In modern-day research, magnetometry provides valuable information for a wide range of studies. Among all the different forms of magnetometers, the nitrogenvacancy (NV) lattice defect in diamond has emerged as a powerful magnetic field sensor thanks to the combination of sensitivity, spatial resolution and versatile capabilities. High-fidelity microwave control and optical readout of the NV spin over a wide range of conditions has enabled applications in condensed matter physics, chemistry, biology, geoscience and many more. In particular, its capability of visualizing magnetic phenomena with high spatial resolution has proven to be a powerful tool in both fundamental physics and applied sciences. Advances in NV magnetometry in the past decade have led to numerous breakthroughs, especially in revealing the nanoscale physics of condensed matter systems. However, the free-space optics generally used for optical interrogation of the NV spins are challenging to realize in cryogenic, intra-cellular, or other hard-to-reach environments. As such, realizing robust all-fiber-based NV probes with efficient optical readout could enable new measurements in low-temperature (quantum) or biological systems...@en

The current frontier of human exploration is space. We have achieved sending rovers to planets and probes throughout the Solar System and beyond into interstellar space, yet the stars seemed to be out of reach. The current fastest man-made object would take thousands of years to travel to the nearest star, Proxima Centauri. However, Breakthrough Starshot presents an engineering challenge to reach it in a generation's time. Recent technological advancements make this possible through ultralight spacecraft equipped with mirror-like sails, known as Lightsails. Due to their structure, these sails could be propelled by lasers up to 20% light speed, or over 200 million km/h.
To achieve the required efficiency, the sails need to exhibit high reflectivity and low mass on the scale of 1g, while covering a surface area of ∼10m2. Some materials and membranes explored in literature have exhibited high reflectivity, but their mass did not comply with the necessary requirements. The most promising current design is a photonic crystal, which interacts with light in a way that maximizes reflectivity and removes material by incorporating cavities into the surface, effectively reducing overall mass.
Current research into photonic crystal Lightsails primarily focuses on the reflectivity of a small segment of a flat lattice. While this is essential, the literature suggests that the full-scale structures will probably behave like traditional sails on sailboats, tending to billow. This prompts us to consider not only the problem of reflectivity on curved surfaces but also mechanical deformations and stresses within a membrane that's 1000 times thinner than a human hair.
This thesis investigates the full-scale design of the Lightsail from both the structural and electromagnetic sides. Firstly, the material, microstructure, and macrostructure of the Lightsail are analyzed to determine stresses and deformations, as well as the optimal shape for stress distribution and efficiency. Then, through topology optimization of shell elements, the main load-carrying "backbone" of the sail is identified. Secondly, optimization of unit cell photonic crystal cavities is performed across the curvature of the sail derived from the previously obtained shape, maximizing the reflectivity. Finally, the thesis proposes a design of the Lightsail that integrates all findings, which is then used to obtain a Figure of Merit value for comparison with designs in existing literature.

This paper presents the design of a lab-scale silicon nitride photonic crystal lightsail, demonstrating the LightSailSim software package. LightSailSim, an open-source analysis pipeline, integrates a custom mechanical solver based on a particle system model with optical simulation results. The resulting design balances dynamic and structural stability with propulsive performance. It consists of an optimised photonic crystal that provides a suitable trade-off between stability and propulsive force. The sail is suspended at its edges by a circular ring made of silicon, providing sufficient boundary tension to prevent sail deformation-induced instability. The design is made such that it can be produced from a single silicon wafer and levitated using a single 400 W laser.

