J.H. Edgar

Kansas State University | KSU · Department of Chemical Engineering

Pressure is a powerful thermodynamic parameter for tuning the magnetic properties of van der Waals magnets owing to their weak interlayer bonding. However, local magnetometry measurements under high pressure still remain elusive for this important class of emerging materials. Here we introduce a method enabling in situ magnetic imaging of van der W...

Deep ultraviolet (UV) photoluminescence (PL) and x-ray photoemission spectroscopy (XPS) were employed to investigate the origin of the atomic-like emission line at 4.09 eV from hexagonal boron nitride (h-BN) single crystals. High resolution XPS spectra analyzed by correlating PL spectra of the h-BN samples with and without the sharp emission line a...

Large, high-purity single-crystals of hexagonal BN (h-BN) are essential for exploiting its many desirable and interesting properties. Here, we demonstrate via X-ray tomography, X-ray diffraction and scanning electron microscopy that h-BN crystals can be grown by traveling-solvent floating-zone (TSFZ). The diameters of grown boules range from 3 -- 5...

Spin defects embedded in solid-state systems are appealing for quantum sensing of materials and for quantum science and engineering. The spin-sensitive photoluminescence of optically active spin defects in Van der Waals based materials, such as the boron-vacancy (V_{B}^{-}) center in hexagonal boron nitride, enables its application as a quantum sen...

One of the most captivating properties of polaritons is their capacity to confine light at the nanoscale. This confinement is even more extreme in two-dimensional (2D) materials. 2D polaritons have been investigated by optical measurements using an external photodetector. However, their effective spectrally resolved electrical detection via far-fie...

Solid-state quantum emitters, pivotal in quantum technology, act as artificial atoms, emitting single photons crucial for quantum computing, communication, and sensing. Integrated into solid matrices, they offer stability and scalability, but face challenges like low-temperature operation and limited light extraction due to high refractive indices....

Field-effect devices constructed by stacking flakes of van der Waals (vdW) materials, with hexagonal boron nitride (hBN) playing the role of gate dielectric, often exhibit virtually no hysteresis in their characteristics. This permits exquisitely detailed studies of diverse gate-voltage-tuned phenomena in vdW devices. Recently, however, a dramatic...

One of the most captivating properties of polaritons is their capacity to confine light at the nanoscale. This confinement is even more extreme in two-dimensional (2D) materials. 2D polaritons have been investigated by optical measurements using an external photodetector. However, their effective spectrally resolved electrical detection via far-fie...

Bound states in the continuum (BICs) garnered significant interest for their potential to create new types of nanophotonic devices. Most prior demonstrations were based on arrays of dielectric resonators, which cannot be miniaturized beyond the diffraction limit, reducing the applicability of BICs for advanced functions. Here, we demonstrate BICs a...

Van-der-Waals materials have been shown to support numerous exotic polaritonic phenomena originating from their layered structures and associated vibrational and electronic properties. This includes emergent polaritonic phenomena, including hyperbolicity and exciton-polariton formation. However, many van-der-Waals materials' unique properties are m...

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently,...

Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelen...

The negatively charged boron vacancy (VB−) center in hexagonal boron nitride (hBN) is currently garnering considerable attention for the design of two-dimensional (2D) quantum sensing units. Such developments require a precise understanding of the spin-dependent optical response of VB− centers, which still remains poorly documented despite its key...

Accessing the low-energy non-equilibrium dynamics of materials and their polaritons with simultaneous high spatial and temporal resolution has been a bold frontier of electron microscopy in recent years. One of the main challenges lies in the ability to retrieve extremely weak signals and simultaneously disentangling the amplitude and phase informa...

Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero‐bicrystals composed of molybdenum oxide (MoO3) and hexagonal boron nitride (hBN), which feature polariton di...

The negatively charged boron vacancy ($\mathrm{V}_{\mathrm{B}}^-$) in hexagonal boron nitride (hBN) has garnered significant attention among defects in two-dimensional materials. This owes, in part, to its deterministic generation, well-characterized atomic structure, and optical polarizability at room temperature. We investigate the latter through...

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimenta...

Electrical readout of magnetic states is a key to realize novel spintronics devices for efficient computing and data storage. Unidirectional magnetoresistance (UMR) in bilayer systems, consisting of a spin source material and a magnetic layer, refers to a change in the longitudinal resistance upon the reversal of magnetization, which typically orig...

