An optomechanical system consists of an optical cavity mode coupled to a mode of a mechanical oscillator. Depending on the configuration of the system, the optomechanical interaction can be used to drive or cool the mechanical mode, coherently swap the optical and mechanical states, or create entanglement. A multimode optomechanical system consists of many optical (mechanical) modes coupled to a mechanical (optical) mode. With the tools of the optomechanical interaction, multimode optomechanical systems provide a rich platform to study new physics and technologies. A central challenge in optomechanical systems is to mitigate the effects of the thermal environment, which remains significant even at cryogenic temperatures, for mechanical oscillators typically used in optomechanical systems. The central theme of this thesis is to study how the properties of multimode optomechanical systems can be used for such mitigation of thermal noise. The most straightforward extension of an optomechanical system to a multimode system is to have a single optical mode couple to two mechanical modes, or a single mechanical mode couple to two optical modes. In this thesis, we study both types of multimode system. In each case, we study the formation of a dark mode, an eigenstate of the three-mode system that is of particular interest. When the system is in a dark state, the two modes of similar character (optical or mechanical) interact with each other through the mode of dissimilar character, but due to interference, the interaction becomes decoupled from the properties of the dissimilar mode. Another interesting application of the three-mode system is two-mode optical entanglement, generated through mechanical motion. Such entanglement tends to be sensitive to thermal noise. We propose a new method for generating two-mode optical entanglement in the three-mode system that is robust against the thermal environment of the mechanical mode. Finally, we propose a novel, scalable architecture for a quantum computer. The architecture makes use of the concepts developed earlier in the thesis, and applies them to a system that on the surface looks quite different from the standard optomechanical system, but is formally equivalent. This dissertation includes previously published and unpublished coauthored material.

La micro-structuration périodique d’un milieu influence fortement la propagation des ondes élastiques. Ces structures, nommées cristaux phononiques, permettent de manipuler les ondes en les ralentissant, voire en interdisant leurs propagations pour certaines gammes de fréquences appelées bandes interdites. Au cours de la thèse, la dispersion des ondes élastiques dans des structures composées de piliers résonants périodiquement attachés à des poutre structurées a été numériquement et expérimentalement étudiée. Basées sur la combinaison de résonances locales avec des réflexions multiples de Bragg, des poutres phononiques avec de larges bandes interdites ont été obtenues. En particulier, une poutre phononique avec une bande interdite présentant une largeur relative de 94% autour du mégahertz a été fabriquée et caractérisée.Les poutres phononiques ont été utilisées pour améliorer les facteurs de qualité des résonances de défauts insérés en fin, ou entre les poutres. Une amélioration d’un facteur 8 d’une résonance d’un défaut encastrée-libre accroché à une poutre phononique a été démontrée. Cette amélioration est attribuée au confinement de l’énergie mécanique par la poutre phononique dont la bande interdite recouvre la fréquence de résonance du défaut. En parallèle de ces activités, une plateforme pompe sonde hétérodyne femtoseconde a été utilisée pour caractériser la propagation des ondes élastiques à la surface de films minces microstructurés. Une forte anisotropie ainsi qu’une réduction des vitesses des ondes a été mis en évidence. Ces observations ont été reliées aux micro-structurations particulières des films, avec des simulations en éléments finis. / Periodic microstructures in media strongly influences the propagation of elastic waves. These structures, known as phononic crystals, allow the manipulation of waves such as their slowdown, or their prohibition for frequency ranges called band gaps.During the thesis, the dispersion of elastic waves propagating through structures composed of resonant pillars periodically attached to tailored strips was studied. Based on the combination of Bragg scattering and local resonances, phononic strips with large band gaps were achieved. Notably, a phononic strip with a band gap displaying a relative width of 94% in the megahertz range was demonstrated.Phononic strips were then used to ameliorate the resonances’ quality factors for defects inserted at the end, or within the strips. An improvement by a factor of 8 of the resonance quality factor of a beam clamped to a phononic strip was achieved. This improvement was assigned to the mechanical energy confinement linked with the phononic strip, as its band gap overlaps the resonance frequency of the defect.In parrallel to these studies, a femtosecond heterodyne pump probe setup was used to characterize the waves propagation on the surface of microstructured thin films. A strong anisotropy as well as a speed reduction were observed. These observations were linked to the particular microstructures of the films, using numerical simulations performed with the Finite Element Method.

