© 2020, The Author(s), under exclusive licence to Springer Nature Limited. 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 (VB−), 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.

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.

© 2019 Zai-Quan Xu, Igor Aharonovich et al., published by De Gruyter 2019. Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) are promising components for on-chip quantum information processing. Recently, large-area hBN films prepared by chemical vapor deposition (CVD) were found to host uniform, high densities of SPEs. However, the purity of these emitters has, to date, been low, hindering their applications in practical devices. In this work, we present two methods for post-growth processing of hBN, which significantly improve SPEs in hBN films that had been transferred from substrates used for CVD. The emitters exhibit high photon purities in excess of 90% and narrow linewidths of ~3 nm at room temperature. Our work lays a foundation for producing high-quality emitters in an ultra-compact two-dimensional material system and paves the way for deployment of hBN SPEs in scalable on-chip photonic and quantum devices.

© 2019 Author(s). Single-photon emitters in gallium nitride (GaN) are gaining interest as attractive quantum systems due to the well-established techniques for growth and nanofabrication of the host material, as well as its remarkable chemical stability and optoelectronic properties. We investigate the nature of such single-photon emitters in GaN with a systematic analysis of various samples produced under different growth conditions. We explore the effect that intrinsic structural defects (dislocations and stacking faults), doping, and crystal orientation in GaN have on the formation of quantum emitters. We investigate the relationship between the position of the emitters - determined via spectroscopy and photoluminescence measurements - and the location of threading dislocations - characterized both via atomic force microscopy and cathodoluminescence. We find that quantum emitters do not correlate with stacking faults or dislocations; instead, they are more likely to originate from point defects or impurities whose density is modulated by the local extended defect density.

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Integrated quantum photonic circuitry is an emerging topic that requires efficient coupling of quantum light sources to waveguides and optical resonators. So far, great effort is devoted to engineering on-chip systems from 3D crystals such as diamond or gallium arsenide. In this study, room-temperature coupling is demonstrated of quantum emitters embedded in layered hexagonal boron nitride to an on-chip aluminum nitride waveguide. 1.35% light coupling efficiency is achieved in the device and transmission of single photons through the waveguide is demonstrated. The results serve as foundation for integrating layered materials to on-chip components and realizing integrated quantum photonic circuitry.

Single-photon sources in solid-state systems are widely explored as fundamental constituents of numerous quantum-based technologies. We report the observation of single-photon emitters in zinc sulfide and present their photophysical properties via established spectroscopy techniques. The emitter behaves like a three-level system with an intermediate metastable state. It emits at ∼640  nm, and its emission is linearly polarized, with a lifetime of (2.2±0.8) ns. The existence of single-photon sources in zinc sulfide is appealing due to the well-established manufacturing techniques of the material, its versatile technological uses, as well as the availability of many zinc isotopes with potential for designing ad hoc emitter-host pairs with tailored properties.

Hexagonal boron nitride (hBN) mono and multilayers are promising hosts for room-temperature single photon emitters (SPEs). In this work we explore high-energy (∼MeV) electron irradiation as a means to generate stable SPEs in hBN. We investigate four types of exfoliated hBN flakes-namely, high-purity multilayers, isotopically pure hBN, carbon-rich hBN multilayers and monolayered material-and find that electron irradiation increases emitter concentrations dramatically in all samples. Furthermore, the engineered emitters are located throughout hBN flakes (not only at flake edges or grain boundaries) and do not require activation by high-temperature annealing of the host material after electron exposure. Our results provide important insights into controlled formation of hBN SPEs and may aid in identification of their crystallographic origin.

The assembly of quantum nanophotonic systems with plasmonic resonators is important for fundamental studies of single photon sources as well as for on-chip information processing. In this work, we demonstrate the controllable nanoassembly of gold nanospheres with ultra-bright narrow-band quantum emitters in 2D layered hexagonal boron nitride (hBN). We utilize an atomic force microscope (AFM) tip to precisely position gold nanospheres to close proximity to the quantum emitters and observe the resulting emission enhancement and fluorescence lifetime reduction. The extreme emitter photostability permits analysis at high excitation powers, and delineation of absorption and emission enhancement caused by the plasmonic resonators. A fluorescence enhancement of over 300% is achieved experimentally for quantum emitters in hBN, with a radiative quantum efficiency of up to 40% and a saturated count rate in excess of 5 × 106 counts per s. Our results are promising for the future employment of quantum emitters in hBN for integrated nanophotonic devices and plasmonic based nanosensors.

