Single-Photon Emission from Point Defects in Hexagonal Boron Nitride Induced by Plasma Treatment

Solid-state quantum emitters are gaining significant attention for many quantum information applications. Hexagonal boron nitride (h-BN) is an emerging host material for generating bright, stable, and tunable single-photon emission with narrow line widths at room temperature. In this work, we present a facile and efficient approach to generate high-density single-photon emitters (SPEs) in mechanically exfoliated h-BN through H- or Ar-plasma treatment followed by high-temperature annealing in air. It is notable that the postannealing is essential to suppress the fluorescence background in photoluminescence spectra and enhance emitter stability. These quantum emitters exhibit excellent optical properties, including high purity, brightness, stability, polarization degree, monochromaticity, and saturation intensity. The effects of process parameters on the quality of quantum emitters were systematic investigated. We find that there exists an optimal plasma power and h-BN thickness to achieve a high SPE density. This work offers a practical avenue for generating SPEs in h-BN and holds promise for future research and applications in quantum photonics.

Cryogenic Thermal Shock Effects on Optical Properties of Quantum Emitters in Hexagonal Boron Nitride

Solid-state quantum emitters are vital building blocks for quantum information science and quantum technology. Among various types of solid-state emitters discovered to date, color centers in hexagonal boron nitride have garnered tremendous traction in recent years, thanks to their environmental robustness, high brightness, and room-temperature operation. Most recently, these quantum emitters have been employed for satellite-based quantum key distribution. One of the most important requirements to qualify these emitters for space-based applications is their optical stability against cryogenic thermal shock. Such an understanding has, however, remained elusive to date. Here, we report on the effects caused by such thermal shock that induces random, irreversible changes in the spectral characteristics of the quantum emitters. By employing a combination of structural characterizations and density functional calculations, we attribute the observed changes to lattice strain caused by cryogenic temperature shock. Our study sheds light on the stability of the quantum emitters under extreme conditions─similar to those countered in outer space.

Ultracompact Single-Photon Sources of Linearly Polarized Vortex Beams

Ultracompact chip-integrated single-photon sources of collimated beams with polarization-encoded states are crucial for integrated quantum technologies. However, most of currently available single-photon sources rely on external bulky optical components to shape the polarization and phase front of emitted photon beams. Efficient integration of quantum emitters with beam shaping and polarization encoding functionalities remains so far elusive. Here, ultracompact single-photon sources of linearly polarized vortex beams based on chip-integrated quantum emitter-coupled metasurfaces are presented, which are meticulously designed by fully exploiting the potential of nanobrick-arrayed metasurfaces. The authors first demonstrate on-chip single-photon generation of high-purity linearly polarized vortex beams with prescribed topological charges of 0, - 1, and +1. The multiplexing of single-photon emission channels with orthogonal linear polarizations carrying different topological charges are further realized and their entanglement is demonstarated. The work illustrates the potential and feasibility of ultracompact quantum emitter-coupled metasurfaces as a new quantum optics platform for realizing chip-integrated high-dimensional single-photon sources.

Manipulating Interlayer Excitons for Near-Infrared Quantum Light Generation

Interlayer excitons (IXs) formed at the interface of van der Waals materials possess various novel properties. In parallel development, strain engineering has emerged as an effective means for creating 2D quantum emitters. Exploring the intersection of these two exciting areas, we use MoS2/WSe2 heterostructure as a model system and demonstrate how strain, defects, and layering can be utilized to create defect-bound IXs capable of bright, robust, and tunable quantum light emission in the technologically important near-infrared spectral range. Our work presents defect-bound IXs as a promising platform for pushing the performance of 2D quantum emitters beyond their current limitations.

