This thesis represents several steps in the development of Nitrogen-Vacancy Nanodiamond as an effective optical label for biological measurements and as a tool for nanometry and quantum information techniques. In this thesis I cover concrete progress in many areas of nanodiamond research, demonstrating that optical interactions alone can be used to control NV spin polarisation and measure NV temperature. I develop a method for high-contrast imaging of NV centres using magnetic modulation, and also show a method for measuring the Raman scattering of water to an unprecedented level of accuracy. This thesis also contains the outlines of two more experiments: one aimed at measuring NV concentration in aqueous nanodiamond suspensions, and another whose aim is to use multiple NV centres in a single nanodiamond to measure the orientation and strength of an external magnetic field.
This thesis represents several steps in the development of Nitrogen-Vacancy Nanodiamond as an effective optical label for biological measurements and as a tool for nanometry and quantum information techniques. In this thesis I cover concrete progress in many areas of nanodiamond research, demonstrating that optical interactions alone can be used to control NV spin polarisation and measure NV temperature. I develop a method for high-contrast imaging of NV centres using magnetic modulation, and also show a method for measuring the Raman scattering of water to an unprecedented level of accuracy. This thesis also contains the outlines of two more experiments: one aimed at measuring NV concentration in aqueous nanodiamond suspensions, and another whose aim is to use multiple NV centres in a single nanodiamond to measure the orientation and strength of an external magnetic field.
Diamond is unique. Whether mechanical, thermal, electrical or optical, its properties are within the most extreme compared to those of any other material. This, together with the fact that in the last fifty years the synthesis of artificial diamond has become a reality, makes it very attractive for engineering and technological purposes. The range of diamond-based applications is very diverse and, among the others, includes its use in mechanical machining, high-power high-frequency electronics and high transmissivity optics. -- Diamond is also host to a wide variety of crystallographic defects. Appropriately controlled, they can be used to tune the characteristics of the material, but even more interestingly they often possess attractive optical properties on their own. In particular, nitrogen-vacancy (NV) centres in diamond have been the subject of extensive study in the last few years. They possess unique room-temperature optical and spin properties which make them promising systems for solid-state qubits in quantum information technologies and for emerging applications in biotechnology, nanomedicine and high resolution magnetometry, such as drug delivery, fluorescent biomarking and spin imaging. -- This thesis is focused on NV centres in diamond. I first present an extensive review on diamond as a material, describing in detail its physical properties, the methods that have been developed to synthesise it and some of its possible applications. I then move the attention specifically to NV centres in nanodiamonds (NDs), i.e. diamond crystals in the nanometre size range (0-100 nm). They are a potential candidate for emerging technologies including high-resolution magnetometry and biomedical imaging. For these applications, specific material requirements, such as the small size of the diamond nanocrystals and the optical stability of the hosted NV centres, are crucial. The NV stability with respect to the nanodiamond size and surface is the central theme of this thesis. I analyse the optical behaviour of NV centres in nanodiamond while varying a number of key parameters such as size, surface termination and diamond origin. Based on the measurements, I conclude whether the nanodiamond size and the NV surface proximity are factors that must be taken into consideration in order to control the material and realise nanotechnologies based on NV centres in nanodiamonds.
This thesis focuses on the spin physics of nitrogen-vacancy (NV) centres in nanodiamonds (NDs) via their coupling to external fields. In the first part, we theoretically investigate the feasibility of coupling a ND with a high density of NV centres to a gap-tunable flux qubit. We calculate the optimum position including the tolerance for maximizing the coupling and calculate its dependence on the orientation of the ND. We demonstrate that the coherent coupling of a 150-nm ND containing many (10³) NV centres and a flux qubit with dephasing time of ~1 μs is feasible. In the second part, we investigate the spin-coherence properties of NV centres in NDs and discuss them in the context of the NVs-to-flux qubit coupling. In particular, we manipulate the NV spins via an external microwave field using two distinct methods. The first, more conventional method employs a thin wire in direct contact with the sample as the microwave antenna. The second method has been developed by our group. It uses a cavity to produce the microwave field at the position of the NV spins with the advantage over the wire architecture of addressing the NV centre spins, without sample heating and over macroscopic spatial distances (milli-meters versus micro-meters).
