SAW based sensors can offer wireless, passive operation in numerous environments and various device embodiments are employed for retrieval of the sensed data information. Single sensor systems can typically use a single carrier frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor and retrieve the sensed information. This dissertation presents the concept of orthogonal frequency coding (OFC) for applications to SAW sensor technology. OFC offers all advantages inherent to spread spectrum communications including enhanced processing gain and lower interrogation power spectral density (PSD). It is shown that the time ambiguity in the OFC compressed pulse is significantly reduced as compared with a single frequency tag having the same code length and additional coding can be added using a pseudo-noise (PN) sequence. The OFC approach is general and should be applicable to many differing SAW sensors for temperature, pressure, liquid, gases, etc. Device embodiments are shown and a potential transceiver is described. Measured device results are presented and compared with COM model predictions to demonstrate performance. Devices are then used in computer simulations of the proposed transceiver design and the results of an OFC sensor system are discussed.

Surface acoustic wave (SAW) technology has been in use for well over one century. In the last few decades, due to its low cost and high performance, this technology has been widely adopted in modern wireless communication systems, to build filtering devices at radio frequency (RF). SAW filters and duplexers can be virtually found inside every mobile handset. SAW devices are traditionally recognized as passive devices with high linear signal processing behavior. However, recent deployments of third generation (3G) and fourth generation (4G) mobile networks require the handsets to handle an increasing number of frequency bands with more complex modulation /demodulation schemes and higher data rate for more subscribers. These requirements directly demand more stringent linearity specifications on the front end devices, including the SAW duplexers. In the past, SAW duplexer design was based on empirically obtained design rules to meet the linearity specifications. Lack of predictability and an understanding of the root cause of the nonlinearity have limited the potential applications of SAW duplexers. Therefore, research on the nonlinearity characterization and an accurate modeling of SAW nonlinearity for mobile device applications are very much needed. The Ph.D. work presented here primarily focuses on developing a general nonlinear model for SAW resonators/duplexers. Their nonlinear characteristics were investigated by measuring the harmonic and intermodulation distortions of resonators. A nonlinear Mason model is developed and the characterization results are integrated into SAW duplexer design flows to help to simulate the nonlinear effects accurately and improve the linearity performance of the products. iv In this dissertation, first, a novel nonlinear Mason equivalent circuit model including a third order nonlinear coefficient in the wave propagation is presented. Next, the nonlinear distortions of SAW resonators are analyzed by measuring large-signal harmonic and intermodulation spurious emission on resonators using a wafer probe station. The influence of the setups on the measurement reliability and reproducibility is discussed. Further, the nonlinear Mason model is validated by comparing its simulation results with harmonic and intermodulation measurements on SAW resonators and a WCDMA Band 5 duplexer. The Mason model developed and presented here is the first and only nonlinear physical model for SAW devices based on the equivalent circuit approach. By using this new model, good simulation measurement agreements are obtained on both harmonic and intermodulation distortions for SAW resonators and duplexers. These outcomes demonstrate the validity of the research on both the characterization and modeling of SAW devices. The result obtained confirms that the assumption of the representation of the 3 rd order nonlinearity in the propagation by a single coefficient is valid

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This report presents a comprehensive investigation into the accuracy and stability of Finite Difference Methods (FDM) when applied to the acoustic wave equation. The analysis focuses on comparing the classical 2nd order FDM with highly-accurate computational stencil of order 2p = 2,4 and 6 with Summation-by-Parts (SBP) and Simultaneous Approximation Term (SAT) technique of the Finite Difference Method. The objective of the study is to investigate complex numerical techniques that contributes to highly-accurate and stable solutions to many hyperbolic PDEs.  The report starts by introducing the governing problem and studies its well-posedness to ensure stable and unique solutions of the governing equations. It continues with basic introduction to the classic spatial discretization of the FDM and introduces the SBP-SAT implementation of the method. The governing equations are rewritten as a semi-discrete problem, such that it can be written as a system of ordinary differential equations (ODEs) only dependent on the temporal evolution. This system can be solved with classic Runge-Kutta methods to ensure robust and accurate time-stepping schemes.  The results show that the implementation of the higher-order SBP-SAT Finite Difference Method provides highly accurate solutions of the acoustic wave equation compared to the classic FDM. The results also show that the method provides stable solutions with no visible oscillations (dispersion), which can be a challenge for higher order methods. Overall, this paper contributes with valuable insights into the analysis of accuracy and stability in finite difference methods for acoustic wave equation.