Artificial intelligence has a strong need for faster and more energy-efficient solutions, especially for computation performed at the sensor edge. On-chip photonic neural networks (PNNs) offer a promising solution for high speeds and energy efficiency. A less explored side of PNNs is their application to time-series data, which is often the case for real-world sensor applications. While PNNs promise high speeds and energy-efficient solutions, no good use cases have been proposed. This report will first review the state of the art of PNNs. It will be seen that to solve time-series tasks, photonic continuous-time recurrent neural networks (CTRNNs) are required. The dynamics of CTRNNs are thoroughly explored to leverage obtained insights to recommend novel practical applications. This is done through simple examples, and applied to a real machine learning task. It was seen that on a real classification task the network learned two distinct fixed points corresponding to the classes. A link between the time constant of the continuous-time neurons and the temporal dynamics of the task is also found. Two general directions for novel applications are then proposed. Firstly, photonic PNNs can be slowed down to match the task. Opto-electronic PNNs allow for more control of the time constant, and on-chip photonic filter neurons are suggested. Secondly, the high speeds of photonic neural networks can also be directly leveraged. Extremely fast convergence of photonic CTRNNs can be utilized for Modern Hopfield networks, pathfinding algorithms, and time-dependent optimization problems such as for example Model Predictive Control.

Electron energy-loss spectroscopy (EELS) is a powerful analytical technique used in transmission electron microscopy (TEM) to investigate the energy loss of electrons as they interact with a specimen. EELS provides valuable information about the electronic structure, composition, and bonding properties of materials at the nanoscale. Despite its tremendous potential, EELS techniques often face challenges related to spectral resolution and signal-to-noise ratio. In this context, the present project aims to significantly enhance the EELS spectra resolution by innovatively designing and fabricating a specialized Transmission Electron Microscope (TEM) holder. By leveraging advances in photonics and micro-resonator technology, this project seeks to revolutionize the quality and accuracy of EELS measurements, opening new avenues for high-resolution material characterization. The motivation behind this project lies in the critical need to overcome existing limitations in EELS techniques and push the boundaries of nanoscale material analysis. Traditional EELS setups, while offering valuable insights, often suffer from challenges associated with background noise, limited energy resolution, and compromised signal quality due to multiple scattering events. These factors hinder the ability to extract precise information about material properties, impeding progress in various scientific and technological fields. Motivated by these challenges, the central aim of this project is to design and develop a TEM holder that incorporates cutting-edge photonic micro-resonator technology. This combi- nation is expected to improve EELS abilities. The proposed holder will enable controlled interactions between the electron beam and the photonic micro-resonator, exploiting quantum optical phenom- ena to enhance the energy-loss signal and reduce unwanted noise. This approach aligns with recent advancements in Photon Induced Near-field Electron Microscopy (PINEM), quantum optics and electron-photon interactions, paving the way for unprecedented sensitivity and resolution in EELS measurements. The outcomes of this project hold immense promise for diverse scientific disciplines. In materials science, researchers will gain deeper insights into the electronic behavior and properties of nanoma- terials, facilitating the design and optimization of advanced materials with tailored functionalities. Additionally, the improved EELS resolution will impact fields such as catalysis, nanoelectronics, and biological imaging, where precise characterization at the nanoscale is essential. To achieve our objectives, we will address the following key research questions: • What are the optimal design parameters for the micro-resonator, facilitating efficient electronphoton interactions and maximal photon generation per electron? • What strategies can be employed to mitigate potential sources of photon loss within the integrated TEM holder, such as losses in coupling, transmission, and detection, thereby maximizing the efficiency of photon collection and measurement? • How do the characteristics of the micro-resonator, such as its size, shape, and material properties, impact the generation and propagation of cavity photons, and how can these parameters be tailored for optimal EELS performance? We expect that the results of this project will revolutionize the field of material characterization by enabling unprecedented high-resolution EELS measurements, poised to impact diverse scientific and technological domains.

This research explores the integration of Neural Architecture Search with Optical Neural Networks to optimize the efficiency and performance in traditional visual image classification tasks. The study introduces a new approach that applies Neural Architecture Search, a technique traditionally used to optimize the performance of Artificial Neural Networks, to the field of Optical Neural Networks.

Research has shown concepts of phononic integrated circuits (PnICs) consisting of multiple acoustic waveguides and beam splitters. Traveling acoustic waves experience energy loss when moving. Minimizing these losses is critical for large and complex PnICs. By fabricating on-chip PnICs, it is possible to operate under vacuum and at low temperatures resulting in lower acoustic losses.
Studies have shown on-chip data transport using straight acoustic waveguides fabricated inside suspended membrane phononic crystals. The use of high stress Silicon Nitride (SiN) has enabled
the manufacturing of high aspect ratio suspended structures. This is achieved by stress keeping the membrane under tension resulting in a flat surface.
Current acoustic beam splitter designs feature liquid and air for the wave supporting material. Making them not suitable for on-chip devices. Therefore, on-chip PnICs require a new type of splitter
design.

