Recent experimental results have shown that the vibration induced by the tire air cavity resonance is transmitted into the vehicle cabin and may be responsible for significant interior noise. The tire acoustic cavity is excited by the road surface through the contact patch on the rotating tire. The effect of the cavity resonance is that results in significant forces developed at the vehicle's spindle, which in turn drives the vehicle's interior acoustic field. This tire-cavity interaction phenomenon is analytically investigated by modeling the fully coupled tire-cavity systems. The tire is modeled as an annular shell structure in contact with the road surface. The rotating contact patch is used as a forcing function in the coupled tire-cavity governing equation of motion. The contact patch is defined as a prescribed deformation that in turn is expanded in its Fourier components. The response of the tire is then separated into static (i.e. static deformation induced by the contact patch) and dynamic components due to inertial effects. The coupled system of equations is solved analytically in order to obtain the tire acoustic and structural responses. The model provides valuable physical insight into the patch-tire-acoustic interaction phenomenon. The influence of the acoustic cavity resonance on the spindles forces is shown to be very important. Therefore, the tire cavity resonance effect must be reduced in order to control the tire contribution to the vehicle interior. The analysis and modeling of two feasible approaches to control the tire acoustic cavity resonances are proposed and investigated. The first approach is the incorporation of secondary acoustic cavities to detune and damp out the main tire cavity resonance. The second approach is the addition of damping directly into the tire cavity. The techniques presented in this dissertation to suppress the adverse effects of the acoustic cavity in the tire response, i.e. forces at the spindle, show to be very effective and can be easily applied in practice. / Ph. D.

InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1−xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1−xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating δ p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room temperature in p- and n-type GaN were 93±7 nm and 432±30 nm, respectively. Moreover, near field scanning optical microscopy was employed to investigate the spatial variations of extended defects and their effects on the optical quality of semipolar and InGaN heterostructures, which are promoted for higher efficiency light emitters owing to reduced internal polarization fields. The near-field PL from the c+ wings in heterostructures was found to be relatively strong and uniform across the sample but the emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations and basal plane stacking faults. In case of heterostructures, striated regions had weaker PL intensities compared to other regions and the meeting fronts of different facets were characterized by higher Indium content due to the varying internal field. Apart from being the part and parcel of blue LEDs, InGaN heterostructures can be utilized in generation of coherent lattice vibrations at terahertz frequencies. In analogy to LASERs based on photon cavities where light intensity is amplified, acoustic nanocavity devices can be realized for sustaining terahertz phonon oscillations which could potentially be used in acoustic imaging at the nanoscale and ultrafast acousto-optic modulation. Using In0.03Ga0.97N/InxGa1-xN MQWs with varying x, coherent phonon oscillations at frequencies of 0.69-0.80 THz were generated, where changing the MQW period (11.5 nm -10 nm) provided frequency tuning. The magnitude of phonon oscillations was found to increase with indium content in quantum wells, as demonstrated by time resolved differential transmission spectroscopy. Design of an acoustic nanocavity structure was proposed based on the abovementioned experimental findings and also supported by full cavity simulations. Optical gap engineering and carrier dynamics in colloidal Ge1−xSnx QDs were investigated in order to explore their potential in optoelectronics. By changing the Sn content from 5% to 23% in 2 nm-QDs, band-gap tunability from 1.88 eV to 1.61 eV, respectively, was demonstrated at 15 K, consistent with theoretical calculations. At 15 K, time resolved PL spectroscopy revealed slow decay (3 − 27 μs) of luminescence, due to recombination of spin-forbidden dark excitons and effect of surface states. Increase in temperature to 295 K led to three orders of magnitude faster decay (9 − 28 ns) owing to the effects of thermal activation of bright excitons and carrier detrapping from surface states. These findings on the effect of Sn incorporation on optical properties and carrier relaxation and recombination processes are important for future design of efficient Ge1−xSnx QDs based optoelectronic devices. This thesis work represents a comprehensive optical study of InGaN heterostructures and colloidal Ge1−xSnx QDs which would pave the way for more efficient InGaN based LEDs, realization of terahertz generating nanocavities, and efficient Ge1−xSnx based silicon compatible optoelectronic devices.

