Color Centers Research Articles

Effect of the nanopillar diameter on diamond silicon vacancy center spin lifetime

Color centers in diamond micro and nano-structures play an important role in a wide range of quantum technologies. However, obtaining high-quality color centers in small structures is challenging, as properties such as spin population lifetimes can be affected by the transition from a bulk to nanostructured crystal host. In this manuscript, we measure how population lifetimes of silicon vacancy center orbital states change when they are created in nanopillars whose diameters vary from 1 μm to 120 nm. We also discuss the influence of annealing methods on the silicon vacancy inhomogeneous linewidth. After selecting a sample with low inhomogeneous broadening and patterning it with nanopillars, we expected that restricted vibrational modes in the smallest structures could extend spin population lifetimes. However, we found that this effect was masked by other effects that reduced population lifetimes, suggesting that imperfections in the crystal lattice or surface damage caused by etching can influence SiV spins.

Charge-state stability of color centers in wide band gap semiconductors

Charge-state stability of color centers in wide band gap semiconductors

The Creation of Multimode Luminescent Phosphor through an Oxygen Vacancy Center for High-Level Anticounterfeiting.

Integrating multimode optical properties into a single material simultaneously is promising for improving the security level of fluorescent anticounterfeiting. However, there has been a lack of affirmative principles and unambiguous mechanisms that guide the design of such material. Herein, we achieve color-tunable photoluminescence, long-lived persistent emission, thermally stimulated luminescence, and reversible photochromism in a Tb3+-activated Mg4Ga8Ge2O20 phosphor by employing the F-like color center as an energy reservoir. It is experimentally revealed that the role of oxygen vacancies in the lattice of Mg4Ga8Ge2O20 is assumed as the main trap for the photogenerated electronic carriers, which is the origin of metastable F-like color centers. The formed color centers with the estimated depths of 0.48-0.95 eV could suppress the recombination of electron-hole pairs, thus giving rise to good photochromism and persistent emission properties, while under various modes of stimulation such as thermal attack or photo radiation, a quick recombination of electron holes happens, accounting for the bright thermally stimulated luminescence and the accompanied color bleaching. Finally, we fabricate a flexible phosphor/polymer composite by encapsulating the developed phosphor into a polydimethylsiloxane matrix, and conceptual demonstration of the composite for the high-security fluorescent anticounterfeiting technology, by virtue of multimode optical phenomena as authentication signals.

Nitrogen Substitutions Aggregation and Clustering in Diamonds as Revealed by High-Field Electron Paramagnetic Resonance.

Diamonds have been shown to be an excellent platform for quantum computing and quantum sensing applications. These applications are enabled by the presence of defects in the lattice, which are also known as color centers. The most common nitrogen-based defect in synthetic diamonds is the paramagnetic nitrogen substitution (P1) center. While the majority of quantum applications rely on nitrogen-vacancy (NV) centers, the properties of the latter are heavily influenced by the presence and the spatial distribution of the P1 centers. Hence, understanding the spatial distribution and mutual interactions of P1 centers is crucial for the successful development of diamond-based quantum devices. Unlike NV centers, P1 centers do not have a spin-dependent optical signature, and their spin-related properties, therefore, have to be detected and characterized using magnetic resonance methods. We show that using high-field (6.9 and 13.8 T) pulsed electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) experiments, we can distinguish and quantify three distinct populations of P1 centers: isolated P1 centers, weakly interacting ones, and exchange-coupled ones that are clustered together. While such clustering was suggested before, these clusters were never detected directly and unambiguously. Moreover, by using electron-electron double resonance (ELDOR) pump-probe experiments, we demonstrate that the latter clustered population does not exist in isolation but coexists with the more weakly interacting P1 centers throughout the diamond lattice. Its presence thus strongly affects the quantum properties of the diamond. We also show that the existence of this population can explain recent hyperpolarization results in type Ib high-pressure, high-temperature (HPHT) diamonds. We propose a combination of high-field pulsed EPR, ELDOR, and DNP as a tool for probing the aggregation state and interactions among different populations of nitrogen substitution centers.