Integrating Fluorescent Nanodiamonds into Polymeric Microstructures Fabricated by Two-Photon Polymerization

Nitrogen-vacancy (NV) and other color centers in diamond have attracted much attention as non-photobleaching quantum emitters and quantum sensors. Since microfabrication in bulk diamonds is technically difficult, embedding nanodiamonds with color centers into designed structures is a way to integrate these quantum emitters into photonic devices. In this study, we demonstrate a method to incorporate fluorescent nanodiamonds into engineered microstructures using two-photon polymerization (2PP). We studied the optimal concentration of nanodiamonds in the photoresist to achieve structures with at least one fluorescent NV center and good structural and optical quality. Fluorescence and Raman spectroscopy measurements were used to confirm the presence and location of the nanodiamonds, while absorbance measurements assessed scattering losses at higher concentrations. Our results show the feasibility of fabricating microstructures embedded within fluorescent nanodiamonds via 2PP for photonics and quantum technology applications.

Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride

Negatively charged boron vacancies (V_B^-) 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 V_B defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states (V_B^- ⇌ V_B⁰ + 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 V_B charge state, and to stabilize the -1 state which is a prerequisite for spin manipulation and optical readout of the defect.

Amplified Plasmonic Forces from DNA Origami-Scaffolded Single Dyes in Nanogaps

Developing highly enhanced plasmonic nanocavities allows direct observation of light-matter interactions at the nanoscale. With DNA origami, the ability to precisely nanoposition single-quantum emitters in ultranarrow plasmonic gaps enables detailed study of their modified light emission. By developing protocols for creating nanoparticle-on-mirror constructs in which DNA nanostructures act as reliable and customizable spacers for nanoparticle binding, we reveal that the simple picture of Purcell-enhanced molecular dye emission is misleading. Instead, we show that the enhanced dipolar dye polarizability greatly amplifies optical forces acting on the facet Au atoms, leading to their rapid destabilization. Using different dyes, we find that emission spectra are dominated by inelastic (Raman) scattering from molecules and metals, instead of fluorescence, with molecular bleaching also not evident despite the large structural rearrangements. This implies that the competition between recombination pathways demands a rethink of routes to quantum optics using plasmonics.

A DFT study of electron-phonon interactions for the C2CNand VNNBdefects in hexagonal boron nitride: investigating the role of the transition dipole direction

Quantum emitters in two-dimensional hexagonal boron nitride (h-BN) have generated significant interest due to observations of ultra-bright emission made at room temperature. The expectation that solid-state emitters exhibit broad zero-phonon lines at elevated temperatures has been put in question by recent observations of Fourier transform (FT) limited photons emitted from h-BN flakes at room temperature. All decoupled emitters produce photons that are directed in-plane, suggesting that the dipoles are perpendicular to the h-BN plane. Motivated by the promise of an efficient and scalable source of indistinguishable photons that can operate at room temperature, we have developed an approach using density functional theory (DFT) to determine the electron- phonon coupling for defects that have in- and out-of-plane transition dipole moments. Our DFT calculations reveal that the transition dipole for the C₂C_Ndefect is parallel to the h-BN plane, and for the V_NN_Bdefect is perpendicular to the plane. We calculate both the phonon density of states and the electron-phonon matrix elements associated with the h-BN defective structures. We find no indication that an out-of-plane transition dipole by itself will result in the low electron-phonon coupling that is expected to produce FT-limited photons at room temperature. Our work provides direction to future DFT software developments and adds to the growing list of calculations relevant to researchers in the field of solid-state quantum information processing.

Quantum Emitters with Narrow Band and High Debye-Waller Factor in Aluminum Nitride Written by Femtosecond Laser

Solid-state quantum emitters (QEs) are central components for photonic-based quantum information processing. Recently, bright QEs in III-nitride semiconductors, such as aluminum nitride (AlN), have attracted increasing interest because of the mature commercial application of the nitrides. However, the reported QEs in AlN suffer from broad phonon side bands (PSBs) and low Debye-Waller factors. Meanwhile, there is also a need for more reliable fabrication methods of AlN QEs for integrated quantum photonics. Here, we demonstrate that laser-induced QEs in AlN exhibit robust emission with a strong zero phonon line, narrow line width, and weak PSB. The creation yield of a single QE could be more than 50%. More importantly, they have a high Debye-Waller factor (>65%) at room temperature, which is the highest result among reported AlN QEs. Our results illustrate the potential of laser writing to create high-quality QEs for quantum technologies and provide further insight into laser writing defects in relevant materials.

