A quantum system with discrete, resolvable energy levels is an ideal system for sensing applications, taking into account that it can strongly interact with its environment. Although the strong response with the target properties such as magnetic or electric field is ideal, it can be a disadvantage when the system is interacting with other physical changes (noise) at the same time. However, the benefits of the strong coupling as well as small size of the sensor has motivated many researchers to explore quantum systems for measuring very small physical quantities. For instance, Nitrogen vacancy center in diamond is a unique quantum system enabling sensing of magnetic field from single molecules or electrons at room temperature.
This thesis has focused on two different quantum emitters: NV centers in nanodiamonds and quantum emitters in hexagonal Boron Nitride (hBN). In the first part a new assembly technique based on electron beam induced deposition and crosslinking chemistry is developed. It is demonstrated that fluorescent nanodiamonds containing NV centers can be assembled into arrays of various size and shapes on any substrate. The produced array is ideal for device fabrication due to its outstanding robustness. A potential application of such arrays as magnetic field sensor with high spatial resolution is shown.
The superior properties of quantum emitters in hBN has been studied in the second part of this thesis, where is has been shown that these emitters are not only stable but also maintain their single photon purity at elevated temperature. The highest measured quantum emission at 800 K in this study is the highest reported temperature for a quantum emitter so far, makes them a suitable candidate for temperature sensing. In addition, the level structure of emitters in hBN has been investigated in detail which reveals the unique photophysical properties of a class of these emitters which result in a nonlinear increase in the emission upon co-excitation with two lasers of different wavelength. Finally the photophysic property of these emitters has been employed to introduce a new modality of super-resolution microscopy with resolution down to about 70 nm. These findings will extend our understanding of quantum emitters in hBN and introduce a new functionality for them which paves the way toward their application in biology and sensing.
A quantum system with discrete, resolvable energy levels is an ideal system for sensing applications, taking into account that it can strongly interact with its environment. Although the strong response with the target properties such as magnetic or electric field is ideal, it can be a disadvantage when the system is interacting with other physical changes (noise) at the same time. However, the benefits of the strong coupling as well as small size of the sensor has motivated many researchers to explore quantum systems for measuring very small physical quantities. For instance, Nitrogen vacancy center in diamond is a unique quantum system enabling sensing of magnetic field from single molecules or electrons at room temperature.
This thesis has focused on two different quantum emitters: NV centers in nanodiamonds and quantum emitters in hexagonal Boron Nitride (hBN). In the first part a new assembly technique based on electron beam induced deposition and crosslinking chemistry is developed. It is demonstrated that fluorescent nanodiamonds containing NV centers can be assembled into arrays of various size and shapes on any substrate. The produced array is ideal for device fabrication due to its outstanding robustness. A potential application of such arrays as magnetic field sensor with high spatial resolution is shown.
The superior properties of quantum emitters in hBN has been studied in the second part of this thesis, where is has been shown that these emitters are not only stable but also maintain their single photon purity at elevated temperature. The highest measured quantum emission at 800 K in this study is the highest reported temperature for a quantum emitter so far, makes them a suitable candidate for temperature sensing. In addition, the level structure of emitters in hBN has been investigated in detail which reveals the unique photophysical properties of a class of these emitters which result in a nonlinear increase in the emission upon co-excitation with two lasers of different wavelength. Finally the photophysic property of these emitters has been employed to introduce a new modality of super-resolution microscopy with resolution down to about 70 nm. These findings will extend our understanding of quantum emitters in hBN and introduce a new functionality for them which paves the way toward their application in biology and sensing.
This thesis decay rate enhancement and photophysics of nitrogen vacancy (NV) centers in diamond and quantum emitters in hexagonal boron nitride (hBN). The NV center in diamond is a unique physical system that finds use in many quantum technologies such as quantum sensing, quantum information processing and single photon generation. Consisting of a substitutional nitrogen atom next to a vacant lattice site, the NV center introduces discrete electronic energy levels within the wide bandgap of diamond. The NV center can be optically excited and will emit single photons upon relaxation that hold information of the spin state of the NV center. In this thesis, we use metallic structures to modify and enhance the decay rate of NV centers in bulk and nano diamonds in three different experiments. Such an enhancement is a way to improve the usability of NV as a single photon source. Using TiN films with metamaterial properties we observe a 2-fold decrease of the total excited state lifetime of NV centers in nano diamonds, when comparing to similar NV centers not influenced by the TiN films. For shallow implanted NV centers in bulk diamond we experimentally investigate how a decay rate enhancement influences the signal-to-noise ratio (SNR) in spin readout measurements. In this experiment we perform spin state readout before and after introducing a rate enhancement and find a 30 % drop in spin contrast. We model the spin readout measurements using a five-level model and use that to predict the SNR under higher rate enhancement. In the last NV related experiment we use a curved metallic mirror on top of shallow implanted NV centers, to measure the continuous lifetime modification as a function of distances to the mirror. This allows us to determine the internal quantum efficiency (IQE) by fitting the experimental data with a model that includes calculating the expected Purcell factor. We find IQE values of 0.70±0.07 and 0.82±0.08 for implantation depths of 4.5±1 and 8±2 nm, respectively. Last but not least, newly discovered bright single photon emitters in two dimensional hBN were investigated. Similar to the NV center they are believed to be crystal defects, but the defect configuration is not fully known. In this thesis, the photophysics of quantum emitters in hBN nano flakes are investigated. Intensity autocorrelation measurements indicate the presence of at least two metastable states, seen by distinct photon bunching levels on long time scales in the g(2)(τ)-functions. We also observe blinking behaviour, which we show can be explained by changes in the IQE. From rate equation fitting we find that transition rates scale linearly with excitation power. Based on the transition rates we estimate the IQE to be close to unity for excitation powers below saturation.