On readout and initialisation fidelity by finite demolition single shot readout

Abstract Ideal projective quantum measurement makes the system state collapse in one of the observable operator eigenstates | ϕ α ⟩ , making it a powerful tool for preparing the system in the desired pure state. Nevertheless, experimental realisations of projective measurement are not ideal. During the measurement time needed to overcome the classical noise of the apparatus, the system state is often (slightly) perturbed, which compromises the fidelity of initialisation. In this paper, we propose an analytical model to analyse the initialisation fidelity of the system performed by the single-shot readout. We derive a method to optimise parameters for the three most used cases of photon counting based readouts for NV colour centre in diamond, charge state, nuclear spin and low temperature electron spin readout. Our work is of relevance for the accurate description of initialisation fidelity of the quantum bit when the single-shot readout is used for initialisation via post-selection or real-time control.

Experimental preparation and electronic structure calculation of vacancy color centers of diamond IVA group

Experimental preparation and electronic structure calculation of vacancy color centers of diamond IVA group

Identification of Optically Active Quartet Spin Centers Based on a Si Vacancy in SiC Promising for Quantum Technologies

Optically active (bright) and optically inactive (dark) quartet S = 3/2 spin color centers including a negatively charged Si vacancy have been identified in silicon carbide using high-frequency electron nuclear double resonance on the nuclei of the 13C isotope, enhanced by a tenfold increase in its content. The alignment of populations of spin levels is optically induced in a bright center promising for quantum technologies, whereas the populations of spin levels in a dark center, which is an isolated negatively charged Si vacancy V-Si, correspond to a Boltzmann distribution and do not change under optical excitation.

The annealing kinetics of the F-type defects in MgAl2O4 spinel single crystals irradiated by swift heavy ions

The annealing kinetics of the F and F+ color centers induced in MgAl2O4 single crystals under irradiation by 156-MeV 132Xe and 2.25-GeV 197Au ions are precisely measured via relevant optical absorption bands and analyzed in terms of the diffusion-controlled reactions between two pairs of anion Frenkel defects which determine thermal stability of irradiated material. It is demonstrated that two types of mobile oxygen interstitials (neutral and charged ones) are involved into recombination process with very different activation energies, which are practically independent on radiation fluencies.

Proton-beam energy diagnostics by color-center photoluminescence imaging in LiF crystals: Implementation of multiple Coulomb scattering into an analytical Bragg-curve model

Lithium fluoride crystals are used to assess proton-beam energy spectra. Proton irradiation induces laser-active color centers in the crystal, whose density correlates with the absorbed dose. The spatial distribution of photoluminescence emitted by these color centers is exploited to estimate the proton-beam energy spectrum using an analytical Bragg-curve model. This study integrates the effects of multiple Coulomb scattering (MCS) into the model. At high enough energies, MCS leads to proton leakage through the crystal faces with a reduction in absorbed dose along the crystal length. The model incorporates MCS using an empirical approach based on Monte Carlo simulations.

Defects distribution and evolution in selected-area helium ion implanted 4H–SiC

An experimental demonstration of the generation of VSi, CSiVC, and NCVSi color centers in 4H–SiC using helium ion microscope irradiation of 5 × 5 μm2 areas with subsequent annealing treatment is presented. With the aid of transmission electron microscopy (TEM), photoluminescence (PL) and cathodoluminescence (CL) spectroscopy and Raman stress analysis, the evolution and distribution of color centers were thoroughly investigated. Cross-sectional TEM revealed the presence of helium bubbles in the center of the regions implanted with high doses which account for the observed quench of PL emission in the center of the implanted regions in both PL and CL measurements. PL spectra from the virgin, implanted and annealed samples proved the appearance of VSi after implantation and the transformation from VSi to CSiVC and NCVSi centers after annealing at 1000 °C. Moreover, with the increase of the implantation dose, the area of NCVSi centers increases whereas that of CSiVC decreases, which provide the first evidence of the competitive relationship between the formation of CSiVC and NCVSi defects. The comparison between stress distribution and CSiVC defect distribution illustrated that CSiVC centers predominantly distributed around the surface rupture region after thermal annealing where significant stress repair occurred. The results show that focused helium ion implantation holds promise for the precise coupling of VSi, CSiVC and NCVSi centers in predefined locations in integrated photonics applications.

