Abstract Effective methods to generate colour centres in diamond and other wide band‐gap materials are essential to the realisation of solid state quantum technologies based on such systems. Such methods have been the subject of intensive research effort in recent years. In this review, we bring together the various techniques used in the generation and positioning of colour centres in diamond: ion implantation, delta‐doping, electron irradiation, laser writing and thermal annealing. We assess the roles and merits of each of these techniques in the formation of colour centres for different quantum technologies and consider future combinations of the techniques to meet the requirements of the most demanding applications.
Abstract Effective methods to generate colour centres in diamond and other wide band‐gap materials are essential to the realisation of solid state quantum technologies based on such systems. Such methods have been the subject of intensive research effort in recent years. In this review, we bring together the various techniques used in the generation and positioning of colour centres in diamond: ion implantation, delta‐doping, electron irradiation, laser writing and thermal annealing. We assess the roles and merits of each of these techniques in the formation of colour centres for different quantum technologies and consider future combinations of the techniques to meet the requirements of the most demanding applications.
Colour centre defects in diamond are promising candidates for many quantum technologies because they act as isolated two level systems in the solid-state that can be controlled and read-out through photonics [1][2][3]. Realisation of these technologies requires the placement of many highly coherent centres in a regular array with high positional accuracy and high fabrication yield [4]. Previous fabrication techniques have lacked the ability to produce high yields with high positional accuracy without causing irreparable damage to the lattice which reduced the quality of the spin and optical properties [5]. Ultra-fast laser writing has been shown to produce highly coherent nitrogen vacancy (NV) colour centres within the diamond lattice with positional accuracies of order hundreds of nanometers and a stochastic yield. In this case, the yield and positional accuracy is held back by the need to thermally anneal the diamond within a furnace to convert laser generated defects into colour centres; a fabrication step in which there is little to no control over individual site [6]. In this thesis, the ultra-fast pulsed laser writing technique is broached in detail from both a theoretical and experimental standpoint. The theory behind the highly non-linear light-matter interactions is studied and a hypothesis is developed for the energy transfer between photo-excited carriers and the lattice via the non-radiative recombination of self-trapped multi-excitonic states [7]. In addition, this thesis presents an expansion on the laser writing of colour centres through broaching the generation of different colour centres that have some preferential properties over the NV [8], and the development of a new engineering technique involving a local laser anneal [9]. The yield of generated centres is increased from stochastic to deterministic by replacing the global thermal anneal with the local laser anneal in combination with a fluorescence feedback mechanism. In addition, the use of the laser to locally anneal generated centres leads to a factor of five improvement in the positional accuracy due to the highly non-linear interaction of the laser with the material. These results culminate in a key step forward in the development of a large scale quantum device and improved understanding of the highly dynamical processes that occur upon femtosecond laser interaction with a wide bandgap material.
Abstract We present a high throughput and systematic method for the screening of colour centres in diamond with the aim of searching for and reproducibly creating new optical centres down to the single defect level, potentially of interest for a wide range of diamond-based quantum applications. The screening method presented here should, moreover, help to identify some already indexed defects among hundreds in diamond (Zaitsev 2001 Optical Properties of Diamond (Berlin: Springer)) but also some promising defects of a still unknown nature, such as the recently discovered ST1 centre (Lee et al 2013 Nat. Nanotechnol . 8 487; John et al 2017 New J. Phys . 19 053008). We use ion implantation in a systematic manner to implant several chemical elements. Ion implantation has the advantage of addressing single atoms inside the bulk with defined depth and high lateral resolution, but the disadvantage of producing intrinsic defects. The implanted samples are annealed in vacuum at different temperatures (between 600 °C and 1600 °C with 200 °C steps) and fully characterised at each step in order to follow the evolution of the defects: formation, dissociation, diffusion, re-formation and charge state, at the ensemble level and, if possible, at the single centre level. We review the unavoidable ion implantation defects (such as the GR1 and 3H centres), discuss ion channeling and thermal annealing and estimate the diffusion of the vacancies, nitrogen and hydrogen. We use different characterisation methods best suited for our study (from widefield fluorescence down to sub-diffraction optical imaging of single centres) and discuss reproducibility issues due to diamond and defect inhomogeneities. Nitrogen is also implanted for reference, taking advantage of the considerable knowledge on NV centres as a versatile sensor in order to retrieve or deduce the conditions and local environment in which the different implanted chemical elements are embedded. We show here the preliminary promising results of a long-term study and focus on the elements O, Mg, Ca, F and P from which fluorescent centres were found.
