This thesis reports research conducted on point defects in single crystal diamond utilising the complementary techniques of electron paramagnetic resonance (EPR), optical absorption and photoluminescence (PL) spectroscopy. Intentional silicon-doping of chemical vapour deposition (CVD) diamond allowed the production of samples grown on differently oriented substrates and containing distinct silicon isotopic abundances. The EPR spectrum of the neutral charge state of the silicon split-vacancy centre in diamond, (Si-V)0, has been characterised in the literature. Evidence for the assignment of the 1.31 eV zero phonon line (ZPL) seen in absorption and PL to the 3A2g -> 3A1u transition arising at (Si-V)0 is presented. Reversible charge transfer between the negatively charged centre, (Si-V)− (ZPL at 1.68 eV), and (Si-V)0 enabled the determination of calibration factors relating defect concentrations to their respective ZPL intensities. Preferential alignment of trigonal centres, such as (Si-V), in CVD material grown on {110}-substrates has been observed. The formation of (Si-V) centres during CVD synthesis and via irradiation and annealing of silicon-doped diamond was studied. Variable temperature EPR spectroscopy under illumination was used to investigate the optical spin polarisation (SP) of the (Si-V)0 3A2g ground state. Two different mechanisms for the SP are considered; selective intersystem crossing of (Si-V)0, and photoionisation of (Si-V)−. The properties of (Si-V)0 are compared to those of the extensively studied negatively charged nitrogen vacancy centre, (N-V)−. The effective spin-lattice relaxation and spin polarisation rates for both centres during continuous illumination are explored using pulsed EPR methods. A new defect, labelledWAR3, with spin S = 1 2 was observed in silicon-doped diamond and characterised using multifrequency EPR. Analysis of the data revealed that WAR3 is the neutral charge state of a silicon divacancy complex decorated by a hydrogen atom, (Si-V2:H)0. The experimentally derived 29Si and 1H hyperfine parameters are in good agreement with the values calculated using the spindensity- functional technique, confirming this model and ruling out a non-planar structure.
This thesis reports research conducted on point defects in single crystal diamond utilising the complementary techniques of electron paramagnetic resonance (EPR), optical absorption and photoluminescence (PL) spectroscopy. Intentional silicon-doping of chemical vapour deposition (CVD) diamond allowed the production of samples grown on differently oriented substrates and containing distinct silicon isotopic abundances. The EPR spectrum of the neutral charge state of the silicon split-vacancy centre in diamond, (Si-V)0, has been characterised in the literature. Evidence for the assignment of the 1.31 eV zero phonon line (ZPL) seen in absorption and PL to the 3A2g -> 3A1u transition arising at (Si-V)0 is presented. Reversible charge transfer between the negatively charged centre, (Si-V)− (ZPL at 1.68 eV), and (Si-V)0 enabled the determination of calibration factors relating defect concentrations to their respective ZPL intensities. Preferential alignment of trigonal centres, such as (Si-V), in CVD material grown on {110}-substrates has been observed. The formation of (Si-V) centres during CVD synthesis and via irradiation and annealing of silicon-doped diamond was studied. Variable temperature EPR spectroscopy under illumination was used to investigate the optical spin polarisation (SP) of the (Si-V)0 3A2g ground state. Two different mechanisms for the SP are considered; selective intersystem crossing of (Si-V)0, and photoionisation of (Si-V)−. The properties of (Si-V)0 are compared to those of the extensively studied negatively charged nitrogen vacancy centre, (N-V)−. The effective spin-lattice relaxation and spin polarisation rates for both centres during continuous illumination are explored using pulsed EPR methods. A new defect, labelledWAR3, with spin S = 1 2 was observed in silicon-doped diamond and characterised using multifrequency EPR. Analysis of the data revealed that WAR3 is the neutral charge state of a silicon divacancy complex decorated by a hydrogen atom, (Si-V2:H)0. The experimentally derived 29Si and 1H hyperfine parameters are in good agreement with the values calculated using the spindensity- functional technique, confirming this model and ruling out a non-planar structure.
The properties of the negatively charged silicon-vacancy (SiV) centre in diamond were investigated in this thesis. Polarisation measurements revealed transition dipoles and the symmetry groups of the centre. In a diamond grown by chemical vapour deposition, a high spectral overlap of SiV centres with nearly transform limited optical line widths was found pointing to a low sensitivity of SiV centres to both strain and local electric fields. This low inhomogeneous distribution allowed for a quantum interference leading to a contrast of over 70 % in a Hong-Ou-Mandel experiment. Additionally, an isotopic shift at single site level could be confirmed and than used to identify the 64meV peak in the phonon sideband as a local vibrational mode. A low temperature investigation of optical line width and orbital relaxation time indicated a direct single-phonon process between the orbital states while at higher temperatures a two-phonon dephasing was found to be the main electron-phonon process. The results were consistently described by the microscopic model developed and additional strength was added by consistent studies of spin-orbit splitting and optical lifetime. By applying a magnetic field the spin degeneracy in the ground state was lifted allowing for spin initialisation with over 95 % fidelity with a spin relaxation time of over 2 milliseconds. Two resonant lasers were used to demonstrate coherent population trapping leading to a coherence time in the order of the orbital relaxation time. The coherence was concluded to be limited by orbital dephasing processes and we propose techniques to overcome them. One of them is to use nanodiamonds with a size smaller than half the phonon wavelength to suppress the dephasing process. Investigating the spectral properties at cryogenic temperatures, nearly lifetime limited line widths have been found, making these nanodiamonds good candidates for further spin applications with SiV centres.
