Spin defects in hexagonal boron nitride for strain sensing on nanopillar arrays.

Two-dimensional hexagonal boron nitride (hBN) has attracted much attention as a platform for studies of light-matter interactions at the nanoscale, especially in quantum nanophotonics. Recent efforts have focused on spin defects, specifically negatively charged boron vacancy (VB-) centers. Here, we demonstrate a scalable method to enhance the VB- emission using an array of SiO2 nanopillars. We achieve a 4-fold increase in photoluminescence (PL) intensity, and a corresponding 4-fold enhancement in optically detected magnetic resonance (ODMR) contrast. Furthermore, the VB- ensembles provide useful information about the strain fields associated with the strained hBN at the nanopillar sites. Our results provide an accessible way to increase the emission intensity as well as the ODMR contrast of the VB- defects, while simultaneously form a basis for miniaturized quantum sensors in layered heterostructures.

Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride.

Optically addressable solid-state spins are important platforms for quantum technologies, such as repeaters and sensors. Spins in two-dimensional materials offer an advantage, as the reduced dimensionality enables feasible on-chip integration into devices. Here, we report room-temperature optically detected magnetic resonance (ODMR) from single carbon-related defects in hexagonal boron nitride with up to 100 times stronger contrast than the ensemble average. We identify two distinct bunching timescales in the second-order intensity-correlation measurements for ODMR-active defects, but only one for those without an ODMR response. We also observe either positive or negative ODMR signal for each defect. Based on kinematic models, we relate this bipolarity to highly tuneable internal optical rates. Finally, we resolve an ODMR fine structure in the form of an angle-dependent doublet resonance, indicative of weak but finite zero-field splitting. Our results offer a promising route towards realising a room-temperature spin-photon quantum interface in hexagonal boron nitride.

Rational Control on Quantum Emitter Formation in Carbon-Doped Monolayer Hexagonal Boron Nitride.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) are promising candidates for quantum light generation. Despite this, techniques to control the formation of hBN SPEs down to the monolayer limit are yet to be demonstrated. Recent experimental and theoretical investigations have suggested that the visible wavelength single-photon emitters in hBN originate from carbon-related defects. Here, we demonstrate a simple strategy for controlling SPE creation during the chemical vapor deposition growth of monolayer hBN via regulating surface carbon concentration. By increasing the surface carbon concentration during hBN growth, we observe increases in carbon doping levels by 2.4-fold for B-C bonds and 1.6-fold for N-C bonds. For the same samples, we observe an increase in the SPE density from 0.13 to 0.30 emitters/µm2. Our simple method enables the reliable creation of hBN SPEs in monolayer samples for the first time, opening the door to advanced two-dimensional (2D) quantum state engineering.

Coupling Spin Defects in a Layered Material to Nanoscale Plasmonic Cavities.

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.

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.

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.

Coupling Hexagonal Boron Nitride Quantum Emitters to Photonic Crystal Cavities.

Quantum photonics technologies require a scalable approach for the integration of nonclassical light sources with photonic resonators to achieve strong light confinement and enhancement of quantum light emission. Point defects from hexagonal boron nitride (hBN) are among the front runners for single photon sources due to their ultra-bright emission; however, the coupling of hBN defects to photonic crystal cavities has so far remained elusive. Here we demonstrate on-chip integration of hBN quantum emitters with photonic crystal cavities from silicon nitride (Si3N4) and achieve an experimentally measured quality factor (Q-factor) of 3300 for hBN/Si3N4 hybrid cavities. We observed 6-fold photoluminescence enhancement of an hBN single photon emission at room temperature. Our work will be useful for further development of cavity quantum electrodynamic experiments and on-chip integration of two-dimensional (2D) materials.

Strain-Induced Modification of the Optical Characteristics of Quantum Emitters in Hexagonal Boron Nitride.

Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for the realization of integrated quantum photonic systems. However, their spectral inhomogeneity currently limits their potential applications. Here, tensile strain is applied to quantum emitters embedded in few-layer hBN films and both red and blue spectral shifts are realized with tuning magnitudes up to 65 meV, a record for any 2D quantum source. Reversible tuning of the emission and related photophysical properties is demonstrated. Rotation of the optical dipole in response to strain is also observed, suggesting the presence of a second excited state. A theoretical model is derived to describe strain-based tuning in hBN, and the rotation of the optical dipole. The study demonstrates the immense potential for strain tuning of quantum emitters in layered materials to enable their employment in scalable quantum photonic networks.

Engineering and Tuning of Quantum Emitters in Few-Layer Hexagonal Boron Nitride.

Quantum technologies require robust and photostable single photon emitters (SPEs). Hexagonal boron nitride (hBN) has recently emerged as a promising candidate to host bright and optically stable SPEs operating at room temperature. However, the emission wavelength of the fluorescent defects in hBN has, to date, been shown to be uncontrolled, with a widespread of zero phonon line (ZPL) energies spanning a broad spectral range (hundreds of nanometers), which hinders the potential development of hBN-based devices and applications. Here we demonstrate chemical vapor deposition growth of large-area, few-layer hBN films that host large quantities of SPEs: ∼100-200 per 10 × 10 µm2. More than 85% of the emitters have a ZPL at (580 ± 10) nm, a distribution that is an order of magnitude narrower than reported previously. Furthermore, we demonstrate tuning of the ZPL wavelength using ionic liquid devices over a spectral range of up to 15 nm-the largest obtained to date from any solid-state SPE. The fabricated devices illustrate the potential of hBN for the development of hybrid quantum nanophotonic and optoelectronic devices based on two-dimensional materials.