Abstract Color centers, optically active defects within solids, are vital for leading quantum information technologies such as quantum computing and quantum sensing. An essential prerequisite for realizing scalable quantum architectures is the ability to create quantum emitters deterministically. In the last decades, significant efforts have been devoted to selectively generating color centers. One of the most used methods is high‐energy ion implantation. However, this method usually causes extended lattice damage along the entire trajectory because of ions bombardment. Moreover, the position depth of color centers is also limited by the ions penetrate length in crystals. The direct laser‐writing (DLW) technique has recently emerged as a powerful tool to create color centers in solid‐state materials. It can define color centers at arbitrary depths inside the substrate and operate at the ambient environment, therefore, providing a feasible 3D fabrication method for integrated quantum photonics. Here, recent advancements of laser writing of color centers in solid‐state materials are reviewed, from bulk crystals, such as diamond and silicon carbide, to nanostructures, involving single‐walled carbon nanotubes, 2D layered materials, and quantum dots.
Quantum photonics is an emerging field of photonics where single photons are used for applications in quantum computation, quantum communication, and quantum sensing. One of the key enablers of these new technologies is colour centres. Colour centres are isolated defects in crystals that are able to emit single photons and have spin properties that can be exploited for quantum technologies. These properties include high brightness, low photo-bleaching, and room-temperature optically accessible spins. Wide-bandgap semiconductors such as diamond, silicon carbide and gallium nitride, and 2D materials such as h-BN and TMDs are of particular interest as host materials for colour centres. Numerous light emitters have been discovered and documented in these materials. Colour centres typically occur naturally in semiconductors and are often introduced during the growth process. This random distribution of defects is clearly not desirable for making quantum devices as it makes it difficult to process the materials and introduce structures that interact with these defects such as electrical contacts. The key technology that allows for the precise engineering of colour centres and other quantum defects in materials is ion implantation. Ion implantation utilizes energetic ion beams for the modification of electrical, optical, and magnetic properties of materials. It is a key processing step that has been widely used in the semiconductor industry for doping of semiconductors and for creating buried oxides. Ion implanters that are currently used in industry are designed for uniform wafer-level implantation of relatively large fluences of ions with high beam currents. The new quantum technologies, such as quantum computing, require a major paradigm shift in the way in which ion implantation is performed.
Colour centres in diamonds represent one of the most actively researched areas in the field of quantum technologies. In this work, we investigate a specific colour centre based on sodium (Na), selected due to promising theoretical predictions suggesting its potential for various quantum applications, despite the current lack of experimental studies. To explore this centre, we conducted two complementary experiments at ISOLDE/CERN. First, we employed the electron emission channeling (EC) technique to determine the most probable lattice sites occupied by sodium in a single-crystal lattice of a diamond implanted with radioactive 24Na. We then used a photoluminescence (PL) set-up to obtain the emission spectrum of a similar sample implanted with stable 23Na. EC measurements revealed that sodium likely adopts a combination of substitutional (S) and bond-centre (BC) sites, maximised after an annealing temperature of 600°C, exhibiting approximately 51% NaV split-vacancy (the highest fraction observed) and 17% substitutional Na (which increases slowly and almost linearly with annealing temperature). However, at higher annealing temperatures, the BC configuration decreases rapidly, and the combined fraction of these two sites also drops, suggesting the emergence of another, yet unidentified, configuration. In the PL experiments, we did not observe a direct optical signal attributable to sodium, but we present the optical characterisation of single-crystal diamond samples fabricated by Element Six and implanted with stable sodium at different annealing temperatures. We found the 3H centre consistently present across nearly all samples followingimplantation and annealing, with its zero-phonon line (ZPL) located around 503 nm.