Multifunctional Core/Shell Diamond Nanoparticles Combining Unique Thermal and Light Properties for Future Biological Applications.

We report the development of multifunctional core/shell chemical vapor deposition diamond nanoparticles for the local photoinduced hyperthermia, thermometry, and fluorescent imaging. The diamond core heavily doped with boron is heated due to absorbed laser radiation and in turn heats the shell of a thin transparent diamond layer with embedded negatively charged SiV color centers emitting intense and narrowband zero-phonon lines with a temperature-dependent wavelength near 738 nm. The heating of the core/shell diamond nanoparticle is indicated by the temperature-induced spectral shift in the intensive zero-phonon line of the SiV color centers embedded in the diamond shell. The temperature of the core/shell diamond particles can be precisely manipulated by the power of the incident light. At laser power safe for biological systems, the photoinduced temperature of the core/shell diamond nanoparticles is high enough to be used for hyperthermia therapy and local nanothermometry, while the high zero-phonon line intensity of the SiV color centers allows for the fluorescent imaging of treated areas.

Deterministic creation of strained color centers in nanostructures via high-stress thin films

Color centers have emerged as a leading qubit candidate for realizing hybrid spin-photon quantum information technology. One major limitation of the platform, however, is that the characteristics of individual color centers are often strain dependent. As an illustrative case, the silicon-vacancy center in diamond typically requires millikelvin temperatures in order to achieve long coherence properties, but strained silicon-vacancy centers have been shown to operate at temperatures beyond 1 K without phonon-mediated decoherence. In this work, we combine high-stress silicon-nitride thin films with diamond nanostructures to reproducibly create statically strained silicon-vacancy color centers (mean ground state splitting of 608 GHz) with strain magnitudes of ∼4×10−4. Based on modeling, this strain should be sufficient to allow for operation of a majority silicon-vacancy centers within the measured sample at elevated temperatures (1.5 K) without any degradation of their spin properties. This method offers a scalable approach to fabricate high-temperature operation quantum memories. Beyond silicon-vacancy centers, this method is sufficiently general that it can be easily extended to other platforms as well.

Resonant Versus Nonresonant Spin Readout of a Nitrogen-Vacancy Center in Diamond under Cryogenic Conditions.

The last decade has seen an explosive growth in the use of color centers for metrology applications, the paradigm example arguably being the nitrogen-vacancy (NV) center in diamond. Here, we focus on the regime of cryogenic temperatures and examine the impact of spin-selective, narrow-band laser excitation on NV readout. Specifically, we demonstrate a more than fourfold improvement in sensitivity compared to that possible with nonresonant (green) illumination, largely due to a boost in readout contrast and integrated photon count. We also leverage nuclear spin relaxation under resonant excitation to polarize the ^{14}N host, which we then prove beneficial for spin magnetometry. These results open opportunities in the application of NV sensing to the investigation of condensed matter systems, particularly those exhibiting superconducting, magnetic, or topological phases selectively present at low temperatures.

Reversible optical data storage below the diffraction limit.

Colour centres in wide-bandgap semiconductors feature metastable charge states that can be interconverted with the help of optical excitation at select wavelengths. The distinct fluorescence and spin properties in each of these states have been exploited to show storage of classical information in three dimensions, but the memory capacity of these platforms has been thus far limited by optical diffraction. Here we leverage local heterogeneity in the optical transitions of colour centres in diamond (nitrogen vacancies) to demonstrate selective charge state control of individual point defects sharing the same diffraction-limited volume. Further, we apply this approach to dense colour centre ensembles, and show rewritable, multiplexed data storage with an areal density of 21 Gb inch-2 at cryogenic temperatures. These results highlight the advantages for developing alternative optical storage device concepts that can lead to increased storage capacity and reduced energy consumption per operation.

