Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancies (VB-) in hexagonal boron nitride (hBN) have recently gained interest as spin defects for quantum information processing and quantum sensing by a layered material. However, the boron vacancy can exist in a number of charge states in the hBN lattice, but only the -1 state has spin-dependent photoluminescence and acts as a spin-photon interface. Here, we investigate the charge state switching of VB defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states (VB- ⇌ VB0 + e-), occurring at rates controlled by excess electrons or holes injected into hBN by a layered heterostructure device. Our work provides a means to monitor and manipulate the VB charge state, and to stabilize the -1 state which is a prerequisite for spin manipulation and optical readout of the defect.

Deterministic Fabrication of a Coupled Cavity-Emitter System in Hexagonal Boron Nitride.

Light-matter interactions in optical cavities underpin many applications of integrated quantum photonics. Among various solid-state platforms, hexagonal boron nitride (hBN) is gaining considerable interest as a compelling van der Waals host of quantum emitters. However, progress to date has been limited by an inability to engineer simultaneously an hBN emitter and a narrow-band photonic resonator at a predetermined wavelength. Here, we overcome this problem and demonstrate deterministic fabrication of hBN nanobeam photonic crystal cavities with high quality factors over a broad spectral range (∼400 to 850 nm). We then fabricate a monolithic, coupled cavity-emitter system designed for a blue quantum emitter that has an emission wavelength of 436 nm and is induced deterministically by electron beam irradiation of the cavity hotspot. Our work constitutes a promising path to scalable on-chip quantum photonics and paves the way to quantum networks based on van der Waals materials.

Quantum Emitters in Hexagonal Boron Nitride.

Hexagonal boron nitride (hBN) has emerged as a fascinating platform to explore quantum emitters and their applications. Beyond being a wide-bandgap material, it is also a van der Waals crystal, enabling direct exfoliation of atomically thin layers─a combination which offers unique advantages over bulk, 3D crystals. In this Mini Review we discuss the unique properties of hBN quantum emitters and highlight progress toward their future implementation in practical devices. We focus on engineering and integration of the emitters with scalable photonic resonators. We also highlight recently discovered spin defects in hBN and discuss their potential utility for quantum sensing. All in all, hBN has become a front runner in explorations of solid-state quantum science with promising future prospects.

Nanoscale 3D Tomography by In-Flight Fluorescence Spectroscopy of Atoms Sputtered by a Focused Ion Beam.

Nanoscale fabrication and characterization techniques critically underpin a vast range of fields, including nanoelectronics and nanobiotechnology. Focused ion beam (FIB) techniques are appealing due to their high spatial resolution and widespread use for processing of nanostructured materials. Here, we introduce FIB-induced fluorescence spectroscopy (FIB-FS) as a nanoscale technique for spectroscopic detection of atoms sputtered by an ion beam. We use semiconductor heterostructures to demonstrate nanoscale lateral and depth resolution and show that it is limited by ion-induced intermixing of nanostructured materials. Sensitivity is demonstrated qualitatively by depth profiling of 3.5, 5, and 8 nm quantum wells and quantitatively by detection of trace-level impurities present at parts-per-million levels. The utility of the FIB-FS technique is demonstrated by characterization of quantum wells and Li-ion batteries. Our work introduces FIB-FS as a high-resolution, high-sensitivity, 3D analysis and tomography technique that combines the versatility of FIB nanofabrication techniques with the power of diffraction-unlimited fluorescence spectroscopy.

Purcell Enhancement of a Cavity-Coupled Emitter in Hexagonal Boron Nitride.

Integration of solid-state quantum emitters into nanophotonic circuits is a critical step towards fully on-chip quantum photonic-based technologies. Among potential materials platforms, quantum emitters in hexagonal boron nitride (hBN) have emerged as a viable candidate over the last years. While the fundamental physical properties have been intensively studied, only a few works have focused on the emitter integration into photonic resonators. Yet, for a potential quantum photonic material platform, the integration with nanophotonic cavities is an important cornerstone, as it enables the deliberate tuning of the spontaneous emission and the improved readout of distinct transitions for a quantum emitter. In this work, the resonant tuning of a monolithic cavity integrated hBN quantum emitter is demonstrated through gas condensation at cryogenic temperature. In resonance, an emission enhancement and lifetime reduction are observed, with an estimate for the Purcell factor of ≈15.

Identifying carbon as the source of visible single-photon emission from hexagonal boron nitride.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered increasing attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN via various bottom-up synthesis methods and directly through ion implantation, we provide direct evidence that the visible SPEs are carbon related. Room-temperature optically detected magnetic resonance is demonstrated on ensembles of these defects. We perform ion-implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of the simplest 12 carbon-containing defect species suggest the negatively charged [Formula: see text] defect as a viable candidate and predict that out-of-plane deformations make the defect environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to the deterministic engineering of these defects for quantum photonic devices.

Integrated room temperature single-photon source for quantum key distribution: publisher's note.

This publisher's note contains a correction to Opt. Lett.47, 1673 (2022)10.1364/OL.454450.

