QUANTUM OPTICS WITH OPTOMECHANICAL SYSTEMS IN THE LINEAR AND NONLINEAR REGIME: WITH APPLICATIONS IN FORCE SENSING AND ENVIRONMENTAL ENGINEERING

Optomechanical system, a hybrid system where mechanical and optical degrees of freedom are mutually coupled, is a new platform for studying quantum optics. In a typical optomechanical setup, the cavity is driven by a large amplitude coherent sate of light to enhance the effective optomechanical coupling. This system can be linearized around its classical steady state, and many interesting effects arise from the linearized optomechanical interaction, such as the dynamical modification of the properties of the mechanical resonator and the modulation of the amplitude and phase of the light coming out the of cavity. When the single-photon optomechanical coupling is comparable to the optical and mechanical loss, we must also keep the nonlinear interactions in the hamiltonian, which make it possible to study optomechanically induced nonlinear phenomena such as photon-blockade, Kerr nonlinearity, etc. In this thesis, we study quantum optics with optomechanical systems both in the linear and nonlinear regime, with emphasis on its applications in force sensing and environmental engineering. We first propose a mirror-in-the-middle system and show that when driving near optomechanical instability, the optomechanical interaction will generate squeezed states of the output light. This system can be used to detect weak forces far below the standard quantum limit. Subsequently, we find that this particular driving scheme can also lead to enhanced optomechanical nonlinearity in a certain regime and by measuring the output field appropriately. We study the photon-blockade effect and discuss the conditions for maximum photon antibunching. We then focus on thermal noise reduction for mechanical resonators, by designing a system of two coupled resonators whose damping is primarily clamping loss. We show that optomechanical coupling to the clamping region enables dynamical control over the coupled mechanical resonator. This leads to the counterintuitive outcome: increasing optical power simultaneously reduces the temperature and linewidth of the mechanical mode, in contrast to direct optomechanical cooling. We also consider the Brillouin scattering induced optomechanical interaction in ring wave-guide resonators where phonon scattering via impurities is present. We find that it is possible to realize chiral transport behavior of phonons by modifying the phonon environment with optomechanics. We study a simple few-mode theory and it can explain experimental data well. Finally, we study a continuum multi-mode theory and calculate the phonon Green's function using a diagrammatic perturbative expansion, showing that a decrease in the phonon diffusion constant is possible with increasing optical pump power

Suppression of disorder-induced scattering in optomechanical systems

High-Q optical and mechanical resonators have been utilized in ultra-high precision metrology, transducers, sensor applications, and even investigating quantum mechanics at mesoscales. However, these low-loss devices are often limited by sub-wavelength fluctuations within the host material, that may be frozen-in or even dynamically induced. Rayleigh scattering is observed in nearly all wave-guiding technologies today and can lead to both irreversible radiative losses as well as undesirable intermodal coupling. The mitigation of disorder-induced scattering is extremely challenging for micro and nanoscale devices, as surface roughness, which causes Rayleigh scattering, is unavoidable in microfabrication processes (Appl Phys Lett 85, 17, 2004; J Lightwave Technol 24, 12, 2006). Minimizing disorder-induced Rayleigh backscattering has thus been a significant challenge until now. It has been shown that backscattering from disorder can be suppressed by breaking time-reversal symmetry in magneto-optic (Sov Phys JETP, 59, 1, 1984; Phys Rev B, 37, 1988) and topological insulator materials (Phys Rev B, 38, 1988; Nature, 461, 7265, 2009). Yet, common monolithic dielectrics, which are basic building ingredients of high-Q resonators, possess neither of these properties. Fortunately, we develop a novel technique to break time-reversal symmetry without magneto-optic in a high-Q optical cavity pumped by a single-frequency laser through parity-selective optomechanics. Such optomechanical interaction is achieved by Brillouin scattering, owing to the phase-matching condition. This method enables complete linear optical isolation without requiring magnetic fields. Instead, the isolation originates from a nonreciprocal induced transparency based on a coherent light-sound interaction, where light and sound are coupled bv a traveling-wave Brillouin scattering interaction. That breaks time-reversal symmetry within the waveguide-resonator system. Our result demonstrates that material agnostic and wavelength-agnostic optical isolation is far more accessible in chip-scale photonics than previously thought. However, isolators block backscattering from systems, but cannot prevent disorder-induced backscattering inherently. In order to minimize disorder-induced backscattering, we experimentally demonstrate robust phonon transport in the presence of material disorder. This is achieved by explicitly inducing chirality through the parity-selective optomechanical coupling. We show that asymmetric optical pumping of a symmetric resonator enables a dramatic chiral cooling of clockwise and counterclockwise phonons, while simultaneously suppressing the hidden action of disorder. Surprisingly, this passive mechanism is also accompanied by a chiral reduction in heat load leading to optical cooling of the mechanics without added damping, an effect that has no optical analog. This technique can potentially improve upon the fundamental thermal limits of resonant mechanical sensors, which cannot be attained through sideband cooling. This new mechanism can be also expanded to the optics domain, where Rayleigh scattering severely limits the performance of devices in the limit of microscale. We have demonstrated an optomechanical approach for dynamically suppressing Rayleigh light backscattering within optical resonators. Similar to the previous method, we achieve this by locally breaking time-reversal symmetry in a silica resonator through a Brillouin scattering interaction that is available in all materials. Near-complete suppression of Rayleigh backscattering is experimentally confirmed through three independent measurements -- the reduction of the back-reflections caused by scatterers, the elimination of a commonly seen normal-mode splitting effect, and by measurement of the reduction in intrinsic optical loss. More broadly, our results suggest that it is possible to dynamically suppress Rayleigh backscattering within any optical dielectric medium using time-reversal symmetry breaking, for achieving robust light propagation in spite of scatterers or defects. Our proposal is not limited by a specific form of time-reversal symmetry breaking through Brillouin scattering in optical cavities. It can be realized in linear waveguides under different time-reversal symmetry approaches such as acousto-optic, nonlinear-optics, and PT symmetry breaking technique