By projecting observed microseismic data backward in time to when fracturing occurred, it is possible to locate the fracture events in space, assuming a correct velocity model. In order to achieve this task in near real-time, a robust computational system to handle backward propagation, or Reverse Time Migration (RTM), is required. We can then test many different velocity models for each run of the RTM. We investigate the use of a Graphics Processing Unit (GPU) based system using Compute Unified Device Architecture for C (CUDA-C) as the programming language. Our preliminary results show a large improvement in run-time over conventional programming methods based on conventional Central Processing Unit (CPU) computing with Fortran. Considerable room for improvement still remains. / text

Ces travaux ont donc visé le développement d’une plateforme complète de détection de gaz pour environnement sévère haute température. Cette plateforme intègre un dispositif à ondes acoustiques de surface sur substrat Langasite, une résistance chauffante, une couche sensible inorganique nanostructurée et est placée dans une enceinte hermétique. Des températures de l’ordre de 450°C ont été atteintes et des tests de cyclages ont démontré un fonctionnement en accord avec les modèles théoriques et une reproductibilité des mesures. Des tests de détection de composés organiques volatils (éthanol et toluène) ont mis en avant des seuils de détection de l'ordre de quelques ppm. / Measuring pollutants concentrations in gas and vapors emissions are important environmental issues. This work presents a stand-alone portable device for high temperature assessment. The system includes a Langasite (LGS) acoustic sensor, a ceramic heater and a platform with RF connections for remote in-situ measurements. The packaging consists in a hermetic stainless steel cell which enables safe gas detection. In situ temperature measurements have been achieved and the thermal behavior was successfully investigated in the temperature range 25-450°C. The designed cell highlights good agreement with theoretical models and reproducibility of the measures. Volatile organic compounds exposures have been investigated and promising ppm level detections have been obtained.