For decades, infrared (IR) spectroscopy has advanced on two distinct frontiers: enhancing spatial resolution and broadening spectroscopic information. Although atomic force microscopy (AFM)-based IR microscopy overcomes Abbe’s diffraction limit and reaches sub-10 nm spatial resolutions, time-domain two-dimensional IR spectroscopy (2DIR) provides in...

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence (CL) and time-resolved CL experiments to stud...

Hexagonal boron nitride (hBN), also known as white graphite, is a transparent layered crystal with a wide bandgap. Its crystal structure resembles graphite, featuring layers composed of honeycomb lattices held together through van der Waals forces. The layered crystal structure of hBN facilitates exfoliation into thinner flakes and makes it highly...

Nano‐ and angstrom‐scale channels fabricated from 2D‐layered materials provide a unique platform for studying fluidic behavior at atomic‐scale confinement, with applications in desalination, osmotic power generation, and fuel cells. While various fabrication methods exist, achieving precision, scalability, and minimal fabrication time is challengin...

We present an experimental protocol using cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of...

In conventional thin materials, the diffraction limit of light constrains the number of waveguide modes that can exist at a given frequency. However, layered van der Waals (vdW) materials, such as hexagonal boron nitride (hBN), can surpass this limitation due to their dielectric anisotropy, exhibiting positive permittivity along one optic axis and...

Single photon emitters from atomic defects in crystals like hexagonal boron nitride (hBN) are vital for quantum technologies. Although various techniques are devised to obtain defects emission in hBN, simultaneous control over defects position, type, and emission spectrum has not been achieved yet. Here, ion implantation with ¹² C, ²⁰ Ne, and ⁶⁹ Ga...

Compressing light into nanocavities substantially enhances light–matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the po...

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy c...

We present free-electron imaging of sub-cycle spatio-temporal dynamics of 2D polariton wavepackets, demonstrating the first simultaneous time-, space-, and phase-resolved measurement of such phenomena, and resolving their novel features like vortex-anti-vortex singularities for record-low intensities.

We experimentally demonstrate the generation of chiral electron beams in an ultrafast transmission electron microscope without the necessity for chiral light or chiral-shaping structures, but by breaking mirror symmetry in the light-electron interaction.

The layered insulator hexagonal boron nitride (hBN) is a critical substrate that brings out the exceptional intrinsic properties of two‐dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). In this work, we demonstrate how hBN slabs tuned to the correct thickness act as optical waveguides, enabling direct optical...

Hyperbolic phonon polaritons (HPhPs) can be supported in materials where the real parts of their permittivities along different directions are opposite in sign. HPhPs offer confinements of long-wavelength light to deeply subdiffractional scales, while the evanescent field allows for interactions with substrates, enabling the tuning of HPhPs by alte...

The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising two‐dimensional material for electronic, optoelectronic, nanophotonic, and quantum devices. Here we report on the changes in hBN's properties induced by isotopic purification in both boron and nitrogen. Previous studies...

Phonon-polaritons are electromagnetic waves resulting from the coherent coupling of photons with optical phonons in polar dielectrics. Due to their exceptional ability to confine electric fields to deep subwavelength scales with low loss, they are uniquely poised to enable a suite of applications beyond the reach of conventional photonics, such as...

We report on electron spin resonance (ESR) spectroscopy of boron-vacancy (VB−) centers hosted in isotopically engineered hexagonal boron nitride (hBN) crystals. We first show that isotopic purification of hBN with N15 yields a simplified and well-resolved hyperfine structure of VB− centers, while purification with B10 leads to narrower ESR linewidt...

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers (VB−) can be detected optically in the...

Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene an...

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility gra...

The ideal mechanical properties and behaviors of materials without the influence of defects are of great fundamental and engineering significance but considered inaccessible. Here, we use single-atom-thin isotopically pure hexagonal boron nitride (hBN) to demonstrate that two-dimensional (2D) materials offer us close-to ideal experimental platforms...

The boron-vacancy spin defect ( $${\,{{\mbox{V}}}}_{{{\mbox{B}}}\,}^{-}$$ V B − ) in hexagonal boron nitride (hBN) has a great potential as a quantum sensor in a two-dimensional material that can directly probe various external perturbations in atomic-scale proximity to the quantum sensing layer. Here, we apply first-principles calculations to dete...