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Materials at the nanoscale show properties uniquely different from the bulk scale which when controlled can be utilized for variety of thermal management applications. Different applications require reduction, increase or directional control of thermal conductivity. This thesis focuses on investigating thermal transport in two such application areas, viz., 1) thermoelectric energy conversion and 2) thermal rectification. Using molecular dynamics simulations, several methods for reducing of thermal conductivity in polyaniline and polyacetylene are investigated. The reduction in thermal conductivity leads to improvement in thermoelectric figure of merit. Thermal diodes allow heat transfer in one direction and prevents in the opposite direction. These materials have potential application in phononics, i.e., for performing logic calculations with phonons. Rectification obtained with existing material systems is either too small or too difficult to implement. In this thesis, a more useful scheme is presented that provides higher rectification using a single wall carbon nanotube (SWCNT) that is covalently functionalized near one end with polyacetylene (PA). Although several thermal diodes are discussed in literature, more complex phononic devices like thermal logic gates and thermal transistors have been sparingly investigated. This thesis presents a first design of a thermal AND gate using asymmetric graphene nanoribbon (GNR) and characterizes its performance. / Ph. D.

The objective of this dissertation is to improve the performance of surface acoustic wave (SAW) biosensors for use in point-of-care-testing (POCT) applications. SAW biosensors have the ability to perform fast, accurate detection of an analyte in real time without the use of labels. However, the technology suffers from the inability to differentiate between specific and non-specific binding. Due to this limitation, direct testing of bodily fluids using SAW sensors to accurately determine an analyte's concentration is difficult. In addition, these sensors are challenged by the need to detect small concentrations of a biomarker that are typically required to give a clinical diagnosis. Sensitivity, selectivity and reliability are three critical aspects for any sensing platform. To improve sensitivity the delay path of a SAW sensor has been modified with microcavities filled with various materials. These filled cavities increased sensitivity by confining wave energy to the surface by way of constructive interference and waveguiding. Thus, the improved sensitivity will result in a lower limit of detection. In addition, insertion loss is decreased as a consequence of increased wave confinement to the surface. Sensor selectivity and reliability are adversely affected by non-specific binding of unwanted species present in a sample. To address this issue a multifunctional SAW sensor is presented. The sensor consists of two SAW delay lines oriented orthogonal to each on ST-quartz in order to generate two distinct wave modes. One wave mode is used for sensing while the other is used to remove loosely bound material. By using the same transduction mechanism for both removal and sensing, the sensor chip is simplified and complex electronics are avoided. The findings of this research involve the technological advances for SAW biosensors that make their use in POCT possible.

Ultrafast spectroscopy allows us to probe and understand material properties. With it, we can measure phonon-polaritons (optical phonons coupled with light) and the resulting dispersion curve in lithium niobate. Customizing the excitation source in ultrafast measurements can excite phonon modes to large amplitudes, allowing the experimental exploration of the Potential Energy Surface in solids. However, stronger pump fluences and bigger signal isn't always the answer in ultrafast spectroscopy. When sample signals and their nonlinear and mechanisms cannot be distinguished with 1D measurements, simple 2D THz measurements are a great place to start searching for distinct factors as was the case in cadmium tungstate. 2D measurements when paired with modeling and first principles calculations can reveal cutting edge information about exciting materials.

Phononic crystals are structures composed of periodically arranged scatterers in a background medium that affect the transmission of elastic waves. They have garnered much interest in recent years for their macro-scale properties that can be modulated by the micro-scale components. The elastic properties of the composite materials, the contrast in the elastic properties of the composite materials, and the material arrangement all directly affect how an elastic wave will behave as it propagates through the sonic structure. The behavior of an elastic wave in a periodic structure is revealed in its transmission bandstructure, and modification of any the elastic parameters will result in tuning of the band structure. In this dissertation, a phononic crystal with properties that can be modulated using electromagnetic radiation, and more specifically, radio-frequency (RF) light will be presented.