© 2017 American Chemical Society. Quantum emitters in layered hexagonal boron nitride (hBN) have recently attracted a great deal of attention as promising single photon sources. In this work, we demonstrate resonant excitation of a single defect center in hBN, one of the most important prerequisites for employment of optical sources in quantum information processing applications. We observe spectral line widths of an hBN emitter narrower than 1 GHz while the emitter experiences spectral diffusion. Temporal photoluminescence measurements reveal an average spectral diffusion time of around 100 ms. An on-resonance photon antibunching measurement is also realized. Our results shed light on the potential use of quantum emitters from hBN in nanophotonics and quantum information processing applications.

Copyright © 2018 American Chemical Society. Single crystal diamond membranes that host optically active emitters are highly attractive components for integrated quantum nanophotonics. In this work we demonstrate bottom-up synthesis of single crystal diamond membranes containing germanium vacancy (GeV) color centers. We employ a lift-off technique to generate the membranes and perform chemical vapor deposition in the presence of a germanium source to realize the in situ doping. Finally, we show that these membranes are suitable for engineering of photonic resonators such as microdisk cavities with quality factors of ∼1500. The robust and scalable approach to engineer single crystal diamond membranes containing emerging color centers is a promising pathway for the realization of diamond integrated quantum nanophotonic circuits on a chip.

Two-dimensional transition-metal dichalcogenides (TMDC) are of great interest
for on-chip nanophotonics due to their unique optoelectronic properties. Here,
we propose and realize coupling of tungsten diselenide (WSe2) monolayers to
circular Bragg grating structures to achieve enhanced emission. The interaction
between WSe2 and the resonant mode of the structure results in Purcell-enhanced
emission, while the symmetric geometrical structure improves the directionality
of the out-coupling stream of emitted photons. Furthermore, this hybrid
structure produces a record high contrast of the spin valley readout (> 40%)
revealed by the polarization resolved photoluminescence (PL) measurements. Our
results are promising for on-chip integration of TMDC monolayers with optical
resonators for nanophotonic circuits.

© 2017 Elsevier B.V. Optical properties of color centers in diamond have been the subject of intense research due to their promising applications in quantum photonics. In this work we study the optical properties of Xe related color centers implanted into nitrogen rich (type IIA) and an ultrapure, electronic grade diamond. The Xe defect has two zero phonon lines at ∼794 nm and 811 nm, which can be effectively excited using both green and red excitation, however, its emission in the nitrogen rich diamond is brighter. Near resonant excitation is performed at cryogenic temperatures and luminescence is probed under strong magnetic field. Our results are important towards the understanding of the Xe related defect and other near infrared color centers in diamond.

© 2018 American Physical Society. Hexagonal boron nitride (h-BN) is an attractive van der Waals material for studying fluorescent defects due to its large band gap. In this work, we demonstrate enhanced pink color due to neutron irradiation and perform electron paramagnetic resonance (EPR) measurements. The point defects are tentatively assigned to doubly occupied nitrogen vacancies with (S=1) and a zero-field splitting (D=1.2GHz). These defects are associated with a broad visible optical absorption band and a near-infrared photoluminescence band centered at ∼490 and 820 nm, respectively. The EPR signal intensities are strongly affected by thermal treatments in the temperature range between 600 °C and 800 °C, where also the irradiation-induced pink color is lost. Our results are important for understanding of point defects in h-BN and their deployment for quantum and integrated photonic applications.

Layered van der Waals materials are emerging as compelling two-dimensional platforms for nanophotonics, polaritonics, valleytronics and spintronics, and have the potential to transform applications in sensing, imaging and quantum information processing. Among these, hexagonal boron nitride (hBN) is known to host ultra-bright, room-temperature quantum emitters, whose nature is yet to be fully understood. Here we present a set of measurements that give unique insight into the photophysical properties and level structure of hBN quantum emitters. Specifically, we report the existence of a class of hBN quantum emitters with a fast-decaying intermediate and a long-lived metastable state accessible from the first excited electronic state. Furthermore, by means of a two-laser repumping scheme, we show an enhanced photoluminescence and emission intensity, which can be utilized to realize a new modality of far-field super-resolution imaging. Our findings expand current understanding of quantum emitters in hBN and show new potential ways of harnessing their nonlinear optical properties in sub-diffraction nanoscopy.

Artificial atomic systems in solids are becoming increasingly important building blocks in quantum information processing and scalable quantum nanophotonic networks. Amongst numerous candidates, 2D hexagonal boron nitride has recently emerged as a promising platform hosting single photon emitters. Here, we report a number of robust plasma and thermal annealing methods for fabrication of emitters in tape-exfoliated hexagonal boron nitride (hBN) crystals. A two-step process comprising Ar plasma etching and subsequent annealing in Ar is highly robust, and yields an eight-fold increase in the concentration of emitters in hBN. The initial plasma-etching step generates emitters that suffer from blinking and bleaching, whereas the two-step process yields emitters that are photostable at room temperature with emission wavelengths greater than ∼700 nm. Density functional theory modeling suggests that the emitters might be associated with defect complexes that contain oxygen. This is further confirmed by generating the emitters via annealing hBN in air. Our findings advance the present understanding of the structure of quantum emitters in hBN and enhance the nanofabrication toolkit needed to realize integrated quantum nanophotonic circuits.

