Physics and Astronomy

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This is the collection for the University of Waterloo's Department of Physics and Astronomy.

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Browsing Physics and Astronomy by Author "Bajcsy, Michal"

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Now showing 1 - 6 of 6
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    Creating and probing laser-cooled atomic ensembles inside a hollow-core optical fibre
    (University of Waterloo, 2024-01-26) Anderson, Paul; Bajcsy, Michal
    A laser-cooled atomic ensemble confined inside a hollow-core optical fiber offers a unique platform for enhanced light-matter interactions and their applications. At the same time, transferring a cloud of laser-cooled atoms from a free space magneto-optical trap into the few-micron diameter core of the optical fiber presents a host of experimental challenges (and requires optimization of in a multidimensional parameter space). This thesis investigates loading of laser-cooled caesium atoms into a hollow-core photonic-crystal fiber, develops diagnostic methods to optimize the process and probe the atoms inside the fiber, presents initial experiments exploring the optical properties of the fiber-confined atomic ensemble, and discusses the potential uses of fiber-confined atomic ensembles in ’hybrid’ quantum repeaters that utilize quantum dots as source of entangled photon pairs. Fluorescence-based methods are also employed to estimate atom numbers and assess temperature of the atomic cloud collected initially in the magneto-optical trap and to aid in the alignment of the atom cloud with the fiber’s core. Machine learning, specifically Gaussian processes, is explored as a means to optimize experimental parameters. M-LOOP, a Python-based tool, is utilized for this purpose, demonstrating its ability to navigate around local minima. The influence of dipole beam characteristics, such as intensity and resonance, on loading efficiency is examined, considering factors like Stark shifts and trap depth. The dissertation also delves into two-photon electromagnetically induced absorption (TPEIA) with cold atomic cesium, highlighting the importance of optical depth for efficient wavelength conversion. The ladder scheme is discussed, showcasing its potential for quantum memory systems with modest delays in electromagnetically induced transparency (EIT) media. The concept of slow light under EIT conditions is presented, illustrating its utility in optical communication traffic buffering and quantum memory. We also discuss the potential uses of this platform in a quantum repeater.
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    Laser-cooled Atomic Ensembles in Hollow Optical Fibers
    (University of Waterloo, 2021-05-06) Venuturumilli, Sai Sreesh; Bajcsy, Michal
    This thesis explores hollow-core fibres as a platform for quantum optics experiments with laser-cooled atomic ensembles. The non-diffracting, tightly-confined guided modes of these fibers grant us a ~µm-wide one-dimensional space to study atom-light interactions. In order to describe on-going experiments, simulations are carried out to understand atomic motion into the hollow fibers. Following which, a preliminary case study of a quantum optics experiment to convert wavelengths of single photons with Cs atomic ensembles inside the hollow fiber is presented. Lastly, basic optical properties of photonic crystal membranes are briefly explored. These can form novel cavities when appended to hollow fibers.
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    Optical Resonators Integrated into a Hollow Core Photonic Crystal Fiber for Enhanced Light-Matter Interactions
    (University of Waterloo, 2019-08-29) Flannery, Jeremy; Bajcsy, Michal
    The focus of this thesis is to investigate and fabricate a platform that can facilitate the enhancement of light-matter interactions. By tightly confining the photons and atoms to the same region of space, the probability of interaction is drastically increased as compared to free space interactions. Specifically, we focus on forming an optical cavity (or resonator) incorporated into a hollow-core photonic crystal fiber (HCPCF) using two distinct types of reflective mirrors along the axis of the fiber while still permitting atoms to be loaded into the region of high field confinement. One means of pursuing this goal that we explored was to propose and numerically simulate two methods for implementing Bragg gratings in a HCPCF. These two methods leave the hollow-core unobstructed and are both based on controlled selective injection of photosensitive polymers into the photonic-crystal region of the hollow-core fiber, followed by interference photolithography. We report the results of numerical simulations for the hollow core fiber with Bragg gratings formed by the two methods. We find that a reflectivity of > 99.99% should be achievable from such fiber-integrated mirrors. Such a device could support high cooperativity and strong coupling regimes to be achieved. We also demonstrate a fiber-integrated Fabry-Pérot cavity formed by attaching a pair of dielectric metasurfaces to the ends of a hollow-core photonic-crystal fiber segment. The metasurfaces consist of perforated membranes designed as photonic-crystal slabs that act as planar mirrors but can potentially allow injection of gases through their holes into the hollow core of the fiber. We have so far observed cavities with