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Motivated by previous relevant research on phononics including both active and passive phononics, the interest of faster turnability and more functions of the active phononics of further study led to this proposing research topic: magnetic field tunable active functional phononics. The first design of magnetic field tunable reciprocal--non-reciprocal transmission acoustic device was established, material was characterized, and numerical simulation has been performed. The simulation results show clear T-symmetric breaking non-reciprocity due to energy level splitting effect with Doppler effect – an acoustic Zeeman effect. Inspired by this preliminary work, further experiments were planned to demonstrate this effective Zeeman effect in phononics and effectively charged phonons in water based ferro-fluid. The objectives of this work as the next series of tasks were to illustrate acoustic Zeeman effect and acoustic Landau levels in various strength of magnetic field to investigate a design non-reciprocal sound device with magnetic field switching, which could be controlled on the amount of non-reciprocity with the strength of magnetic field. Once this new field first discovered by the proposed study tasks, more active tunable magnetic field phononics devices could be designed and exemplified in terms of both simulations and experiments. Faster and more controllable active phononic devices could be designed and made based on this study. The experimental maximum non-reciprocity was measured as 22 dB difference and the amount of the non-reciprocity can be further controlled by adjusting the strength of the external magnetic field. The remote pumping system in the device worked as expected and did not introduce any impact of the cavity properties.

This thesis theoretically and experimentally studies the characteristics of integrated microresonators (MRs) built by passive (no gain) and non-magnetic materials and characterized by both Hermitian and non-Hermitian Hamiltonians. In particular, I have studied three different microresonators: a typical Microring Resonator (MR), a Taiji Microresonator (TJMR), which consists of a microresonator with an embedded S-shaped waveguide, and a new geometry called the Infinity-Loop Microresonator (ILMR), which is characterized by a microresonator shaped like the infinity symbol coupled at two points to the bus waveguide. To get an accurate picture of the three devices, they were modeled using both the transfer matrix method and the temporal coupled mode theory. Neglecting propagation losses, the MR is described by a Hermitian Hamiltonian, while the TJMR and the ILMR are described by a non-Hermitian one. An important difference between Hermitian and non-Hermitian systems concerns their degeneracies. Hermitian degeneracies are called Diabolic Points (DPs) and are characterized by coincident eigenvalues and mutually orthogonal eigenvectors. In contrast, non-Hermitian degeneracies are called Exceptional Points (EPs). At the EP, both the eigenvalues and the eigenvectors coalesce. The MR is at a DP instead, and the TJMR and the ILMR are at an EP. Since the TJMR and ILMR are at an EP, they have interesting features such as the possibility of being unidirectional reflectors. Here, it is shown experimentally how in the case of the TJMR this degeneracy can also be used to break Lorentz reciprocity in the nonlinear regime (high incident laser powers), discussing the effect of the Fabry-Perot of the bus waveguide facets. The effect of backscattering, mainly due to the waveguide surface-wall roughness, on the microresonators is also studied. This phenomenon induces simultaneous excitation of the clockwise and counterclockwise modes, leading to eigenvalue splitting. This splitting makes the use of typical quality factor estimation methods unfeasible. To overcome this problem and mitigate the negative effects of backscattering, a new experimental technique called interferometric excitation is introduced. This technique involves coherent excitation of the microresonator from both sides of the bus waveguide, allowing selective excitation of a single supermode. By adjusting the relative phase and amplitude between the excitation fields, the splitting in the transmission spectrum can be eliminated, resulting in improved quality factors and eigenvalue measurements. It is shown that this interferometric technique can be exploited under both stationary and dynamic conditions of time evolution. The thesis also investigates the sensing performance of the three microresonators as a function of a backscattering perturbation, which could be caused, for example, by the presence of a molecule or particle near the microresonator waveguide. It is shown that the ILMR has better performance in terms of responsivity and sensitivity than the other two microresonators. In fact, it has both the enhanced sensitivity due to the square root dependence of the splitting on the perturbation (characteristic of EPs) and the ability to completely eliminate the region of insensitivity as the backscattering perturbation approaches zero, which is present in both the other two microresonators. To validate the models used, they were compared with experimental measurements both in the linear regime and, for TJMR, also in the nonlinear regime, with excellent agreement.