The neutrally charged silicon-vacancy defect (SiV 0 ) is a colour centre in diamond with spin S = 1, a zero-phonon line (ZPL) at 946 nm and long spin coherence, which makes it a promising candidate for quantum network applications. For the proper performance of such colour centres, all of them must have identical optical characteristics. However, in practice, there are factors that influence each individual centre. One of these factors is non-uniform isotope composition for both carbon atoms in diamond lattice and silicon atoms of dopant. In this work, we studied the isotopic shifts of SiV 0 centres for CVD-grown epitaxial layers of isotopically enriched 12 C and 13 C diamonds, as well as for diamond with natural isotope composition but doped only with one isotope of Si ( 28 Si, 29 Si and 30 Si). The detected shift was 1.60 meV for 12 C/ 13 C couple and 0.33 meV for 28 Si/ 29 Si and 29 Si/ 30 Si couples, which are close to the previously obtained values of the isotopic shift for the negatively charged silicon vacancy (SiV − ), which indicates a similar model of interaction with the environment for these two charge states of the SiV colour centres. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.
The electronic spins of the nitrogen-vacancy (NV) centers in chemical-vapor-deposition (CVD) grown diamonds form ideal probes of magnetic fields and temperature, as well as promising qubits for quantum information processing. Studying and controlling the magnetic environment of NV centers in such high-purity crystals is thus essential for these applications. We demonstrate optical detection of paramagnetic species, such as hydrogen-related complexes, in a CVD-grown diamond. The resonant transfer of the NV centers' polarized electronic spins to the electronic spins of these species generates conspicuous features in the NV photoluminescence when employing magnetic field scans along the [100] crystal direction. Our results offer prospects for more detailed studies of CVD-grown processes as well as for coherent control of the spin of novel classes of hyperpolarized paramagnetic species.
Optical experiments, involving photoluminescence (PL), the PL excitation and quenching spectra, as well as transmission and its quenching, were used to analyse the photochromic behaviour of some vacancy-related complexes in diamond. The 2.156 eV, 1.945 eV and 1.68 eV optical centres in CVD diamond are attributed to the neutral nitrogen-vacancy, negative nitrogen-vacancy and neutral silicon-vacancy ([Si-V](0)) centres, respectively. Oscillatory behaviour in the excitation spectrum of the 1.68 eV luminescence is observed and from the threshold of the oscillations a position of Ec - 2.05 eV is suggested for the ground state of the [Si-V](0) centre.
The fluorescence of the nitrogen-vacancy (NV) centre in artificial diamond has been widely studied for applications such as quantum communication protocols, quantum computation, fluorescence-based biolabelling, and optical sensing. Optical sensing by itself encompasses a wide range of possible applications: from magnetic and electric field sensing to temperature sensing and enabling, for example, the detection of paramagnetic ions and the identification of molecules by nuclear magnetic resonance. Compared to many other systems capable of optical sensing, the NV centre works at room temperature, and its spin-dependent fluorescence emission is the key property to achieve a high sensitivity to the surrounding environment. Additionally, the NV centre emission is highly photostable, has a relatively short lifetime (~ 12ns) and spans a broad wavelength range, from the red to the near-infrared region of the electromagnetic spectrum (600 - 800nm). While the common denominator of all mentioned applications is the NV centre, the properties of the artificial diamond surrounding the NV centre have a significant impact on its sensing performance. At present, the processing of artificial diamond has not been fully optimised, and research and development towards the realisation of the `best¿ diamond sample for optical sensing is far from over. Specifically, the role of other defects in diamond and their effects on the photo-physical properties of the NV centres are not entirely understood. In this thesis, I will discuss the potential benefits of a novel processing technique to create ensembles of NV centres, and the importance that surrounding nitrogen defects has on the achievable magnetic field sensitivity. Additionally, I will show both theoretical and experimental investigations of the NV centres photo-physical processes, with focus on magnetic field sensing. I will initially explore a protocol for microwave-free magnetometry. This protocol predicts a theoretical magnetic field sensitivity <10µT/√Hz in the range 0.3 - 30mT and I will demonstrate it experimentally with a maximum sensitivity of 3.6mT/Hz. Following, I will focus on exploring techniques to increase the magnetic field sensitivity by improving the material properties of the diamond samples. I will demonstrate that the simultaneous electron irradiation and annealing improves the creation efficiency of an ensemble of NV centres by up to 117%. Finally, I will investigate how substitutional nitrogen defects in the surrounding lattice affects the charge state of the NV centre, its emission intensity, its lifetime and its magnetic field sensitivity. I will study a sample characterised by multiple regions across its volume, with nitrogen concentration changing between 10ppm and 100ppm in each region. There si an optimal region with ~ 40ppm of substitutional nitrogen which demonstrates the highest fluorescence emission, mainly from the negatively-charged NV centre, and the highest magnetic field sensitivity of 1.6 ± 0.2µT/√Hz. However, both the regions with lower and higher substitutional nitrogen concentration than the optimal region show lower emission intensity and worse magnetic field sensitivity. Following is a summary