Coherent dynamics of strongly interacting electronic spin defects in hexagonal boron nitride

Optically active spin defects in wide-bandgap materials have many potential applications in quantum information and quantum sensing. Spin defects in two-dimensional layered van der Waals materials are just emerging to be investigated. Here we demonstrate that optically-addressable spin ensembles in hexagonal boron nitride (hBN) can be generated by femtosecond laser irradiation. We observe optically detected magnetic resonance (ODMR) of hBN spin defects created by laser irradiation. We show that the creation of spin defects in hBN is strongly affected by the pulse energy of the femtosecond laser. When the laser pulse number is less than a few thousand, the pulse number only affects the density of defects but not the type of defects. With proper laser parameters, spin defects can be generated with a high probability of success. Our work provides a convenient way to create spin defects in hBN by femtosecond laser writing, which shows promising prospects for quantum technologies.

Spin-active defects in layered hexagonal boron nitride (hBN) crystals have attracted increased attention in quantum sensing. Notably, the recently discovered negatively charged boron vacancy (V B −) center stands out due to its optical addressability and coherent controllability. Among the various methods reported for generating such defects, ion implantation is notable as a readily accessible technique. In this paper, the properties of V B − defects in hBN generated via ion implantation are extensively studied. We achieve a ubiquitous distribution of highly stable defects across the crystal sample, and find that the ion beam current density, rather than fluence, plays a critical role in determining the uniformity and density of defects. The generated defects display bright and stable photoluminescence, and we explicitly investigate the dependence of spin properties on factors such as laser, microwave power, and duration. An intriguing phenomenon is observed wherein the peak contrast exceeds 20% without any enhancing techniques in the optically detected magnetic resonance spectrum for some special defects. Our results provide valuable insights and suggestions for the controlled generation of V B − defects in hBN through ion implantation.

Quantum technology grown out of quantum information theory, including quantum communication, quantum computation and quantum sensing, not only provides powerful research tools for numerous fields, but also is expected to go to civilian use in the future. Solid-state spin-active defects are one of promising platforms for quantum technology, and the host materials include three-dimensional diamond and silicon carbide, and the emerging two-dimensional hexagonal boron nitride (hBN) and transition-metal dichalcogenides. In this review, we will focus on the spin defects in hBN, and summarize theoretical and experimental progresses made in understanding properties of these spin defects. In particular, the combination of theoretical prediction and experimental verification is highlighted. We also discuss the future advantages and challenges of solid-state spins in hBN on the path towards quantum information applications.

Understanding and engineering atomic defects in hexagonal boron nitride (hBN) provides a powerful platform for realizing solid-state quantum emitters and spin qubits, advancing the field of quantum information science and technologies. However, the full potential of such quantum defects remains locked by the critical lack of a deterministic structure-property relationship at the atomic scale. Here, we demonstrate a strategy to atomically engineer and decipher quantum defects in hBN by integrating scanning tunneling microscopy/spectroscopy (STM/STS) and noncontact atomic force-microscopy with a CO-functionalized tip. We implemented controllable argon ion bombardment to create both boron vacancies (VB) and nitrogen vacancies (VN) in submonolayer hBN grown on Cu(111). Simultaneously, encapsulated Ar species trapped between hBN and Cu(111) locally lift the hBN to form nanobubbles, thereby decoupling atomic vacancies from the metal substrate and enabling direct probing of their electronic states. For the on-bubble VN, STS measurement reveals a prominent in-gap state with a phonon replica. Furthermore, with aid of STM tip-assisted manipulation, we demonstrate that the tuning of nanobubble sizes modulates their strain profile, thereby modulating the energetic positions of electronic states in on-bubble defects, corroborated by density functional calculations. Our studies offer insight into the intrinsic defect structures in hBN and quantum defect engineering via local strain engineering.