Integrated room temperature single-photon source for quantum key distribution.

High-purity single-photon sources (SPS) that can operate at room temperature are highly desirable for a myriad of applications, including quantum photonics and quantum key distribution. In this work, we realize an ultra-bright solid-state SPS based on an atomic defect in hexagonal boron nitride (hBN) integrated with a solid immersion lens (SIL). The SIL increases the source efficiency by a factor of six, and the integrated system is capable of producing over ten million single photons per second at room temperature. Our results are promising for practical applications of SPS in quantum communication protocols.

Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride.

We demonstrate the fabrication of large-scale arrays of single-photon emitters (SPEs) in hexagonal boron nitride (hBN). Bottom-up growth of hBN onto nanoscale arrays of dielectric pillars yields corresponding arrays of hBN emitters at the pillar sites. Statistical analysis shows that the pillar diameter is critical for isolating single defects, and diameters of ∼250 nm produce a near-unity yield of a single emitter at each pillar site. Our results constitute a promising route toward spatially controlled generation of hBN SPEs and provide an effective and efficient method to create large-scale SPE arrays. The results pave the way to scalability and high throughput fabrication of SPEs for advanced quantum photonic applications.

Coupling Spin Defects in Hexagonal Boron Nitride to Monolithic Bullseye Cavities.

Color centers in hexagonal boron nitride (hBN) are becoming an increasingly important building block for quantum photonic applications. Herein, we demonstrate the efficient coupling of recently discovered spin defects in hBN to purposely designed bullseye cavities. We show that boron vacancy spin defects couple to the monolithic hBN cavity system and exhibit a 6.5-fold enhancement. In addition, by comparative finite-difference time-domain modeling, we shed light on the emission dipole orientation, which has not been experimentally demonstrated at this point. Beyond that, the coupled spin system exhibits an enhanced contrast in optically detected magnetic resonance readout and improved signal-to-noise ratio. Thus, our experimental results, supported by simulations, constitute a first step toward integration of hBN spin defects with photonic resonators for a scalable spin-photon interface.

Bottom-Up Synthesis of Hexagonal Boron Nitride Nanoparticles with Intensity-Stabilized Quantum Emitters.

Fluorescent nanoparticles are widely utilized in a large range of nanoscale imaging and sensing applications. While ultra-small nanoparticles (size ≤10 nm) are highly desirable, at this size range, their photostability can be compromised due to effects such as intensity fluctuation and spectral diffusion caused by interaction with surface states. In this article, a facile, bottom-up technique for the fabrication of sub-10-nm hexagonal boron nitride (hBN) nanoparticles hosting photostable bright emitters via a catalyst-free hydrothermal reaction between boric acid and melamine is demonstrated. A simple stabilization protocol that significantly reduces intensity fluctuation by ≈85% and narrows the emission linewidth by ≈14% by employing a common sol-gel silica coating process is also implemented. This study advances a promising strategy for the scalable, bottom-up synthesis of high-quality quantum emitters in hBN nanoparticles.

Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature.

Optically addressable spins in wide-bandgap semiconductors are a promising platform for exploring quantum phenomena. While colour centres in three-dimensional crystals such as diamond and silicon carbide were studied in detail, they were not observed experimentally in two-dimensional (2D) materials. Here, we report spin-dependent processes in the 2D material hexagonal boron nitride (hBN). We identify fluorescence lines associated with a particular defect, the negatively charged boron vacancy ([Formula: see text]), showing a triplet (S = 1) ground state and zero-field splitting of ~3.5 GHz. We establish that this centre exhibits optically detected magnetic resonance at room temperature and demonstrate its spin polarization under optical pumping, which leads to optically induced population inversion of the spin ground state-a prerequisite for coherent spin-manipulation schemes. Our results constitute a step forward in establishing 2D hBN as a prime platform for scalable quantum technologies, with potential for spin-based quantum information and sensing applications.

Photonic Nanobeam Cavities with Nanopockets for Efficient Integration of Fluorescent Nanoparticles.

Integrating fluorescent nanoparticles with high-Q, small mode volume cavities is indispensable for nanophotonics and quantum technologies. To date, nanoparticles have largely been coupled to evanescent fields of cavity modes, which limits the strength of the interaction. Here, we developed both a cavity design and a fabrication method that enable efficient coupling between a fluorescent nanoparticle and a cavity optical mode. The design consists of a fishbone-shaped, one-dimensional photonic crystal cavity with a nanopocket located at the electric field maximum of the fundamental optical mode. Furthermore, the presence of a nanoparticle inside the pocket reduces the mode volume substantially and induces subwavelength light confinement. Our approach opens exciting pathways to achieve tight light confinement around fluorescent nanoparticles for applications in energy, sensing, lasing, and quantum technologies.

Single Photon Sources in Atomically Thin Materials.