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.
Abstract Powered by the mutual developments in instrumentation, materials and theoretical descriptions, sensing and imaging capabilities of quantum emitters in solids have significantly increased in the past two decades. Quantum emitters in solids, whose properties resemble those of atoms and ions, provide alternative ways to probing natural and artificial nanoscopic systems with minimum disturbance and ultimate spatial resolution. Among those emerging quantum emitters, the nitrogen vacancy (NV) color center in diamond is an outstanding example due to its intrinsic properties at room temperature (highly-luminescent, photo-stable, biocompatible, highly-coherent spin states). This review article summarizes recent advances and achievements in using NV centers within nano- and single crystal diamonds in sensing and imaging. We also highlight prevalent challenges and material aspects for different types of diamond and outline the main parameters to consider when using color centers as sensors. As a novel sensing resource, we highlight the properties of NV centers as light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g. graphene.
Fluorescent milled nanodiamonds are one of the most promising physical bases for quantum devices. The nitrogen vacancy (NV) center in diamond acts a room temperature, optically addressable quantum system which significantly decreases the infrastructure costs of developing quantum devices. Since nanodiamonds are nontoxic and colloidally stable, the nanoparticles are particularly attractive to the fields of sensing and nanomedicine where environmental changes must be detected with nanoscale resolution. Particle to particle irregularity, however, alters the spin and optical properties of each nanodiamond and complicates their integration into more complex systems. In this dissertation, we address the inhomogeneity of nanodiamonds and provide multiple paths towards fully utilizing these quantum emitters. First, we develop a technique for the self-assembly of nanodiamonds into arrays. We then utilize these arrays to perform a statistical study of the nanodiamonds and characterize the impact of the material’s surface and crystalline structure on the NV center’s optical and spin properties. Next, we establish a surface modification method that both improves the general properties of the nanodiamonds and enables chemical conjugation reactions. Together, these results deliver a newfound level of control and uniformity over an otherwise uncontrollable and polydisperse material.
Room temperature solid-state emitters provide a promising source of single photons for use in quantum technologies. The emitters studied in this thesis are the nitrogen-vacancy (NV) centre in diamond, and defects in two-dimensional hexagonal boron nitride (hBN). In this thesis, I present investigations into the coupling of these defects to open-access microcavities at room temperature. The optical microcavity serves to confine the electric field down to mode volumes on the order of λ3, increasing the spontaneous emission rate of these emitters and channelling this emission into well defined cavity modes. The general properties of both emitters will be presented, including examples of single photon emission, emission spectra, saturation behaviour and photo-stability. A few examples highlighting the long-term photo-stability of the NV centre under pulsed excitation will be displayed. This will highlight the suitability of the NV centre for a practical device. Following the preparation and characterisation of each emitter, single photon emission into the cavity modes is verified, reaching single photon purities greater than 97%. Enhancements in the spectral density of photons were observed for both the NV centre (~25×) and defect in hBN (~3×), along with decreases in the excited state lifetimes. While the NV centre provides a stable source of single photons, the cavity enhancement is limited by its broad emission spectrum. Defects in hBN present near-ideal optical properties in the absence of the cavity, and were expected to deliver superior results within the cavity. However, the poor photo-stability of these defects did not allow complete characterisation of the cavity coupled emission. FDTD simulations were combined with changes in the excited state lifetime to infer the quantum efficiency of these defects. The results of this thesis form the foundation for the realisation of a robust room temperature single photon source.
Powered by the mutual developments in instrumentation, materials andtheoretical descriptions, sensing and imaging capabilities of quantum emitters insolids have significantly increased in the past two decades. Quantum emitters insolids, whose properties resemble those of atoms and ions, provide alternative waysto probing natural and artificial nanoscopic systems with minimum disturbance andultimate spatial resolution. Among those emerging quantum emitters, the nitrogen-vacancy (NV) color center in diamond is an outstanding example due to its intrinsicproperties at room temperature (highly-luminescent, photo-stable, biocompatible,highly-coherent spin states). This review article summarizes recent advances andachievements in using NV centers within nano- and single crystal diamonds in sensingand imaging. We also highlight prevalent challenges and material aspects for differenttypes of diamond and outline the main parameters to consider when using color centersas sensors. As a novel sensing resource, we highlight the properties of NV centersas light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g.graphene.