Sounding the reef: comparative acoustemologies of underwater noise pollution / Pejling af revet: komparativ akustemologi af undersøisk støjforurening

Matthew Buttacavoli studied the development and detection of the category of underwater noise pollution in the Great Barrier Reef. He examined the phenomenon using a multi-species ethnographic approach. He found that bodily affordances and species’ boundaries that make sensing underwater noise difficult, can be overcome through technology and skilled practice. [Extract from Danish abstract] Denne afhandling vælger en etnografisk strategi til at undersøge, hvordan interesserede lyttere forsøger at opfatte og rekonstruere det akustiske havmiljø. Observation og interviews af deltagere sættes sammen med optagelse og kreative metoder for at kortlægge de (ufuldkomne) måder, hvorpå lyttere (herunder forfatteren) forsøger at forstå havskabningers lydverdener. I fokus er de lyttemetoder, der blev udviklet af dykkere, havforskere, akustikere og interesseorganisationer

Strategies of empirical justification in experimental science

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

The Impact of a Visual Approach Used in the Teaching of Grammar When Embedded into Writing Instruction: A Study on the Writing Development of Chinese First Year University Students in a British University in China

Born into a visual culture, today’s generation of learners generally prefer a visually-rich multimodal learning environment. Tapping into the potential of visuals in language pedagogy, this study was aimed at discovering the impact of a visual presentation of grammatical concepts related to sentence structure on student writing. The study used a mixed methods design to analyse the impact of the visual approach first by statistically measuring sentence variety and syntactic complexity of student pre and post intervention texts and then using interviews to explain the nature of the impact of visuals on student conceptual understanding and its effect on their writing development. Statistical findings reveal that the experimental groups of Chinese students who were taught grammatical concepts in the context of writing instruction using a visual approach outperformed the students in the control groups who were given similar lessons in the context of writing instruction but using traditional printed hand-outs. Qualitative findings suggest that the visuals seems to have increased these students’ conceptual understanding of grammatical items that were taught, and this resulted in more sophisticated and syntactically complex texts after the intervention. The study supports the theory of contextualized teaching of grammar and proposes the use of external visuals that lead to internal visualization based on the cognitive theory of multimodal learning. In so doing, it extends the use of visual learning to grammar pedagogy. However, the findings also suggest that the visual approach would not work effectively in cultures that promote rote learning and decontextualized exercises in grammar with the sole aim of passing the exams. A shift in attitude towards grammar pedagogy in China is deemed necessary