finesse of 11 and Q-factors of ~ 4.5 × 10^5, but much higher values should be achievable with improved fabrication procedures. We expect this device to enable the advancement of new fiber lasers, enhanced gas spectroscopy, and studies of fundamental light-matter interactions and nonlinear optics. These mirrors can be designed to be polarization dichroic — transparent for one polarization and reflective for another. This unique property can be exploited to allow for all signals to be directed along the high optical depth axis of the cavity and may provide a excellent platform for applications such as optical switching. Finally, we develop a novel protocol for a single photon all-optical transistor and how it may be implemented in the above mentioned fiber cavity systems. This unique scheme utilizes a far off-resonant vacuum cavity mode to stimulate a Raman absorption process of a source photon which may be switched off by the insertion of a single gate photon into the cavity mode. Relatively high switching contrasts and ratios for the source photon transmission can be obtained in our system.
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    Spin-Preserving Metasurface-Based Focusing Mirror for Enhancing Light-Matter Interactions
    (University of Waterloo, 2021-10-01) Kuru, Fehime Sema; Bajcsy, Michal
    We propose a dielectric metasurface mirror that focuses one spin state while diverging the other state and preserves the spin state upon reflection, unlike conventional mirrors. First, we discuss the working principle of the mirror and introduce an earlier version of the design to discuss important potential drawbacks to a metasurface design. Then, we simulate a mirror design that can preserve the spin state up to 99.6%. Overall, the simulations give 81% reflectivity for the desired spin state, half of which is due to material loss. A Fabry Pérot optical cavity formed by a pair of such mirrors would have a finesse of 15 and Q value of 1964. We find the focusing of the mirror to have good quality, with a Strehl ratio of 0.88. We simulate a cavity numerically to find the mode profile after 120 roundtrips. We estimate a mode volume of 725 μm³ for a cavity with length 56μm and mirror size 15μm. Our metasurface design has potential to be used in quantum optics to enhance light-matter interactions and optical nonlinearities. The reflectivity of the mirror can be further enhanced by overcoming material loss, which would allow a high finesse cavity for single spin state to be built. Last, we construct and characterize with broadband polarization tomography a fiber integrated quarter-waveplate formed by misaligning and splicing a short section of polarization maintaining fiber with precise length.
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    Superradiance and its implementation in cold atoms inside a hollow-core waveguide
    (University of Waterloo, 2019-05-23) Ding, Zhenghao; Bajcsy, Michal
    In this thesis, I am intending to understand the cooperative effect of an ensemble of quantum emitters, which constitutes the preliminary elements of our current experimental investigations towards realization of an ultra-narrow linewidth superriant laser. In the first part of the thesis, I investigate the basics of the theory of superradiance (SR), which includes the full derivation of the Hamiltonian and the Lindblad equation for an ensemble of two-level atoms in both free-space and a single-mode waveguide. In addition, I construct the simulations for observing the transition from single-atom uncorrelated spontaneous emission to superradiance in various physical settings, as well as a simulation for the understanding of the cooperative effects of an ensemble of two-level atoms inside an optical cavity. Then, in the second part of the thesis, I introduce the experimental progress we have been making to observe SR with an ensemble of laser-cooled Cs atoms inside a hollow-core photonic crystal fiber (HCPCF). In our experiment, the Cs atoms, initially cooled using a magneto-optical trap (MOT), are guided and confined inside a short piece of HCPCF with a magic-wavelength dipole trap. Currently we have successfully implemented a novel detection methods for studying superradiance.
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    Techniques for Coherent Control of Effective Photon-Photon Interactions at Low Light Levels
    (University of Waterloo, 2019-02-01) Vickers, Cameron; Bajcsy, Michal
    This thesis proposes improvements on techniques for the coherent control of photon-photon interactions mediated by a cloud of cold atoms. Atoms cooled and trapped in a MOT can be loaded into a hollow, photonic crystal waveguide to ensure light coupled into the waveguide is localized on the tightly confined atoms. The low temperature of the atoms decreases decoherence rates, and improves coherent light-matter interactions, such as electromagnetically induced transparency and slow light. Predictions are made for two future experiments using this system, modulating of the phase of a laser beam with a much weaker one, and modulating the transmission of a laser through two photon absorption. These predictions indicate that the phase of a 0.1μW laser beam can be modulated by as much as one milliradian per weak laser photon, and that a 800pW laser beam can modulate the transmission of a comparably powerful laser by as much as 60%. Both of these predictions would be improvements on results for similar experiments performed with warm atoms.