Layered materials are very attractive for studies of light-matter interactions at the nanoscale. In particular, isolated quantum systems such as color centers and quantum dots embedded in these materials are gaining interest due to their potential use in a variety of quantum technologies and nanophotonics. Here, we review the field of nonclassical light emission from van der Waals crystals and atomically thin two-dimensional materials. We focus on transition metal dichalcogenides and hexagonal boron nitride and discuss the fabrication and properties of quantum emitters in these systems and proof-of-concept experiments that provide a foundation for their integration in on-chip nanophotonic circuits. These experiments include tuning of the emission wavelength, electrical excitation, and coupling of the emitters to waveguides, dielectric cavities, and plasmonic resonators. Finally, we discuss current challenges in the field and provide an outlook to further stimulate scientific discussion.

Hexagonal Boron Nitride Cavity Optomechanics.

Hexagonal boron nitride (hBN) is an emerging layered material that plays a key role in a variety of two-dimensional devices, and has potential applications in nanophotonics and nanomechanics. Here, we demonstrate the first cavity optomechanical system incorporating hBN. Nanomechanical resonators consisting of hBN beams with average dimensions of 12 µm × 1.2 µm × 28 nm and minimum predicted thickness of 8 nm were fabricated using electron beam induced etching and positioned in the optical near-field of silicon microdisk cavities. Of the multiple devices studied here a maximum 0.16 pm/[Formula: see text] sensitivity to the hBN nanobeam motion is demonstrated, allowing observation of thermally driven mechanical resonances with frequencies between 1 and 23 MHz, and largest mechanical quality factor of 1100 for a 23 MHz mode, at room temperature in high vacuum. In addition, the role of air damping is studied via pressure dependent measurements. Our results constitute an important step toward realizing integrated optomechanical circuits employing hBN.

Facile Production of Hexagonal Boron Nitride Nanoparticles by Cryogenic Exfoliation.

Fluorescent nanoparticles with optically robust luminescence are imperative to applications in imaging and labeling. Here we demonstrate that hexagonal boron nitride (hBN) nanoparticles can be reliably produced using a scalable cryogenic exfoliation technique with sizes below 10 nm. The particles exhibit bright fluorescence generated by color centers that act as atomic-size quantum emitters. We analyze their optical properties, including emission wavelength, photon-statistics, and photodynamics, and show that they are suitable for far-field super-resolution fluorescence nanoscopy. Our results provide a foundation for exploration of hBN nanoparticles as candidates for bioimaging, labeling, as well as biomarkers that are suitable for quantum sensing.

Deterministic Coupling of Quantum Emitters in 2D Materials to Plasmonic Nanocavity Arrays.

Quantum emitters in two-dimensional materials are promising candidates for studies of light-matter interaction and next generation, integrated on-chip quantum nanophotonics. However, the realization of integrated nanophotonic systems requires the coupling of emitters to optical cavities and resonators. In this work, we demonstrate hybrid systems in which quantum emitters in 2D hexagonal boron nitride (hBN) are deterministically coupled to high-quality plasmonic nanocavity arrays. The plasmonic nanoparticle arrays offer a high-quality, low-loss cavity in the same spectral range as the quantum emitters in hBN. The coupled emitters exhibit enhanced emission rates and reduced fluorescence lifetimes, consistent with Purcell enhancement in the weak coupling regime. Our results provide the foundation for a versatile approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays and 2D materials.

Dynamic Pattern Formation in Electron-Beam-Induced Etching.

We report highly ordered topographic patterns that form on the surface of diamond, span multiple length scales, and have a symmetry controlled by the precursor gas species used in electron-beam-induced etching (EBIE). The pattern formation dynamics reveals an etch rate anisotropy and an electron energy transfer pathway that is overlooked by existing EBIE models. We, therefore, modify established theory such that it explains our results and remains universally applicable to EBIE. The patterns can be exploited in controlled wetting, optical structuring, and other emerging applications that require nano- and microscale surface texturing of a wide band-gap material.

Engineering Quantum Light Sources with Flat Optics.

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field are highlighted.

Optically addressable spin defects coupled to bound states in the continuum metasurfaces.

Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are layered crystalline solids with appealing properties for investigating light-matter interactions at the nanoscale. hBN has emerged as a versatile building block for nanophotonic structures, and the recent identification of native optically addressable spin defects has opened up exciting possibilities in quantum technologies. However, these defects exhibit relatively low quantum efficiencies and a broad emission spectrum, limiting potential applications. Optical metasurfaces present a novel approach to boost light emission efficiency, offering remarkable control over light-matter coupling at the sub-wavelength regime. Here, we propose and realise a monolithic scalable integration between intrinsic spin defects in hBN metasurfaces and high quality (Q) factor resonances, exceeding 102, leveraging quasi-bound states in the continuum (qBICs). Coupling between defect ensembles and qBIC resonances delivers a 25-fold increase in photoluminescence intensity, accompanied by spectral narrowing to below 4 nm linewidth and increased narrowband spin-readout efficiency. Our findings demonstrate a new class of metasurfaces for spin-defect-based technologies and pave the way towards vdW-based nanophotonic devices with enhanced efficiency and sensitivity for quantum applications in imaging, sensing, and light emission.