Spin defects in hexagonal boron nitride, and specifically the negatively charged boron vacancy (VB - ) centers, are emerging candidates for quantum sensing. However, the VB - defects suffer from low quantum efficiency and, as a result, exhibit weak photoluminescence. In this work, a scalable approach is demonstrated to dramatically enhance the VB - emission by coupling to a plasmonic gap cavity. The plasmonic cavity is composed of a flat gold surface and a silver cube, with few-layer hBN flakes positioned in between. Employing these plasmonic cavities, two orders of magnitude are extracted in photoluminescence enhancement associated with a corresponding twofold enhancement in optically detected magnetic resonance contrast. The work will be pivotal to progress in quantum sensing employing 2D materials, and in realization of nanophotonic devices with spin defects in hexagonal boron nitride.

Hexagonal boron nitride(hBN) has emerged as a promising materialplatform for nanophotonics and quantum sensing, hosting opticallyactive defects with exceptional properties such as high brightnessand large spectral tuning. However, precise control over deterministicspatial positioning of emitters in hBN remained elusive for a longtime, limiting their proper correlative characterization and applicationsin hybrid devices. Recently, focused ion beam (FIB) systems provedto be useful to engineer several types of spatially defined emitterswith various structural and photophysical properties. Here we systematicallyexplore the physical processes leading to the creation of opticallyactive defects in hBN using FIB and find that beam–substrateinteraction plays a key role in the formation of defects. These findingsare confirmed using transmission electron microscopy, which revealslocal mechanical deterioration of the hBN layers and local amorphizationof ion beam irradiated hBN. Additionally, we show that, upon exposureto water, amorphized hBN undergoes a structural and optical transitionbetween two defect types with distinctive emission properties. Moreover,using super-resolution optical microscopy combined with atomic forcemicroscopy, we pinpoint the exact location of emitters within thedefect sites, confirming the role of defected edges as primary sourcesof fluorescent emission. This lays the foundation for FIB-assistedengineering of optically active defects in hBN with high spatial andspectral control for applications ranging from integrated photonics,to nanoscale sensing, and to nanofluidics.

Spin defects existing in van der Waals materials attract wide attention thanks to their natural advantages for in situ quantum sensing, especially the negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (h-BN). Here we systematically investigate the laser and microwave power broadening in continuous-wave optically detected magnetic resonance (ODMR) of the VB- ensemble in h-BN, by revealing the behaviors of ODMR contrast and line width as a function of the laser and microwave powers. The experimental results are well explained by employing a two-level simplified model of ODMR dynamics. Furthermore, with optimized power, the DC magnetic field sensitivity of VB- ensemble is significantly improved up to 2.87 ± 0.07 μT/. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Recently, Stone–Wales (SW) defects gradually attracted people’s research interest because of their unique properties. The theoretical research indicated that the SW defect in hexagonal boron nitride (h-BN) can lead to new defect levels in bandgap, making h-BN apply in ultraviolet emitters. However, the SW defect is always observed in graphene and rarely observed in h-BN in the experiments. Here, we confirmed the SW defects are not easily formed in h-BN under thermodynamic conditions by first-principles calculations. Specifically, the monolayer h-BN with SW defect (h-BN-SW) has the weak bond strength, dynamic stability and high-temperature thermal stability, facilitating the healing of SW defects under high-temperature conditions and the role of hydrogen. Additionally, we found the SW defect in AB stacked h-BN (AB-h-BN) have good mechanical stability, dynamic stability and thermodynamic stability than h-BN-SW, especially for AB-h-BN-2SW (2SW defects formed in upper and lower layer of AB-h-BN, respectively), which can meet the requirements for its application in electronic devices. Even under thermodynamic conditions, the formation of SW defects is extremely challenging. Electron beam irradiation technology provides a window for the generation of SW defects in h-BN. This offers opportunities for the introduction and control of SW defects, while also creating potential for their application in electronic devices. Moreover, we found that the absorption peak broadens, and a new absorption peak appears with the generation of SW defects, which is mainly induced by the decrease of bandgap and the generation of defect levels. Our research can provide theoretical guidance at atomic scale for designing and applying h-BN with SW defect in the experiments.