In this study, finite element methods are used to investigate if suspended SiN membranes offer a solution for on-chip acoustic beam splitting. The wave confinement of square and hexagonal lattice splitters are also compared to find the best performing design. Phononic bandstructures are constructed by performing eigenfrequency studies on various 2D phononic crystal unit cells. These bandstructures reach full GHz bandgaps with relative bandgap sizes of 57.3%.
Symmetric beam splitters are then made by introducing line defects into the crystal structures. Multiple wave excitation locations, frequencies and waveguide widths are investigated to establish the highest wave confining splitter design. The final design consists of a y-shaped splitter inside a suspended SiN hexagonal lattice phononic crystal and achieves a wave confinement of 85.03% at an excitation frequency of 3.25GHz.
The results show that suspended SiN membranes featuring hexagonal lattice phononic crystals offer a promising solution for on-chip beam splitting.

Due to the demand for renewable energies, gasses likeH2 need to be detected with sensitive, accurate and fast detectors. The US Department of Energy has made a list of requirements that H2 sensors need to fullfill of which the minimum detectable concentration (0.1%) and the reaction time (one second) are challenging for current hydrogen sensors, particularly for low-power and low-cost mass-produced sensors. This thesis covers the design and simulation of a photonic crystal nanobeam cavity sensor, using the Pt-catalyzed WO3 as a H2 sensitive material. The thicknesses of both layers show a trade-off between the minimally detectable concentration and the reaction times, resulting in a cavity with sensitivity of almost 40 nm/RIU and a Q factor varying between 5e5 and 5e4 depending on how much catalyst is required to meet the one second reaction time performance target. Based on the conservative assumptions regarding the reaction times of the sensor, the 0.1% performance target is almost achieved. This means that if in practice, any of the layer thicknesses show to be more favourable than assumed in this thesis, together with the fact that for optical sensors there is no risk of sparks, the photonic crystal nanobeam cavity as a H2 sensor shows to be a good fit for a high performance, low-cost, mass-produced H2 sensor that meets the DoE performance targets.

Nanophotonics is the study of structures’ interaction with light with features at or below the nanometer scale. It has gained the interest of many researchers, as it can be used to control the flow of light very effectively in the design of, e.g., solar cells, highly efficient biosensors or lasers. The design of such devices can be non-intuitive and complex and therefore computational tools like topology optimization techniques have been used to improve their designs. However, the topology optimization methods used in the literature often use a density-based representation of the geometry, which often leads to jagged edges. It has been shown in the literature that jagged edges can deteriorate the accuracy of simulation results. Using a level-set method in combination with an enriched finite element method offers a smoother boundary representation than the often used density-based methods. This work aims to develop an analysis and level set optimization for 2D electromagnetic scattering and eigenvalue problems using an enriched finite element method. Furthermore, we showcase that even for a non-conforming discretization, the enriched finite element method achieves the same convergence properties as the standard finite element method with fitted meshes. Finally, we perform topology optimization on the design of both a 2D meta lens and 2D reflector, maximizing their ability to focus light onto a point, using a level set method to define the geometry in combination with the enriched method used in the analysis.

Suspended microchannel resonators are a kind of mechanical resonator with an embedded microchannel inside. This is done as for some samples it is convenient or even necessary to put the sample into a fluid. The sample can then travel through the channel and its mass can be determined. The channel also simplifies placement of the samples compared to regular nanomechanical resonators. By having this channel embedded within the resonator, it is then possible to place the resonator as a whole in a vacuum and so eliminate medium losses. The main purpose of these suspended microchannel resonators is mass spectrometry which has already been performed on viruses and biological cells and so could potentially be very useful for the medical field as well. Especially if suspended microchannel resonators can be used to measure individual proteins which has not been done before with these resonators. It would also be interesting to use these kinds of resonators in quantum mechanical experiments. This means that the performance of these suspended microchannel resonators needs to be improved.