Element isotopes are characterized by distinct atomic masses and nuclear spins, which can significantly influence material properties. Notably, however, isotopes in natural materials are homogenously distributed in space. Here, we propose a method to configure material properties by repositioning isotopes in engineered van der Waals (vdW) isotopic...

We report on electron spin resonance (ESR) spectroscopy of boron-vacancy (V$_\text{B}^-$) centers hosted in isotopically-engineered hexagonal boron nitride (hBN) crystals. We first show that isotopic purification of hBN with $^{15}$N yields a simplified and well-resolved hyperfine structure of V$_\text{B}^-$ centers, while purification with $^{10}$...

Different from hexagonal boron nitride (hBN) sheets, the bandgap of hBN nanoribbons (BNNRs) can be changed by spatial/electrostatic confinement. It has been predicted that a transverse electric field can narrow the bandgap and even cause an insulator-metal transition in BNNRs. However, experimentally introducing an overhigh electric field across th...

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy c...

The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising two-dimensional material for electronic, optoelectronic, nanophotonic, and quantum devices. Here we report on the changes in hBN's properties induced by isotopic purification in both boron and nitrogen. Previous studies...

Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene an...

Quantized vortices are topological defects found in different two-dimensional geometries, from liquid crystals to ferromagnets, famously involved in spontaneous symmetry breaking and phase transitions. Their optical counterparts appear in planar geometries as a universal wave phenomenon, possessing topologically protected orbital angular momentum (...

Accessing the low-energy non-equilibrium dynamics of materials with simultaneous spatial and temporal resolutions has been a bold frontier of electron microscopy in recent years. One of the main challenges is the ability to retrieve extremely weak signals while simultaneously disentangling amplitude and phase information. Here, we present an algori...

Strong coupling (SC) between light and matter excitations bears intriguing potential for manipulating material properties. Typically, SC has been achieved between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. However, simultaneously achieving SC in both frequency bands remains unexplored. Here, we intro...

Optically-active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this work, we first demonstrate that the electron spin resonance frequencies of boron vacancy centres (V$_\text{B}^-$) can be detected opticall...

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow the fabrication of high carrier mobili...

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. By combining these approaches, hypercrystals can be created, which are hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. Despite several attempts, there has been limit...

The boron-vacancy spin defect ($\text{V}_\text{B}^{-}$) in hexagonal boron nitride (hBN) has a great potential as a quantum sensor in a two-dimensional material that can directly probe various external perturbations in atomic-scale proximity to the quantum sensing layer. Here, we apply first principles calculations to determine the coupling of the...

Citation: Gil, B.; Desrat, W.; Rousseau A.; Elias C.; Valvin, P.; Moret, M.; Li, J.; Janzen, E.; Edgar, J. H.; Cassabois G. Polytypes of sp 2-Bonded Boron Nitride. Crystals 2022, 12, 782. https://doi.

B oron nitride (BN) was first synthesised in 1842 as a powdery solid and is characterised by a layered honeycomb structure resembling graphite, micas, and other semiconductor materials. In its most common form, BN consists of layers of hexagonal units made of alternating atoms of boron and nitrogen. Commonly known as 'white graphite', hexagonal BN...

Wavelength‐selective absorbers (WS‐absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography‐free fabrication of WS‐absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBR) on plasmonic materials. While multi‐frequency and nearly arbitrary spect...

Hyperbolic phonon polaritons (HPhPs) can be supported in highly anisotropic materials, where the real parts of their permittivities along different directions are opposite in sign as a result of spectrally offset optical phonons. Compared to surface polaritons, HPhPs offer further confinement of long-wavelength light to deeply subdiffractional scal...

Graphite is one of the most chemically inert materials. Its elementary constituent, monolayer graphene, is generally expected to inherit most of the parent material's properties including chemical inertness. Here, we show that, unlike graphite, defect-free monolayer graphene exhibits a strong activity with respect to splitting molecular hydrogen, w...

Strong coupling (SC) between light and matter excitations such as excitons and molecular vibrations bear intriguing potential for controlling chemical reactivity, conductivity or photoluminescence. So far, SC has been typically achieved either between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. Achiev...