. At the example of polymorphic phase transitions, dehydration processes and crystallization experiments from the amorphous phase the potential of terahertz spectroscopy for pharmaceutical analysis was systematically investigated. In addition, a novel concept for using terahertz radiation in structural imaging of pharmaceutical dosage forms was developed. The technology, thus far predominantly used for the analysis of inorganic materials and semiconductors in particular, is now mature enough for its application to a wider field and to help with the understanding of fundamental and exciting new challenges at the interface between physics and the life sciences. Together with a comparison of this new technology to the established techniques in physical characterisation an initial attempt to understand and interpret the spectral information provided is presented. The potential for future applications is discussed.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: James Kolodzey, Electrical and Computer Engineering. Includes bibliographical references.

Thesis (M.E.E.)--University of Delaware, 2006. / Principal faculty advisor: James Kolodzey, Electrical and Computer Engineering. Includes bibliographical references.

Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T ≈ ħω/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/ħω factor will be compared. We could improve the kBT/ħω factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

Terahertz (THz) frequency range (wavelength of 300-30 μm, frequency of 1-10 THz and photon energy of ~4-40meV), the gap between infrared and microwave electromagnetic waves, have remained relatively unexplored for a long time, due to lack of a high power, coherent, and compact source, as well as the lack of an appropriate detector and the transmission devices. THz wave has recently received considerable attention for potential applications in non-invasive medical imaging, detecting trace of gases in the environment, sensing of organic and biological molecules, security controls, local oscillators for heterodyne receiver systems, free space communication, etc. THz quantum cascade laser (QCL), as the relatively high power and coherent THz radiation source, was demonstrated in 2002. After near a decade of intense research, THz QCLs operate only up to 186K in pulse mode with maximum power of 250 mW at 10 K. This thesis discusses many aspects of theoretical and experimental design considerations for THz QCLs. The objective is to obtain a laser device that emits high powers and works towards the temperatures achievable by thermoelectric coolers. This work includes designing the active gain medium, and the engineering of the waveguide and heat removal structures. A density matrix based model is developed to explain the charge transport and gain mechanism in the intersubband devices, particularly for three well resonant phonon based THz QCLs. The model allows for designing of the optimum and novel active gain mediums that work at higher temperatures. The designed active gain mediums are fabricated using discussed low loss waveguide and efficient heat removal structures. The maximum operating temperatures as high as ~176 K is achieved. Finally a promising lasing scheme based on phonon-photon-phonon emissions is proposed that improves the population inversion and offers high gain peak.

The potential of terahertz time domain spectroscopy has until recently been neglected in the field of the ultrafast magnetism. At the same time this technique can serve as a useful complementary tool with respect with conventional methods to investigate ultrafast magnetization dynamics. This thesis aims to implement time domain terahertz spectroscopy to observe high frequency spin waves excited optically in different magnetic systems. This work covers several distinct phenomena related to the study of spin waves (magnonics) at terahertz frequencies. The generation of transient broadband nonlinear magnetization via inverse Faraday effect in terbium gallium garnet is described in chapter 4. We demonstrate a remarkable discrepancy of at least two orders of magnitude between the strengths of the direct and inverse Faraday effects, thereby challenging the commonly accepted understanding of their relationship. Additionally, a striking nonlocality of the optical response is found. In chapter 5 the results of THz absorption spectroscopy of the terbium gallium garnet are reported. The garnet exhibits an intricate paramagnetic state with several magnetic sub-lattices at cryogenic temperatures under the application of strong magnetic fields. Some precessional modes of these sub-lattices were measured. The components of the g-tensor of terbium ions were extracted from the data. In chapter 6 the ultrafast magnetization dynamics of thulium orthoferrite, studied my means of terahertz spectroscopy, is described. It is demonstrated that terahertz response of the orthoferrite provides crucial additional information with respect to the optical pump-probe signal. A novel exchange driven mechanism of optical manipulation of the magnetic state is demonstrated. Finally, chapter 7 is a theoretical discussion of so called planar magnonic metamaterials. It is shown that the arrays of ferromagnetic films may exhibit negative refraction index at sub-terahertz frequencies, provided the mechanism of spin wave quantization is introduced. The thesis ends with a brief conclusions chapter where a short summary of the results is given. Some possible future extensions of the conducted research are drawn as well.