Multiphoton fluorescence microscopy (MPM), using near infrared excitation light, provides increased penetration depth, decreased detection background, and reduced phototoxicity. Using stimulated emission depletion (STED) approach, MPM can bypass the diffraction limitation, but it requires both spatial alignment and temporal synchronization of high power (femtosecond) lasers, which is limited by the inefficiency of the probes. Here, we report that upconversion nanoparticles (UCNPs) can unlock a new mode of near-infrared emission saturation (NIRES) nanoscopy for deep tissue super-resolution imaging with excitation intensity several orders of magnitude lower than that required by conventional MPM dyes. Using a doughnut beam excitation from a 980 nm diode laser and detecting at 800 nm, we achieve a resolution of sub 50 nm, 1/20th of the excitation wavelength, in imaging of single UCNP through 93 μm thick liver tissue. This method offers a simple solution for deep tissue super resolution imaging and single molecule tracking.

© 2017 Elsevier B.V. Remotely triggered drug delivery using nanoparticles is an area of great interest for targeted therapy to fight cancer. In this work, we synthesized photoresponsive nanoparticles to remotely initiate the delivery of doxorubicin (DOX) to 3D cultured human breast cancer cells (MCF-7) via NIR two-photon excitation (TPE) using nitrogen-doped and surface passivated (PEG 200 ) carbon nanohybrid dots (CNDs). On-demand drug delivery relies on bio-compatible, photo-responsive nano-carriers with high quantum yields. A facile (5 min synthesis) one-pot hot plate method was used to synthesize functionalized and surface passivated carbon nanohybrid dots (CND-P) having 53% quantum yield (QY). Compared to CNDs prepared from citric acid (CND-C) and citric acid plus urea (CND-N), CND-P had QY enhanced by factors of 12.6 and 4.4, respectively. The up-converted emission intensity of CND-P was strengthened by a factor of 4.5 over that of CND-N from nitrogen doped CNDs for similar test conditions (wavelength, excitation power and concentration). The drug loading capacity of CND-P was measured to be 0.98 w/w with the ability to release DOX via two-photon excitation (TPE). Intense green luminescence was observed under both 360 and 780 nm lasers using single and two-photon excitations. The highly biocompatible CND-P showed 88% cell viability at concentrations as high as 1100 µg/mL. The combined chemo- and photothermal therapeutic effect of the DOX-loaded CND-P (CND-P@DOX) complex resulted in the death of 78% of the MCF-7 cells compared to 59% with DOX alone.

Single-photon emitters with narrow linewidths are highly sought after for applications
in quantum information processing and quantum communications. In this
letter, we report on a bright, highly polarized near infrared single photon emitter
embedded in diamond nanocrystals with a narrow, sub-GHz optical linewidth
at 10 K. The observed zero-phonon line at ∼780 nm is optically stable under low
power excitation and blue shifts as the excitation power increases. Our results highlight
the prospect for using new near infrared color centers in nanodiamonds for
quantum applications

© 2017 American Chemical Society. Realization of quantum information and communications technologies requires robust, stable solid-state single-photon sources. However, most existing sources cease to function above cryogenic or room temperature due to thermal ionization or strong phonon coupling, which impedes their emissive and quantum properties. Here we present an efficient single-photon source based on a defect in a van der Waals crystal that is optically stable and operates at elevated temperatures of up to 800 K. The quantum nature of the source and the photon purity are maintained upon heating to 800 K and cooling back to room temperature. Our report of a robust high-temperature solid-state single photon source constitutes a significant step toward practical, integrated quantum technologies for real-world environments.

Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.

Arrays of fluorescent nanoparticles are highly sought after for applications in sensing, nanophotonics and quantum communications. Here we present a simple and robust method of assembling fluorescent nanodiamonds into macroscopic arrays. Remarkably, the yield of this directed assembly process is greater than 90% and the assembled patterns withstand ultra-sonication for more than three hours. The assembly process is based on covalent bonding of carboxyl to amine functional carbon seeds and is applicable to any material, and to non-planar surfaces. Our results pave the way to directed assembly of sensors and nanophotonics devices.