Two-dimensional (2D) Van der Waals materials have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to enhance and control this interaction by integrating these 2D materials with microcavities, and metamaterials. I will first discuss the formation of strongly coupled half-light half-matter quasiparticles (microcavity excitonpolaritons) [1], their spin-optic [2], and electrical control/excitation (Fig. 1) [3], in the 2D transition metal dichalcogenide (TMD) systems. Prospects for enhanced nonlinear optical interaction using Rydberg states in TMDs will be discussed. Use of photonic hypercrystals and hyperbolic media to enhance the spontaneous emission from the TMDs will also be presented. Following this, I will discuss the routing of valley excitons in 2D TMDs using chiral metasurfaces (Fig. 2) [4]. Finally, time permitting, I will briefly talk about developing deterministic quantum emitters at room temperature using defects in hexagonal boron nitride [5] and prospects of coupling these emitters to high Q resonators.

Spin defects in solid-state materials are strong candidate systems for quantum information technology and sensing applications. Here we explore in details the recently discovered negatively charged boron vacancies (VB−) in hexagonal boron nitride (hBN) and demonstrate their use as atomic scale sensors for temperature, magnetic fields and externally applied pressure. These applications are possible due to the high-spin triplet ground state and bright spin-dependent photoluminescence of the VB−. Specifically, we find that the frequency shift in optically detected magnetic resonance measurements is not only sensitive to static magnetic fields, but also to temperature and pressure changes which we relate to crystal lattice parameters. We show that spin-rich hBN films are potentially applicable as intrinsic sensors in heterostructures made of functionalized 2D materials.

Single-photon emitters play an essential role in quantum technologies, including quantum computing and quantum communications. Atomic defects in hexagonal boron nitride ( h-BN) have recently emerged as new room-temperature single-photon emitters in solid-state systems, but the development of scalable and tunable h-BN single-photon emitters requires external methods that can control the emission energy of individual defects. Here, by fabricating van der Waals heterostructures of h-BN and graphene, we demonstrate the electrical control of single-photon emission from atomic defects in h-BN via the Stark effect. By applying an out-of-plane electric field through graphene gates, we observed Stark shifts as large as 5.4 nm per GV/m. The Stark shift generated upon a vertical electric field suggests the existence of out-of-plane dipole moments associated with atomic defect emitters, which is supported by first-principles theoretical calculations. Furthermore, we found field-induced discrete modification and stabilization of emission intensity, which were reversibly controllable with an external electric field.

We present temperature-dependent micro-PL studies on a single defect in hexagonal boron nitride. A zero-phonon line emission accompanied by Stokes and anti-Stokes phonon sidebands (≈ 6.5 meV) with a Debye-Waller factor of 0.59 is observed.

The atomic structure, stability, and dynamics of defects in hexagonal boron nitride (h-BN) are investigated using an aberration-corrected transmission electron microscope operated at 80 kV between room temperature and 1000 °C. At temperatures above 700 °C, parallelogram- and hexagon-shaped defects with zigzag edges become prominent, in contrast to the triangular defects typically observed at lower temperatures. The appearance of 120° corners at defect vertices indicates the coexistence of both N- and B-terminated zigzag edges in the same defect. In situ dynamics studies show that the hexagonal holes grow by electron-induced sputtering of B-N chains, and that at high temperatures these chains can migrate from one defect corner to another. We complement the experiments with first-principles calculation which consider the thermal equilibrium formation energy of different defect configurations. It is shown that, below a critical defect size, hexagonal defects have the lowest formation energy and therefore are the more-stable configuration, and triangular defects are energetically metastable but can be "frozen in" under experimental conditions. We also discuss the possible contributions of several dynamic processes to the temperature-dependent defect formation.

Electron paramagnetic resonance (EPR) has been used to investigate defects in hexagonal boron nitride (BN), polycrystalline cubic BN, BN and BN: C films produced by reactive sputtering. The dominant paramagnetic centre is attributed to the N vacancy. The Epr relaxation parameters have been deduced from the saturation data. In the investigated temperature range the spin-lattice relaxation rate was found to follow an almost linear temperature dependence possibly related to local modes.

Review for "Optimization of Sensing Sensitivity and Coherence Properties of Spin Defects in Hexagonal Boron Nitride"