of the content in each chapter: Chapter 1 will present an introduction to the diamond material and the classification of artificial diamond samples. It will introduce the NV centre, its fluorescence properties and the process to create NV centres in diamond. The chapter will also introduce the concepts around optical magnetometry and the working principle that allows the NV centre to sense magnetic fields as well as other external quantities, such as temperature or pressure. Finally, it will include a brief overview of the defects that are found in artificial diamond or are created during the sample processing. Chapter 2 will describe the working principle of the instruments utilised in this work. I will list the details of each instrument and describe the methods used for the analysis of the data. Chapter 3 will introduce the background of modelling the fluorescence emission from NV centres. I will discuss the implementation of a computational model based on the Lindblad master equation to simulate the photo-physical dynamics of the NV centre. I will demonstrate the successful simulation of the saturation emission, fluorescence lifetime and optically detected magnetic resonance using the theoretical model. Finally, I will explore the theoretically predicted sensitivity of microwave-free magnetometry, based on the NV centre emission intensity change under an external magnetic field. Chapter 4 will demonstrate experimentally the microwave-free magnetometry technique. In relation to the study of microwave-free magnetometry, this chapter will explore an unexpected increase in emission intensity at high magnetic fields > 100mT. Combining the experimental observations and the theoretical simulation developed in the previous chapter, I will identify and discuss a phenomenological decrease in the NV centre transition rate from the triplet state to the singlet state. The chapter will conclude comparing the experimental and theoretical sensitivities associated with the microwave-free magnetometry technique. Chapter 5 will show the characterisation of a diamond sample at each step of the process to create NV centres. I will study the electron irradiation followed by separate annealing on a high-pressure high-temperature diamond sample. I will then characterise the sample employing visible and infrared absorption spectroscopy, as well as measure the fluorescence spectra and lifetime of the defects identified at each step. Chapter 6 will examine an alternative method to create NV centres in diamond. I will compare diamond samples irradiated and annealed separately with samples simultaneously irradiated and annealed (referred to as 'annealing in situ'). To compare the two processes, I will estimate the concentration of nitrogen defects and NV centres and calculate their N-to-NV conversion efficiency. Chapter 7 will discuss the effects of substitutional nitrogen on the properties of the NV centres in diamond. For this discussion, I will use a single-crystal diamond that shows the presence of 4 growth sectors. Each growth sector is characterised by a different nitrogen concentration, as I will demonstrate at the beginning of the chapter. I will focus on 4 main properties affected by the concentration of nitrogen in the different sectors: the balance between the charge states of the NV centre, the overall fluorescence intensity from each sector, the fluorescence lifetime and its relation to non-radiative processes, and the magnetic field sensitivity obtained with the optically detected magnetic resonance technique. Chapter 8 will gather the results and conclusions of the entire body of work. It will also discuss future avenues of research in the topic of diamond processing and characterisations, to improve the existing optical sensing applications.
The negative Nitrogen-Vacancy (NV) colour centre in diamond is an atomic-level impurity that is remarkably well suited for quantum information processing and quantum metrology applications. Underlying these properties is the fact that optical illumination causes the NV centre to preferentially populate one of its three spin levels. The mechanism that gives rise to this spin polarisation has not been well understood, despite intense research interest in the NV centre and its applications. This thesis arrives at an account of the spin polarisation mechanism, although a detailed theoretical treatment remains outstanding. The energy level structure of the NV centre is investigated using techniques of solid state spectroscopy. These include absorption and photoluminescence measurements made on diamonds at cryogenic and room temperatures in the presence of uniaxial stress, magnetic fields, and microwaves. Pulsed light experiments allow the dynamic processes within the NV centre to be measured. It is shown that at room temperature the orbital doublet excited state behaves like an orbital singlet, and this description is elegantly confirmed by single-site observations. This picture of the excited state is inconsistent with cryogenic measurements, and its behaviour is examined across a range of temperatures in order to resolve this tension. It is shown that electron-vibration interaction accounts for the quenching of orbital properties in the excited state with temperature. A new optical transition within the NV centre is reported at a wavelength of 1042 nm. It is established that this transition occurs between the intermediate singlet levels which are thought to provide the decay pathway that produces spin polarisation. The results are consistent with this understanding, and the order and properties of these singlet levels are established. Electron-vibration interaction is observed to be important within the singlet system. It is shown that electron-vibration interaction is an important detail in the NV optical pumping cycle. Taking it into consideration leads to the first plausible physical description of the spin polarisation mechanism. -- provided by Candidate.