The boron-vacancy spin defect (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 determine the coupling of the V − B electronic spin to strain and electric fields. Our work unravels the interplay between local piezoelectric and elastic effects contributing to the final response to the electric fields. The theoretical predictions are then used to analyse optically detected magnetic resonance (ODMR) spectra recorded on hBN crystals containing different densities of V − B centres. We prove that the orthorhombic zero-field splitting parameter results from local electric fields produced by surrounding charge defects. By providing calculations of the spin-strain and spin-electric field couplings, this work paves the way towards applications of V − B centres for quantitative electric field imaging and quantum sensing under pressure.

Nanoporous GaN layers were fabricated using selective area sublimation through a self-organized AlN nanomask in a molecular beam epitaxy reactor. The obtained pore morphology, density and size were measured using plan-view and cross-section scanning electron microscopy experiments. It was found that the porosity of the GaN layers could be adjusted from 0.04 to 0.9 by changing the AlN nanomask thickness and sublimation conditions. The room temperature photoluminescence properties as a function of the porosity were analysed. In particular, a strong improvement (>100) of the room temperature photoluminescence intensity was observed for porous GaN layers with a porosity in the 0.4–0.65 range. The characteristics of these porous layers were compared to those obtained with a Si$_x$ N$_y$ nanomask. Furthermore, the regrowth of p-type GaN on light emitting diode structures made porous by using either an AlN or a Si$_x$ N$_y$ nanomask were compared.

Optically active spin defects hosted in hexagonal boron nitride (h-BN) are promising candidates for the development of a two-dimensional (2D) quantum sensing unit. Here, we demonstrate quantitative magnetic imaging with h-BN flakes doped with negatively charged boron-vacancy (VB-) centers throughneutron irradiation. As a proof-of-concept, we image the magnetic field produced by CrTe2 , a van der Waals ferromagnet with a Curie temperature slightly above 300 K. Compared to other quantum sensors embedded in 3D materials, the advantages of the h-BN-based magnetic sensor described in this work areits ease of use, high flexibility, and, more importantly, its ability to be placed in close proximity to a target sample. Such a sensing unit will likely find numerous applications in 2D materials research by offering a simple way to probe the physics of van der Waals heterostructures.

We report hyperspectral microscopy in sp2-bonded boron nitride polytypes with a simultaneous inspection of the photoluminescence signal in the UV-C and UV-B spectral ranges. By comparing two different polytypes extracted from the same polycrystalline sample, we reveal that the well-known “4 eV defect” does not emit at the same energy for polytypes in the AB and AA′ stackings. The zero-phonon line is either at 4.14 or 4.16 eV for the noncentrosymmetric AB stacking, instead of the usual 4.09 eV energy in the AA′ stacking. Our results open the way for different characterization methods of the stacking order in sp2-bonded boron nitride, and bring alternative inputs for the elucidation of the atomistic configuration of points defects by advanced ab initio calculations.

We study the superradiance of optical phonons during the two- to three-dimensional (2D-3D) crossover of the light-matter interaction in multilayers of atomic crystals. We show the emergence of a superradiant regime with a mode having a linewidth first increasing linearly with the number N of monolayers, and then decreasing as N^{−3} to zero because of the formation of stationary phonon polaritons. The linewidth culminates to values of the order of the longitudinal-transverse splitting. We estimate the extremum of the radiative efficiency for various 2D materials in the superradiant regime. We predict radiative efficiencies larger than 50% for optical phonons emitting between 6 and 165 μm. Superradiance appears as a key resource for mid- and far-infrared optophononics and advanced thermal management using multilayers of 2D materials as the active medium.

The value and the nature of the bandgap of In4Se3 are still not well defined, with a large spread of the experimental data between 0.42 and 1.68 eV and an uncertain nature, predicted to be indirect by ab initio band structure calculations. Here we report on the optical transmission and photoluminescence (PL) performed in In4Se3 thin films grown by coevaporation on (0001)-oriented sapphire wafers. The quality of the polycrystalline layers allows the first detection of the excitonic-like transition in the optical absorption of this compound at low temperature. The PL detected under weak laser excitation shows a bound exciton emission at 0.75 eV. Strong laser irradiation reveals a quadratic dependence of the PL intensity on the optical excitation, which demonstrates a stimulated emission at 0.79 eV in relation with an exciton–exciton scattering process. On the basis of a reasonable estimate of the exciton energy, equal to 10 − 15 meV, we evaluate the direct bandgap of In4 Se3 to 0.82 ± 0.01 eV at low temperature.