Optical Properties of Concentric Nanorings of Quantum Emitters

A ring of sub-wavelength spaced dipole-coupled quantum emitters features extraordinary optical properties when compared to a one-dimensional chain or a random collection of emitters. One finds the emergence of extremely subradiant collective eigenmodes similar to an optical resonator, which features strong 3D sub-wavelength field confinement near the ring. Motivated by structures commonly appearing in natural light-harvesting complexes (LHCs), we extend these studies to stacked multi-ring geometries. We predict that using double rings allows us to engineer significantly darker and better confined collective excitations over a broader energy band compared to the single-ring case. These enhance weak field absorption and low-loss excitation energy transport. For the specific geometry of the three rings appearing in the natural LH2 light-harvesting antenna, we show that the coupling between the lower double-ring structure and the higher energy blue-shifted single ring is very close to a critical value for the actual size of the molecule. This creates collective excitations with contributions from all three rings, which is a vital ingredient for efficient and fast coherent inter-ring transport. This geometry thus should also prove useful for the design of sub-wavelength weak field antennae.

Enhancement of Single-Photon Purity and Coherence of III-Nitride Quantum Dot with Polarization-Controlled Quasi-Resonant Excitation

III-Nitride semiconductor-based quantum dots (QDs) play an essential role in solid-state quantum light sources because of their potential for room-temperature operation. However, undesired background emission from the surroundings deteriorates single-photon purity. Moreover, spectral diffusion causes inhomogeneous broadening and limits the applications of QDs in quantum photonic technologies. To overcome these obstacles, it is demonstrated that directly pumping carriers to the excited state of the QD reduces the number of carriers generated in the vicinities. The polarization-controlled quasi-resonant excitation is applied to InGaN QDs embedded in GaN nanowire. To analyze the different excitation mechanisms, polarization-resolved absorptions are investigated under the above-barrier bandgap, below-barrier bandgap, and quasi-resonant excitation conditions. By employing polarization-controlled quasi-resonant excitation, the linewidth is reduced from 353 to 272 µeV, and the second-order correlation value is improved from 0.470 to 0.231. Therefore, a greater single-photon purity can be obtained at higher temperatures due to decreased linewidth and background emission.

Planar Optical Cavities Hybridized with Low-Dimensional Light-Emitting Materials

Low-dimensional light-emitting materials have been actively investigated due to their unprecedented optical and optoelectronic properties that are not observed in their bulk forms. However, the emission from low-dimensional light-emitting materials is generally weak and difficult to use in nanophotonic devices without being amplified and engineered by optical cavities. Along with studies on various planar optical cavities over the last decade, the physics of cavity-emitter interactions as well as various integration methods are investigated deeply. These integrations not only enhance the light-matter interaction of the emitters, but also provide opportunities for realizing nanophotonic devices based on the new physics allowed by low-dimensional emitters. In this review, the fundamentals, strengths and weaknesses of various planar optical resonators are first provided. Then, commonly used low-dimensional light-emitting materials such as 0D emitters (quantum dots and upconversion nanoparticles) and 2D emitters (transition-metal dichalcogenide and hexagonal boron nitride) are discussed. The integration of these emitters and cavities and the expect interplay between them are explained in the following chapters. Finally, a comprehensive discussion and outlook of nanoscale cavity-emitter integrated systems is provided.

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.

Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications

Optical quantum technologies promise to revolutionize today’s information processing and sensors. Crucial to many quantum applications are efficient sources of pure single photons. For a quantum emitter to be used in such application, or for different quantum systems to be coupled to each other, the optical emission wavelength of the quantum emitter needs to be tailored. Here, we use density functional theory to calculate and manipulate the transition energy of fluorescent defects in the two-dimensional material hexagonal boron nitride. Our calculations feature the HSE06 functional which allows us to accurately predict the electronic band structures of 267 different defects. Moreover, using strain-tuning we can tailor the optical transition energy of suitable quantum emitters to match precisely that of quantum technology applications. We therefore not only provide a guide to make emitters for a specific application, but also have a promising pathway of tailoring quantum emitters that can couple to other solid-state qubit systems such as color centers in diamond.