Quantum photonics technologies require a scalable approach for integration of
non-classical light sources with photonic resonators to achieve strong light
confinement and enhancement of quantum light emission. Point defects from
hexagonal Boron Nitride (hBN) are amongst the front runners for single photon
sources due to their ultra bright emission, however, 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 experimentally measured Q-factor of 3,300
for hBN/Si3N4 hybrid cavities. We observed 9-fold photoluminescence enhancement
of a hBN single photon emission at room temperature. Our work paves the way
towards hybrid integrated quantum photonics with hBN, and outlines an excellent
path for further development of cavity quantum electrodynamic experiments and
on-chip integration of 2D materials.

© 2019 IEEE. Quantum optical information systems offer the potential for secure communication and fast quantum computation. To fully characterise a quantum optical system one has to use quantum tomography [1]. Integration of quantum optics onto photonic chips provides advantages such as miniaturisation and stability, and also significantly improves quantum tomography using both re-configurable [2], and more recently, simpler static designs [3,4]. These on-chip designs have, so far, only used probabilistic single photon sources. Here we are working towards quantum tomography using a true deterministic source - a quantum dot. The scheme of the proposed experiment is shown in Fig. 1A. So far we have fabricated and characterised the performance of an InGaAs quantum dot monolithically integrated into a microlens [5], and completed the design, fabrication and classical characterisation of a photonic chip for quantum tomography.

© COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only. Quantum optical information systems offer the potential for secure communication and fast quantum computation. To fully characterise a quantum optical system one has to use quantum tomography.1 The integration of quantum optics onto photonic chips provides advantages such as miniaturisation and stability, significantly improving quantum tomography using both re-configurable, and more recently, simpler static designs. These on-chip designs have, so far, only used probabilistic single photon sources. Here we are working towards quantum tomography using a true deterministic source-an InGaAs quantum dot.

Layered van der Waals materials are emerging as compelling two-dimensional platforms for nanophotonics, polaritonics, valleytronics and spintronics, and have the potential to transform applications in sensing, imaging and quantum information processing. Amongst these, hexagonal boron nitride (hBN) is known to host ultra-bright, room-temperature quantum emitters, whose nature is yet to be fully understood. Here, we present a set of measurements which give unique insight into the photophysical properties and level structure of hBN quantum emitters. Specifically, we report the existence of a class of hBN quantum emitters with a fast-decaying intermediate and a long-lived metastable state accessible from the first excited electronic state. Furthermore, by means of a two-laser repumping scheme, we show an enhanced photoluminescence and emission intensity which can be utilized to realize a new modality of far-field super-resolution imaging. Our findings expand current understanding of quantum emitters in hBN and show new potential ways of harnessing their nonlinear optical properties in sub-diffraction nanoscopy.

Single crystal diamond membranes that host optically active emitters are
highly attractive components for integrated quantum nanophotonics. In this work
we demonstrate bottom-up synthesis of single crystal diamond membranes
containing the germanium vacancy (GeV) color centers. We employ a lift-off
technique to generate the membranes and perform chemical vapour deposition in a
presence of germanium oxide to realize the insitu doping. Finally, we show that
these membranes are suitable for engineering of photonic resonators such as
microring cavities with quality factors of 1500. The robust and scalable
approach to engineer single crystal diamond membranes containing emerging color
centers is a promising pathway for realization of diamond integrated quantum
nanophotonic circuits on a chip.

Quantum technologies require robust and photostable single photon emitters
(SPEs) that can be reliably engineered. Hexagonal boron nitride (hBN) has
recently emerged as a promising candidate host to 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. The
emitters usually display a large spread of zero phonon line (ZPL) energies
spanning over a broad spectral range (hundreds of nanometers), which hinders
the potential development of hBN-based devices and applications. We demonstrate
bottom-up, chemical vapor deposition growth of large-area, few layer hBN that
hosts large quantities of SPEs: 100 per 10x10 {\mu}m2. Remarkably, more than 85
percent of the emitters have a ZPL at (580{\pm}10)nm, a distribution which is
over an order of magnitude narrower than previously reported. Exploiting the
high density and uniformity of the emitters, we demonstrate electrical
modulation and tuning of the ZPL emission wavelength by up to 15 nm. Our
results constitute a definite advancement towards the practical deployment of
hBN single photon emitters in scalable quantum photonic and hybrid
optoelectronic devices based on 2D materials.

Artificial atomic systems in solids such as single photon emitters are
becoming increasingly important building blocks in quantum information
processing and scalable quantum nanophotonic networks. Here, we report on a
controllable way to engineer emitters in two-dimensional (2D) hexagonal boron
nitride (hBN) crystals using plasma processing. The method is robust, and
yields a 7-fold increase in the density of emitters in hBN, which is promising
for their deployment in practical devices. While as-fabricated emitters suffer
from blinking and bleaching, a subsequent annealing step yields photo-stable
emitters. The presented process is the first step towards controllable
placement of quantum emitters in hBN for integrated on-chip quantum
nanophotonics based on 2D materials.