Pour élargir le domaine fréquentiel d’analyse vibroacoustique des méthodes éléments finis et de la SEA (Statistical Energy Analysis), la Méthode Énergétique Simplifiée MES utilisant des variables quadratiques permet de considérer aussi bien les moyennes que les hautes fréquences. Cette méthode est basée sur une analyse des ondes propagatives vibratoires et acoustiques. Dans ce travail, la formulation inverse de la MES est proposée. A partir d’un vecteur de densité d’énergie et intensité vibroacoustique données, la formulation MES inverse permet de remonter aux sources. Elle permet d’identifier les sources de vibration dans des systèmes bidimensionnels (plaque excitée en flexion,...) et les sources acoustiques dans des systèmes tridimensionnels (cavité acoustique excitée,...). La formulation MES inverse est numériquement validée dans plusieurs cas de figures. Une analyse paramétrique est effectuée afin de tester la robustesse et l’efficacité de cette approche. Par exemple, la sensibilité avec les données d’entrée ou à la nature des sources envisagées est traitée.Une comparaison entre les résultats numériques obtenus par la MES et ceux obtenus par la SEA est abordée permettant de présenter les avantages de la MES au niveau de l’identification des sources. Une application industrielle de la MES inverse est réalisée dans le cadre de ce travail. Elle montre la fiabilité de la méthode pour le cas d’une cabine excitée par un bruit blanc. Finalement, la MES est utilisée pour réduire les nuisances sonores détectées dans les cabines. Un programme d’optimisation est développé permettant de trouver la meilleure répartition des absorbants et de définir leurs caractéristiques. / To widen the frequency domain of vibroacoustic analysis of finite elements methods and the SEA (Statistical Energy Analysis), Simplified Energy Method MES (french abbreviation) using quadratic variables can cover as well medium as high frequencies. This method is based on an analysis of the vibratory and acoustic propagative waves. In this work, Inverse MES formulation is proposed. From a vector of energy density and vibroacoustic intensity data, inverse MES formulation can raise sources. It makes it possible to identify vibration sources in two-dimensional systems (excited plate in inflection...) and acoustic sources in three-dimensional systems (excited acoustic cavity...). Inverse MES formulation is numerically validated in several cases. A parametric analysis is carried out in order to test the robustness and the effectiveness of this approach. For example, the sensitivity with the data input or with nature of considered sources is treated. A comparison between numerical results founded by MES and those founded by SEA are discussed to present MES advantages at identifying sources. An industrial application ofthe inverse formulation of the method is carried out within the framework of this work. It shows its reliability in the case of a cabin excited by a white noise. Finally, MES shows its utility to reduce harmful sound detected in cabins. An optimization program is developed making it possible to find the best distribution of the poroelastic layers and to define their characteristics.

In this thesis, an output-only experimental modal testing and analysis technique known as the Natural Excitation Technique (NExT) is examined for use with large residential structures and interior cavities. The technique which assumes a random, stationary input causing the response data is reviewed and extended for the first time to include the assumption of an impulse input. This technique is examined with respect to the experimental modal analysis of single and two room residential structures. Each structure is first tested using conventional modal testing methods. Then, NExT is applied using each structure's response to a simulated sonic boom, an impulsive input. The results of these analyses along with the results obtained from a finite element model are compared. Then, the interior cavities enclosed by the residential structures are examined using NExT. Therefore, this thesis also demonstrates the successful use of NExT on acoustic systems for the first time. Three configurations of the interconnected cavities enclosed by the two room structure are considered to study physical phenomena. Both interior pressure response to random, stationary inputs and the sonic boom response are used with NExT to determine modal properties. The results of these analyses are compared to a theoretical analysis. Advantages to using NExT with both the response to a random, stationary input and an impulsive input are demonstrated for structural and acoustic systems. / Master of Science