Diese Arbeit befaßt sich mit der SAW-STM-Methode, einer Abwandlung der Rastertunnelmikroskopie (engl. scanning tunneling microscopy) zur hochauflösenden Abbildung akustischer Oberflächenwellen (engl. surface acoustic wave). Das Meßprinzip des SAW-STM beruht auf der Modulation des Tunnelabstandes und der hieraus resultierenden Modulation des Tunnelstroms durch die oszillatorische Bewegung der Probenoberfläche. Zur einfacheren Signaldetektion wird ein Heterodyn-Verfahren verwendet, das über eine zusätzliche Modulation der Tunnelspannung das SAW-STM Signal in den kHz-Frequenzbereich verschiebt. Dieses trägt die komplette Information über die Amplitude und die Phase der Tunnelabstandsmodulation. Im Rahmen dieser Arbeit wird die Abhängigkeit des SAW-STM Signals von den experimentellen Paramtern und von der Topographie der Probe theoretisch beschrieben und experimentell untersucht. Dies führt zu einer kalibrierten Vermessung der oszillatorischen Bewegungsbahn der SAW-tragenden Probenoberfläche. Das untersuchte Materialsystem ist ein 40 nm dicker Goldfilm auf einem LiNbO3-Substrat im Y-Schnitt, das eine Welle vom Rayleigh-Typ in Z-Richtung trägt. Indem ein SAW-STM entwickelt wird, das im Ultrahochvakuum (UHV) arbeitet, wird die SAW-STM Methode hinsichtlich des Spektrums untersuchbarer reaktiver Materialien und hinsichtlich der Signalstabilität. Es wird der Umbau eines kommerziellen Rastertunnelmikroskops zum SAW-STM beschrieben. Zu diesem Zweck wird eine Probenhalterung konstruiert, die den Transfer der Probe zwischen der Bedampfungseinrichtung und dem STM ohne Belüftung der Kammer erlaubt. Gleichzeitig stellt sie automatisch die fünf notwendigen elektrischen Kontakte zum Probenchip her, wenn die Probenhalterung in das STM gesetzt wird. Weiterhin werden die Konstruktion eines UHV-tauglichen Sytems von Hochfrequenz-Signalleitungen und der Bedampfungseinrichtung zur in-situ Probenpräparation beschrieben. Mit diesem UHV-SAW-STM können SAWs einer Amplitude im Bereich von 0.001 Å bis 1 Å angeregt und detektiert werden. Die maximale Frequenz, die eine im UHV-SAW-STM nachweisbare SAW haben kann, beträgt mindestens 360 MHz. Weiterhin wird das in LabVIEW geschriebenen Softwarepacket zur Auswertung der SAW-STM-Daten vorgestellt. Um die SAW-STM-Methode auf ein sicheres Fundament zu stellen, wurde erstmals systematisch die Abhängigkeit des SAW-STM-Signals von den Meßparametern experimentell untersucht. Die im Rahmen dieser Arbeit entwickelte Theorie des SAW-STM-Signals beschreibt die experimenell gefundenen Abhängigkeiten hinreichend gut. Zentraler Punkt dieser Theorie ist dabei die Berücksichtigung der Abstandsregelung des STM, die im Constant-Current-Modus die Spitzenposition so regelt, daß der mittlere Tunnelstrom konstant ist. Der Vergleich der gemessenen und der theoretischen Abhängigkeit der Signalamplitude von der Amplitude der Modulation des Tunnelabstandes ermöglicht dabei die kalibrierte Messung der Amplitude der vertikale Auslenkung der Rayleighwelle. Scans der SAW-STM-Methode liefern Bilder der Topographie sowie der Amplitude und der Phase des SAW-STM-Signals. Mit der Theorie des SAW-STM-Signal der korrugierten Probenoberfläche werden aus der gemessenen Topographie simulierte Amplituden- und Phasebilder erstellt und mit den gemessenen Bildern verglichen. Während die Übereinstimmung mit den gemessenen Amplitudenbildern nur qualitativer Art ist, erlaubt der quantitative Vergleich zwischen simulierten und gemessenen Phasenbildern die Bestimmung der Exzentrizität der Oszillationsellipse. Zusammen mit der oben erläuterten Messung der vertikalen Auslenkungsamplitude ist somit in dieser Arbeit die Oszillationsellipse der Rayleighwelle vollständig ausgemessen. / This thesis deals with the SAW-STM method, which is a technique based on scanning tunneling microscopy (STM) for the high-resolution mapping of surface acoustic waves (SAW). The measurement principle of the SAW-STM utilizes the modulation of the tunneling distance and the resulting modulation of the tunneling current due to the mechanic oscillation of the sample surface. To facilitate signal detection a heterodyn technique is employed which shifts the measured signal into the kHz-range by adding modulation to the tunneling voltage. The signal contains the entire information about the amplitude and the phase of the SAW-induced tunneling distance modulation. Experiments are presented to investigate the dependence of the signal amplitude on the experimental parameters and on the sample topography. This data is compared to a theory developed to describe the SAW-STM signal, leading to a calibrated measurement of the trajectory of the sample surface which carries the wave. The sample we investigated was a 40 nm gold film deposited on a LiNbO3 substrate in the Y-cut, deformed by a Rayleigh-type wave in the z-direction. To increase signal stability and to extend the range of reactive sample materials we constructed a SAW-STM operating in the ultra-high-vacuum (UHV). In this thesis the conversion of a commercial STM to a SAW-STM is described. A sample holder is constructed, which allows the transfer of the sample between the evaporation stage and the STM without venting the UHV chamber. It provides a contact spring mechanism for the automatical electric contactation of the sample chip, when the sample holder is inserted into the STM. Moreover, we installed a UHV-compatible wiring system for SAW-excitation and for signal detecting and an evaporation stage for in-situ sample preparation. We demonstrate that the UHV-SAW-STM is capable of exiting and detecting surface acoustic waves with an amplitude in the range 0.001 Å to 1 Å. The maximal frequency of SAW which can be studied with the UHV-SAW-STM is found to be at least 360 MHz. For the analysis of SAW-STM data a LabVIEW software package was implemented. To put the SAW-STM technique on a strong basis, we systematically studies the dependence of the SAW-STM signal on the various measurement parameters. The theory of the SAW-STM signal developed in this work is in good agreement with this experimental data. In this theory, we take into account that the STM is operated in the constant-current mode, i.e. the tip position is controlled to keep the average tunneling current constant. The comparison of the measured and the simulated dependence of the signal amplitude on the amplitude of the tunneling distance modulation allows the calibrated measurement of the vertical displacement amplitude of the Rayleigh wave. SAW-STM scans yield images of the topography and of the amplitude and the phase of the SAW-STM signal. Employing the theory of the SAW-STM signal on the corrugated surface, we simulated amplitude and phase imaged based on the measured topography. The agreement between simulated and measured amplitude images is only qualitative. In contrast, the comparison of simulated and measured phase images allows the determination of the excentricity of the oscillation ellipse. Having determined this excentricity and the vertical displacement amplitude of the Rayleigh wave, we have gaind complete knowledge about the geometry of the Rayleigh wave oscillation ellipse.