The NV defect center in diamond forms a pseudo-atomic quantum system with discreet optically excitable transitions between ground and excited states in the gap between valence and conduction bands, making the NV center a deep-level defect center in diamond. For the negatively charged NV center (NV-), both the ground and excited states are spin triplets (S=1) and coupling between optical and spin states provides unique opportunities for optical detection of magnetic resonance (ODMR) and quantum metrology under ambient conditions. As an atomically sized point defect that is stable in single nanodiamonds as small as a few nanometers, the NV center enables spin based imaging of temperature, electric fields, magnetic fields and mechanical strain with nanoscale spatial resolution. Made of carbon, nanodiamonds are also ideal nanoparticles for use in biological systems, exhibiting extremely low cytotoxicity, no photo bleaching and exceptional contrast in transmission electron and light microscopy. With the ability to act as both a passive, non perturbing sensor (e.g. a diamagnetic material used for sensing magnetic fields) and a connection point for two way, direct coupling of far field optics to NV− spins and indirectly to the highly localized neighborhood of atoms, electronic states and phonon modes in the local vicinity, NV diamond provides a route to nanoscale field sensing and spin based optical microscopy. These sensors are also stable in nanodiamonds. For nanoparticles, the reduction in particle diameter dramatically enhances the proportion of the material that is surface exposed, enhancing any surface related properties. Nitrogen Vacancy centers in nanodiamonds provide another example of material properties that vary with particle size. In chapter 5, evidence that modulation of nanodiamonds fluorescent emissions by electrical perturbations in their surrouding environment allows transduction of local electrodynamics into a far field optical signal capable of mapping action potentials in cardiomyocytes in presented. The unifying theme for all of the work presented in this dissertation is to enable the use of magnetic resonance signals and interactions for imaging and sensing in previously inaccessible regimes. The specific focus is on methods and techniques to enable nanoscale physical sensing and functional imaging in live tissues, with the ultimate aim to lay the predicate for the combination of both without compromise.
The nitrogen-vacancy (NV) centre in diamond gained popularity as a probe for nanoscale sensing applications in research thanks to the array of sensing modalities available and the bio-compatibility of the material itself, however the utility of NV sensing is limited by the lack of suitable strategies to control them spatially. By confining single nanodiamonds using an optical tweezers (OT) we are able to combine the sensing opportunities of the NV centre with the precision control of position and orientation afforded by OT. This work is an investigation of the interaction of the trapping laser with the spin-based photoluminescence of the NV centre, further it is a demonstration of an all-optical sensing protocol which eliminates the spin depolarisation effects of the trapping laser and allows for NV spin relaxometry in an optically trapped nanodiamond. This relaxometry protocol can determine spin lattice relaxation times on the order of ms and requires relatively low trapping powers < 50 mW, making it particularly applicable to biological systems.
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 confluence of quantum physics and biology is driving a new generation of quantum-based sensing and imaging technology capable of harnessing the power of quantum effects to provide tools to understand the fundamental processes of life. One of the most promising systems in this area is the nitrogen–vacancy centre in diamond—a natural spin qubit which remarkably has all the right attributes for nanoscale sensing in ambient biological conditions. Typically the nitrogen–vacancy qubits are fixed in tightly controlled/isolated experimental conditions. In this work quantum control principles of nitrogen–vacancy magnetometry are developed for a randomly diffusing diamond nanocrystal. We find that the accumulation of geometric phases, due to the rotation of the nanodiamond plays a crucial role in the application of a diffusing nanodiamond as a bio-label and magnetometer. Specifically, we show that a freely diffusing nanodiamond can offer real-time information about local magnetic fields and its own rotational behaviour, beyond continuous optically detected magnetic resonance monitoring, in parallel with operation as a fluorescent biomarker.