Terahertz (THz) frequency range (wavelength of 300-30 μm, frequency of 1-10 THz and photon energy of ~4-40meV), the gap between infrared and microwave electromagnetic waves, have remained relatively unexplored for a long time, due to lack of a high power, coherent, and compact source, as well as the lack of an appropriate detector and the transmission devices. THz wave has recently received considerable attention for potential applications in non-invasive medical imaging, detecting trace of gases in the environment, sensing of organic and biological molecules, security controls, local oscillators for heterodyne receiver systems, free space communication, etc. THz quantum cascade laser (QCL), as the relatively high power and coherent THz radiation source, was demonstrated in 2002. After near a decade of intense research, THz QCLs operate only up to 186K in pulse mode with maximum power of 250 mW at 10 K. This thesis discusses many aspects of theoretical and experimental design considerations for THz QCLs. The objective is to obtain a laser device that emits high powers and works towards the temperatures achievable by thermoelectric coolers. This work includes designing the active gain medium, and the engineering of the waveguide and heat removal structures. A density matrix based model is developed to explain the charge transport and gain mechanism in the intersubband devices, particularly for three well resonant phonon based THz QCLs. The model allows for designing of the optimum and novel active gain mediums that work at higher temperatures. The designed active gain mediums are fabricated using discussed low loss waveguide and efficient heat removal structures. The maximum operating temperatures as high as ~176 K is achieved. Finally a promising lasing scheme based on phonon-photon-phonon emissions is proposed that improves the population inversion and offers high gain peak.

Quantum cascade laser (QCL), as a unipolar semiconductor laser based on intersubband transitions in quantum wells, covers a large portion of the Mid and Far Infrared electromagnetic spectrum. The frequency of the optical transition can be determined by engineering the layer sequence of the heterostructure. The focus of this work is on Terahertz (THz) frequency range (frequency of 1 - 10 THz and photon energy of ~ 4 - 40 meV), which is lacking of high power, coherent, and efficient narrowband radiation sources. THz QCL, demonstrated in 2002, as a perfect candidate of coherent THz source, is still suffering from the empirical operating temperature limiting factor of T ≈ ħω/kB, which allows this source to work only under a cryogenic system. Most of high performance THz QCLs, including the world record design which lased up to ~ 200 K, are based on a resonant phonon (RP) scheme, whose population inversion is always less than 50%. The indirectly-pumped (IDP) QCL, nicely implemented in MIR frequency, starts to be a good candidate to overcome the aforementioned limiting factor of RP-QCL. A rate equation (RE) formalism, which includes both coherent and incoherent transport process, will be introduced to model the carrier transport of all presented structures in this thesis. The second order tunneling which employed the intrasubband roughness and impurity scattering, was implemented in our model to nicely predict the behavior of the QCL designs. This model, which is easy to implement and fast to calculate, could help us to engineer the electron wavefunctions of the structure with optimization tools. We developed a new design scheme which employs the phonon scattering mechanism for both injecting carrier to the upper lasing state and extracting carrier from lower lasing state. Since there is no injection/extraction state to be in resonance with lasing states, this simple design scheme does not suffer from broadening due to the tunneling. Finally, three different THz IDP-QCLs, based on phonon-photon-phonon (3P) scheme were designed, grown, fabricated, and characterized. The performance of those structures in terms of operating temperature, threshold current density, maximum current density, output optical power, lasing frequency, differential resistance at threshold, intermediate resonant current before threshold, and kBT/ħω factor will be compared. We could improve the kBT/ħω factor of the 3P-QCL design from 0.9 in first iteration to 1.3 and the output optical power of the structure from 0.9 mW in first design to 3.4 mW. The performance of the structure in terms of intermediate resonant current and the change in differential resistance at threshold was improved.

Quantum dots and nanorods are becoming increasingly important structures due to their potential applications that range from photovoltaic devices to medicine. The majority of the research on carrier dynamics in these structures has been in the optical regime, with little work performed at Terahertz frequencies where excitonic dynamics can be more directly probed. In this work, we examine theoretically the interaction of Terahertz radiation with colloidal CdSe nanorods to determine the dynamics of excitons generated via a short optical pulse. We calculate the energies and wavefunctions for the excitons within the envelope function approximation in the low density limit where there is at most one exciton per nanorod. The linear Terahertz transmittance and absorbance is found for nanorods that are approximately 70 nm in length and 7 nm in diameter and are compared with experimental results that have shown the first observation of intra-excitonic transitions in nanorods. We find absorbance peaks at 8.5 THz and 11 THz that result from polarizations in the longitudinal (rod axis) and transverse directions respectively. Our theoretical results show that the 8.5 THz and 11 Thz peaks are due to 1s-2pz and 1s-2px transitions respectively. The theoretical absorbance spectra is in good agreement with the experimental one and show that only the ground state is significantly populated 1 ps after optical excitation. This provides strong evidence of rapid trapping of excited holes into the ligand used to passivate the nanorods. A full set of dynamical equations were then constructed from Heisenberg's equation of motion, and were used to model the excitonic correlations as a function of time. Transmittance and absorbance were calculated for different nanorod orientations and electric field strengths in both the linear and nonlinear regime. These results were then averaged over nanorod orientation in order to more accurately reflect experimental conditions. Nonlinearity was found to become significant at peak pulse field strengths of 7 kV/cm and greater. Due to two-photon processes, we predict the 2pz-3dz transition that is not observed in the linear regime will be clearly seen in the nonlinear absorbance spectrum. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2014-05-14 23:37:58.604