SiV0 centres in diamond: effect of isotopic substitution in carbon and silicon.

The neutrally charged silicon-vacancy defect (SiV0) 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 SiV0 centres for CVD-grown epitaxial layers of isotopically enriched 12C and 13C diamonds, as well as for diamond with natural isotope composition but doped only with one isotope of Si (28Si, 29Si and 30Si). The detected shift was 1.60 meV for 12C/13C couple and 0.33 meV for 28Si/29Si and 29Si/30Si 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'.

Narrowband photoluminescence of Tin-Vacancy colour centres inSn-doped chemical vapour deposition diamond microcrystals.

Tin-Vacancy (Sn-V) colour centres in diamond have a spin coherence time in the millisecond range at temperatures of 2 K, so they are promising to be used in diamond-based quantum optical devices. However, the incorporation of large Sn atoms into a dense diamond lattice is a non-trivial problem. The objective of our work is to use microwave plasma-assisted chemical vapour deposition (CVD) to grow Sn-doped diamond with submicron SnO2 particles as a solid-state source of impurity. Well-faceted diamond microcrystals with sizes of afewmicrometres were formed on AlN substrates. The photoluminescence (PL) signal with zero-phonon line (ZPL) peak for Sn-V centre at ≈620 nm was measured at room temperature (RT) and at 7 K. The peak width (full width at half-maximum) was measured to be 1.1-1.7 nm at RT and ≈0.05 nm at 7 K. The observed variations of ZPL shape and position, in particular, narrowing of PL peak at RT and formation of single-line fine structure at low-T, are attributed to strain in the crystallites. The diamond doping with Sn via CVD process offers a new route to from Sn-V colour centre in the bulk of the diamond crystallites. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

A special issue preface: diamond for quantum applications.

This special issue discusses current progress in the utilization of defect centres in diamond as spin-photon interfaces for quantum applications. This issue is based on the discussions of the Theo Murphy meeting 'Diamond for quantum applications' which covered the recent progress of diamond growth and engineering for the creation and optimization of colour centres, toward the integration of diamond-based qubits in quantum systems. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

Absorption and birefringence study for reduced optical losses in diamond with high nitrogen-vacancy concentration.

The use of diamond colour centres such as the nitrogen-vacancy (NV) centre is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser-threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields. Enabling material properties for these techniques and their further improvements are low optical material losses via optical absorption of signal light and low birefringence. Here, we study systematically the behaviour of absorption around 700 nm and birefringence with increasing nitrogen- and NV-doping, as well as their behaviour during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen doping yet substitutional nitrogen does not seem to be the direct absorber. Birefringence reduces with increasing nitrogen doping. We identify multiple crystal defect concentrations via absorption spectroscopy and their changes during the material processing steps and thus identify potential causes of absorption and birefringence as well as strategies to fabricate chemical vapour deposition diamonds with high NV density yet low absorption and low birefringence. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

Low-temperature characteristics of an AlN/Diamond surface acoustic wave resonator

Phonons confined in mechanical resonators can be coupled to a variety of quantum systems and are expected to be applied to hybrid quantum systems. Diamond surface acoustic wave (SAW) devices are capable of high efficiency in phonon interaction with color centers in diamond. The temperature dependence of the quality factor is crucial for inferring the governing mechanism of coupling efficiency between phonons and color centers in diamond. In this paper, we report on the temperature dependence of the quality factor of an AlN/diamond SAW device from room temperature to 5 K. The temperature dependence of the quality factor and resonant frequency suggests that the mechanism of SAW dissipation in the AlN/diamond SAW resonator at 5 GHz is the phonon–phonon scattering in the Akheiser regime and that further cooling can be expected to improve the quality factor. This result provides a crucial guideline for the future design of AlN/diamond SAW devices.

Diamonds with Color Centers—A Novel Type of Functional Materials

Diamonds with Color Centers—A Novel Type of Functional Materials