Mes travaux de thèse portent sur la construction de conditions aux limites absorbantes (CLAs) pour des problèmes de propagation d'ondes posés dans des milieux limités par des surfaces régulières. Ces conditions sont nouvelles car elles prennent en compte non seulement les ondes proagatives (comme la plupart des CLAs existantes) mais aussi les ondes évanescentes et rampantes. Elles sont donc plus performantes que les conditions existantes. De plus, elles sont facilement implémentables dans un schéma d'éléments finis de type Galerkine Discontinu (DG) et ne modifie pas la condition de stabilité de Courant-Friedrichs-Lewy (CFL). Ces CLAs ont été implémentées dans un code simulant la propagation des ondes acoustiques ainsi que dans un code simulant la propagation des ondes en régime harmonique. Les comparaisons réalisées entre les nouvelles conditions et celles qui sont les plus utilisées dans la littérature montrent que prendre en compte les ondes évanescentes et les ondes rampantes permet de diminuer les réflexions issues de la frontière artificielle et donc de rapprocher la frontière artificielle du bord de l'obstacle. On limite ainsi les coûts de calcul, ce qui est un des avantages de mes travaux. De plus, compte tenu du fait que les nouvelles CLAs sont écrites pour des frontières quelconques, elles permettent de mieux adapter le domaine de calcul à la forme de l'obstacle et permettent ainsi de diminuer encore plus les coûts de calcul numérique. / In my PhD, I have worked on the construction of absorbing boundary conditions (ABCs) designed for wave propagation problems set in domains bounded by regular surfaces. These conditions are new since they take into account not only propagating waves (as most of the existing ABCs) but also evanescent and creeping waves. Therefore, they outperform the existing ABCs. Moreover, they can be easily implemented in a discontinuous Galerkin finite element scheme and they do not change the Courant-Friedrichs-Lewy stability condition. These ABCs have been implemented in two codes that respectively simulate the propagation of acoustic waves and harmonic waves. The comparisons performed between these ABCs and the ABCs mostly used in the litterature show that when we take into account evanescent and creeping waves, we reduce the reflections coming from the artificial boundary. Therefore, thanks to these new ABCs, the artificial boundary can get closer to the obstacle. Consequently, we reduce the computational costs which is one of the advantages of my work. Moreover, since these new ABCs are written for any kind of boundary, we can adapt the shape of the computational domain and thus we can reduce again the computational costs.

The non-hydrostatic dynamical core ENDGame (Even Newer Dynamics for the General Atmospheric Modelling of the Environment) is extended into the thermosphere to test its feasability as a whole-atmosphere dynamical core that can simulate the large scale fluid dynamics of the whole atmosphere from the surface to the top of the thermosphere at 600km. This research may have applications in the development of a Sun-to-Earth modelling system involving the Met Office Unified Model, which will be useful for space weather forecasting and chemical climate modelling. Initial attempts to raise the top boundary of ENDGame above ∼100km give rise to instabilities. To explore the potential causes of these instabilities, a one dimensional column version of ENDGame: ENDGame1D, is developed to study the effects of vertically propagating acoustic waves in the dynamical core. A 2D ray-tracing scheme is also developed, which accounts for the numerical effects on wave propagation. It is found that ENDGame’s numerics have a tendency towards the excessive focussing of wave energy towards vertical propagation, and have poor handling of large amplitude waves, also being unable to handle shocks. A key finding is that the physical processes of vertical molecular viscosity and diffusion prevent the excessive growth of wave amplitudes in the thermosphere in ENDGame, which may be crucial to improving ENDGame’s stability as it is extended upwards. Therefore, a fully implicit-in-time implementation of vertical molecular viscosity and diffusion is developed in both ENDGame1D and the full three-dimensional version of ENDGame: ENDGame3D. A new scheme is developed to deal with the viscous and diffusive terms with the dynamics terms in a fully coupled way to avoid time-splitting errors that may arise. The combination of a small amount of off-centring of ENDGame’s semi-implicit formulation and the inclusion of vertical molecular viscosity and diffusion act to make ENDGame significantly more stable, as long as the simulation is able to remain stable up to the molecularly diffused region above an altitude of ∼130km.

We studied the possibility of achieving an order of magnitude reduction in the energy dissipation needed to write bits in perpendicular magnetic tunnel junctions (p-MTJs) by simulating the magnetization dynamics under a combination of resonant surface acoustic waves (r-SAW) and spin-transfer-torque (STT). The magnetization dynamics were simulated using the Landau-Lifshitz-Gilbert equation under macrospin assumption with the inclusion of thermal noise. We studied such r-SAW assisted STT switching of nanomagnets for both in-plane elliptical and circular perpendicular magnetic anisotropy (PMA) nanomagnets and show that while thermal noise affects switching probability in in-plane nanomagnets, the PMA nanomagnets are relatively robust to the effect of thermal noise. In PMA nanomagnets, the resonant magnetization dynamics builds over few 10s of cycles of SAW application that drives the magnetization to precess in a cone with a deflection of ~45⁰ from the perpendicular direction. This reduces the STT current density required to switch the magnetization direction without increasing the STT application time or degrading the switching probability in the presence of room temperature thermal noise. This could lead to a pathway to achieve energy efficient switching of spin-transfer-torque random access memory (STT-RAM) based on p-MTJs whose lateral dimensions can be scaled aggressively despite using materials with low magnetostriction by employing resonant excitation to drive the magnetization away from the easy axis before applying spin torque to achieve a complete reversal.