Engineering Novel Optical Sensors for Magnetic Field Sensing Applications

Abstract

Research aimed at engineering magnetic detectors will continue to gain wide interest due to the numerous applications of field sensors. Over the years, several methods have been developed to sense and image magnetic fields with a majority of these methods further integrated with medical imaging modalities operated at low fields e.g., magnetic resonance imagers (MRIs). Classical methods of detection making use of inductive coils, based on Faraday's law of induction, or superconducting quantum interference devices (SQUIDs), based on the Josephson effect in superconducting materials are well researched. In addition, optical means of magnetic detection, for example, atomic vapour magnetometers (AVMs) based on alkali spins in massive vapour cells, have shown improved sensitivity over other methods. Nonetheless, the significant drawbacks these methods including the need for cryogenic temperature operations, and size, limits practical applications for sensing in compact medical devices. Magnetic field detectors based on nitrogen vacancies (NV) centres in diamond render an alternative to replace other detection protocols with a potential sensitivity well above the classical limit. The NV centres in diamond have numerous properties making it a sought after candidate for magnetic field sensing applications. Its long spin coherence time at room temperature and an efficient method of initializing with optical readout of electronic spins make it an ideal candidate for magnetometry. Above all, NV-based sensors can be operated over a wide range of temperatures with high spatial resolutions; therefore, it is the aim of this dissertation to provide an all-around exploratory study to address specific problems in the material science, microwave engineering, and magnetometry aspect of the NV centre discipline.

First, the potential of holding large ensembles of NV− centres in polycrystalline diamonds (PCDs) grown over non-diamond substrates was explored. A high concentration of NV− centres are a prerequisite to improving the sensitivity of NV sensing protocol, thus making PCDs a favourable candidate for wide-field magnetic field sensing. In addition, PCDs with the desired concentration of NV− centres can be obtained in large quantities and at a lower cost, hence this is a cost-effective option in applications where NV sensor arrays are needed. Results obtained from this part of the thesis showed the formation of both neutral and negatively charged NV centres at an optimum nitrogen flow rate of 10 sccm. This study is essential in benchmarking an optimal parameter space for the growth of nitrogen-doped polycrystalline diamonds suitable for sensing applications. Further, the magnetometry applications of diamonds rely on the incorporation of negatively charged NV optical centres in proximity to the diamond surface; there is however a limited understanding of the effect of nitrogen flow rate on the surface charge of NV centres at the surface of PCDs. An exhaustive study on the contributions of nitrogen flow rate on the surface morphology, grain orientations, and the formation of bonded carbon found at the grain/grain boundaries was carried out to understand the correlation between ingrained surface properties of nitrogen-doped PCDs and the formation of NV− centres in PCDs. Based on these experimental observations, a mechanism for the changes in the surface morphology of films grown under step-wise nitrogen doping was proposed. Consequently, this model was used to explain the possible dominance of NV$^{0}$ centres in PCDs in the low-pressure growth regime. The results obtained from this study contribute to a better understanding of the process for the formation of NV− centres in PCDs deposited at low pressure.

For sensing applications, a novel NV sensor utilizing a different illumination scheme when compared with the confocal system of sensing was designed and constructed. The performance of the designed system was tested by comparing experimental fluorescence (FL) maps obtained with the confocal system set-up under varying optical powers. The designed system status and proposed performance for sensitivity to fluorescence emission were accessed in terms of signal-to-noise ratio (SNR) and signal-to-background ratio (SBR). Although the conventional confocal set-up outperforms our new design in terms of their SNR in different optical excitation regimes, the new design meets the performance requirement in the undersaturation optical excitation regime. Subsequently, the newly designed system was tested for its magnetic field detection potentials and sensitivity in a Halbach magnet configuration using the optically detected magnetic resonance (ODMR) protocol. According to the absorbtion profiles observed in the measured ODMR spectrum, each NV centre experiences a static magnetic field in the range 2.14 - 6.07 mT of the Halbach magnet. The sensitivity was estimated and found to be approximately 0.2 $\mu$T/$\sqrt{Hz}$. These results pave way for the development of scalable NV sensors using the novel excitation and collection methods developed in this thesis.