Smooth Sidewalls on Crystalline Gold through Facet-Selective Anisotropic Reactive Ion Etching: Toward Low-Loss Plasmonic Devices

Quantum plasmonics aims to harness the deeply subwavelength confinement provided by plasmonic devices to engineer more efficient interfaces to quantum systems in particular single emitters. Realizing this vision is hampered by the roughness-induced scattering and loss inherent in most nanofabricated devices. In this work, we show evidence of a reactive ion etching process to selectively etch gold along select crystalline facets. Since the etch is facet selective, the sidewalls of fabricated devices are smoother than the lithography induced line-edge roughness with the prospect of achieving atomic smoothness by further optimization of the etch chemistry. This opens up a route toward fabricating integrated plasmonic circuits that can achieve loss metrics close to fundamental bounds.

Engineering Optically Active Defects in Hexagonal Boron Nitride Using Focused Ion Beam and Water

Hexagonal boron nitride (hBN) has emerged as a promising material platform for nanophotonics and quantum sensing, hosting optically active defects with exceptional properties such as high brightness and large spectral tuning. However, precise control over deterministic spatial positioning of emitters in hBN remained elusive for a long time, limiting their proper correlative characterization and applications in hybrid devices. Recently, focused ion beam (FIB) systems proved to be useful to engineer several types of spatially defined emitters with various structural and photophysical properties. Here we systematically explore the physical processes leading to the creation of optically active defects in hBN using FIB and find that beam-substrate interaction plays a key role in the formation of defects. These findings are confirmed using transmission electron microscopy, which reveals local mechanical deterioration of the hBN layers and local amorphization of ion beam irradiated hBN. Additionally, we show that, upon exposure to water, amorphized hBN undergoes a structural and optical transition between two defect types with distinctive emission properties. Moreover, using super-resolution optical microscopy combined with atomic force microscopy, we pinpoint the exact location of emitters within the defect sites, confirming the role of defected edges as primary sources of fluorescent emission. This lays the foundation for FIB-assisted engineering of optically active defects in hBN with high spatial and spectral control for applications ranging from integrated photonics, to nanoscale sensing, and to nanofluidics.

Understanding Heterogeneities in Quantum Materials

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. The understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

Generation of High-Density Quantum Emitters in High-Quality, Exfoliated Hexagonal Boron Nitride

Single-photon emitters in hexagonal boron nitride (hBN) are promising constituents for integrated quantum photonics. Specifically, engineering these emitters in large-area, high-quality, exfoliated hBN is needed for their incorporation into photonic devices and two dimensional heterostructures. Here, we report on two different routes to generate high-density quantum emitters with excellent optical properties-including high brightness and photostability. We study in detail high-temperature annealing and plasma treatments as an efficient means to generate dense emitters. We show that both an optimal oxygen flow rate and annealing temperature are required for the formation of high-density quantum emitters. In parallel, we demonstrate that the plasma treatment in various environments, followed by standard annealing is also an effective route for emission engineering. Our work provides vital information for the fabrication of quantum emitters in high-quality, exfoliated hBN flakes and paves the way toward the integration of the quantum emitters with photonic devices.

Millimeter-Wave-to-Terahertz Superconducting Plasmonic Waveguides for Integrated Nanophotonics at Cryogenic Temperatures

Plasmonics, as a rapidly growing research field, provides new pathways to guide and modulate highly confined light in the microwave-to-optical range of frequencies. We demonstrated a plasmonic slot waveguide, at the nanometer scale, based on the high-transition-temperature (T_c) superconductor Bi₂Sr₂CaCu₂O_8+δ (BSCCO), to facilitate the manifestation of chip-scale millimeter wave (mm-wave)-to-terahertz (THz) integrated circuitry operating at cryogenic temperatures. We investigated the effect of geometrical parameters on the modal characteristics of the BSCCO plasmonic slot waveguide between 100 and 800 GHz. In addition, we investigated the thermal sensing of the modal characteristics of the nanoscale superconducting slot waveguide and showed that, at a lower frequency, the fundamental mode of the waveguide had a larger propagation length, a lower effective refractive index, and a strongly localized modal energy. Moreover, we found that our device offered a larger SPP propagation length and higher field confinement than the gold plasmonic waveguides at broad temperature ranges below BSCCO’s T_c. The proposed device can provide a new route toward realizing cryogenic low-loss photonic integrated circuitry at the nanoscale.