In order to accomplish this, a new kind of suspended microchannel resonator is introduced. The determination of these designs is strictly FEM based. The new resonator design, which is a doubly clamped beam, is made from pre-stressed silicon nitride which enables high Q-factors caused by an effect known as dissipation dilution. Dissipation dilution means diluting the energy losses of the system by increasing the stored energy which is increased by the initial stress in the silicon nitride. To increase this effect, the resonator is tapered towards the center. This is called strain engineering. Silicon nitride has been used in suspended microchannel resonators before but it did not result in higher Q-factors. Possibly due to clamping losses. The designs presented here offer a solution to that in the form of soft-clamping. This reduces the curvature near the boundaries to a minimum which means that the clamping losses are also reduced or eliminated. The combined effect of these features results in very high Q-factors as has already been reported for nanomechanical resonators. Through machine learning optimization it is possible to achieve 6E12 Hz in Qf product required for quantum mechanical experiments at room temperature and more than one order of magnitude improvement in mass sensitivity.

In the early years of numerical simulation methods, Fermi, Pasta, Ulam and Tsingou (FPUT) discovered that an undamped, weakly nonlinear equation describing the motion of a chain of masses and springs could show complex dynamics. Integration of these equations froma n initial displacement in the formof the fundamental mode resulted in significant mode coupling: energy was transferred from the fundamental mode to several other modes, before the energy would return to the initial condition. To date, very little observations of such behavior in mechanical vibrations have been reported. Recent developments in fabrication of high stress Silicon-Nitride (Si3N4) string resonators have shown that it is possible to generate resonators with extremely high Q-factors, proving a potential testbed for these mechanics. This research shows, through modal conversionof the FPUT potential, that one may observe significant FPUT behavior in systems with non-integer frequency ratios and certain coupling coefficients. In addition, it is shown that for the default FPUT beta-model, the effect of damping is negligible for fundamental mode Q-factors higher than 10,000. Simulations of the experimental
frequency response of a high-Q Si3N4 string resonator show that the nonlinear dynamics of these resonators may be approximated by an analytical model that does not possess the required frequency ratios and coupling coefficients for FPUT behavior. Another string model, for which no mechanical equivalent
has (yet) been found, may potentially show FPUT behavior. Several string-like resonator designs are tested using a numerical tool which can extract the modal coefficients. These resonators are modelled using simplified
deformation models, which account only for axial deformation of the structure. The results for various string-like designs show that the eigenfrequencies and nonlinearity may be engineered easily, but these do
not generate the required coupling coefficient for FPUT behavior.

Nanomechanical resonators with low dissipation rates are ideal tools in fundamental science applications. They have been used in the field of cavity optomechanics for example in ground-state cooling, and in sensing applications, such as atomic resolution mass sensors. Their great sensitivity is due to their high Q-factor, which is a metric that shows how fast a system loses its energy. In ultra-high Q nanomechanical resonators, energy loss is limited to intrinsic and radiation losses, the latter is due to energy dissipation from the resonator into the substrate. Experiments have shown that the Q-factor of resonators with thin substrates are limited by radiation loss. However, the precise role of the substrate remains a topic that has not received much attention, but has significant implications for how we design nanomechanical microchips. Here we show that the resonator mode can couple to nearby substrate modes, which reduces the Q-factor. We found that the strength of this mode-coupling depends on the mode-shape of the substrate, with stronger coupling at anti-nodes of the mode-shape and hardly any coupling at the nodes. Furthermore, we show that clamping down the substrate with double-sided tape reduces the Q-factor of the resonators, this is explained by a reduction in Q of substrate modes due to the tape. Lastly, we found that in thin substrates, which have a higher density of modes, the Q-factor can be limited due to mode-coupling with the substrate. Our results demonstrate that the substrate choice, as it can strongly affect the Q-factor of resonators, should become an integral part of the resonator design phase. These results can likely be used by all types of nanomechanical resonators limited by radiation loss. We can use this knowledge to design chips with resonators that have an even higher Q-factor.