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.

Polarization and Localization of Single-Photon Emitters in Hexagonal Boron Nitride Wrinkles

Color centers in two-dimensional hexagonal boron nitride (h-BN) have recently emerged as stable and bright single-photon emitters (SPEs) operating at room temperature. In this study, we combine theory and experiment to show that vacancy-based SPEs selectively form at nanoscale wrinkles in h-BN with its optical dipole preferentially aligned to the wrinkle direction. By using density functional theory calculations, we find that the wrinkle's curvature plays a crucial role in localizing vacancy-based SPE candidates and aligning the defect's symmetry plane to the wrinkle direction. By performing optical measurements on SPEs created in h-BN single-crystal flakes, we experimentally confirm the wrinkle-induced generation of SPEs and their polarization alignment to the wrinkle direction. Our results not only provide a new route to controlling the atomic position and the optical property of the SPEs but also revealed the possible crystallographic origin of the SPEs in h-BN, greatly enhancing their potential for use in solid-state quantum photonics and quantum information processing.

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.

Photolithographic Patterning of Organic Color-Centers

Organic color-centers (OCCs) have emerged as promising single-photon emitters for solid-state quantum technologies, chemically specific sensing, and near-infrared bioimaging. However, these quantum light sources are currently synthesized in bulk solution, lacking the spatial control required for on-chip integration. The ability to pattern OCCs on solid substrates with high spatial precision and molecularly defined structure is essential to interface electronics and advance their quantum applications. Herein, a lithographic generation of OCCs on solid-state semiconducting single-walled carbon nanotube films at spatially defined locations is presented. By using light-driven diazoether chemistry, it is possible to directly pattern p-nitroaryl OCCs, which demonstrate chemically specific spectral signatures at programmed positions as confirmed by Raman mapping and hyperspectral photoluminescence imaging. This light-driven technique enables the fabrication of OCC arrays on solid films that fluoresce in the shortwave infrared and presents an important step toward the direct writing of quantum emitters and other functionalities at the molecular level.

Metasurface-Enabled Generation of Circularly Polarized Single Photons

Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures, are at the core of chiral quantum optics enabling nonreciprocal single-photon configurations and deterministic spin-photon interfaces. Here, a conceptually new approach to the room-temperature generation of SAM-coded single photons (SSPs) is described, which entails QE nonradiative coupling to surface plasmons being transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. Design, fabrication, and characterization of SSP sources, consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric-protected silver film around a nanodiamond containing a nitrogen-vacancy center, are reported. With properly engineered phases of QE-originated fields scattered by nanoridges, the outcoupled photons are characterized by a well-defined SAM (with the chirality >0.8) and high directionality (collection efficiency up to 92%).

Converting Plasmonic Light Scattering to Confined Light Absorption and Creating Plexcitons by Coupling a Gold Nano-pyramid Array onto a Silica-Gold Film

This report presents a facile microfabrication-compatible approach to fabricate a large area of plasmonic nano-pyramid array-based antennas and demonstrates effective light management by tailoring the architecture. First, a long-range ordered gold nano-pyramid array is fabricated, which exhibits strong light scattering. The maximum electric field enhancement (|E|/|E ₀ |) of 271 is achieved at the corner but decays rapidly away from the pyramid bottom. After the gold nanopyramid array is coupled to a gold film, strong light scattering is converted into strong light absorption due to the excitation of a spectrally tunable plasmonic gap mode, where an intense electric field enhancement of 233 and a strong magnetic field enhancement (|H|/|H ₀ |) of 25 are simultaneously excited for a 10 nm silica gap. The electric field decays much slower away from the pyramid bottom while the magnetic field keeps almost constant. In addition, both experiments and finite-difference time-domain (FDTD) simulation have confirmed that strong plasmon-exciton coupling between the plasmonic gap mode and the J-aggregates can take place when the quantum emitters such as J-aggregates are embedded in the gap, creating plexcitons. This can overcome the problems of high energy loss and weak nonlinearity, which are typically associated with surface plasmon polariton (SPP) supported on plasmonic metallic nanostructures. The coherent plasmon-exciton coupling (plexciton) generated by the film-coupled nano-pyramid nanostructure is expected to find promising applications in light-emitting devices, photodetectors, photovoltaics and photoelectrochemical cells.

Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy

Chiral light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe₂ nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe₂ NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

Effects of High-Energy Electron Irradiation on Quantum Emitters in Hexagonal Boron Nitride

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.

Self-Assembled Nanocrystals of Polycyclic Aromatic Hydrocarbons Show Photostable Single-Photon Emission

Quantum technologies could largely benefit from the control of quantum emitters in sub-micrometric size crystals. These are naturally prone to integration in hybrid devices, including heterostructures and complex photonic devices. Currently available quantum emitters in nanocrystals suffer from spectral instability, preventing their use as single-photon sources for most quantum optics operations. In this work we report on the performances of single-photon emission from organic nanocrystals (average size of hundreds of nm), made of anthracene (Ac) and doped with dibenzoterrylene (DBT) molecules. The source has hours-long photostability with respect to frequency and intensity, both at room and at cryogenic temperature. When cooled to 3 K, the 00-zero phonon line shows linewidth values (50 MHz) close to the lifetime limit. Such optical properties in a nanocrystalline environment recommend the proposed organic nanocrystals as single-photon sources for integrated photonic quantum technologies.

Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation

Defect-based color centers in wide-bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite-vacancy pairs (CAV centers) in 4H-SiC, which serve as single-photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect-based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.

Photochemical Creation of Fluorescent Quantum Defects in Semiconducting Carbon Nanotube Hosts

Quantum defects are an emerging class of synthetic single-photon emitters that hold vast potential for near-infrared imaging, chemical sensing, materials engineering, and quantum information processing. Herein, we show that it is possible to optically direct the synthetic creation of molecularly tunable fluorescent quantum defects in semiconducting single-walled carbon nanotube hosts through photochemical reactions. By exciting the host semiconductor with light that resonates with its electronic transition, we find that halide-containing aryl groups can covalently bond to the sp² carbon lattice. The introduced quantum defects generate bright photoluminescence that allows tracking of the reaction progress in situ. We show that the reaction is independent of temperature but correlates strongly with the photon energy used to drive the reaction, suggesting a photochemical mechanism rather than photothermal effects. This type of photochemical reactions opens the possibility to control the synthesis of fluorescent quantum defects using light and may enable lithographic patterning of quantum emitters with electronic and molecular precision.

Quantum Dot Emission Driven by Mie Resonances in Silicon Nanostructures

Resonant dielectric nanostructures represent a promising platform for light manipulation at the nanoscale. In this paper, we describe an active photonic system based on Ge(Si) quantum dots coupled to silicon nanodisks. We show that Mie resonances govern the enhancement of the photoluminescent signal from embedded quantum dots due to a good spatial overlap of the emitter position with the electric field of Mie modes. We identify the coupling mechanism, which allows for engineering the resonant Mie modes through the interaction of several nanodisks. In particular, the mode hybridization in a nanodisk trimer results in an up to 10-fold enhancement of the luminescent signal due to the excitation of resonant antisymmetric magnetic and electric dipole modes.

Engineering and Localization of Quantum Emitters in Large Hexagonal Boron Nitride Layers

Hexagonal boron nitride is a wide-band-gap van der Waals material that has recently emerged as a promising platform for quantum photonics experiments. In this work, we study the formation and localization of narrowband quantum emitters in large flakes (up to tens of micrometers wide) of hexagonal boron nitride. The emitters can be activated in as-grown hexagonal boron nitride by electron irradiation or high-temperature annealing, and the emitter formation probability can be increased by ion implantation or focused laser irradiation of the as-grown material. Interestingly, we show that the emitters are always localized at the edges of the flakes, unlike most luminescent point defects in three-dimensional materials. Our results constitute an important step on the roadmap of deploying hexagonal boron nitride in nanophotonics applications.