Atomically thin monolayers of transition metal dichalcogenides (TMDs) have emerged as a promising class of novel materials for optoelectronics and nonlinear optics. However, the intrinsic nonlinearity of TMD monolayers is weak, limiting their functionalities for nonlinear optical processes such as frequency conversion. Here we boost the effective nonlinear susceptibility of a TMD monolayer by integrating it with a resonant dielectric metasurface that supports pronounced optical resonances with high quality factors: bound states in the continuum (BICs). We demonstrate that a WS2 monolayer combined with a silicon metasurface hosting BICs exhibits enhanced second-harmonic intensity by more than 3 orders of magnitude relative to a WS2 monolayer on top of a flat silicon film of the same thickness. Our work suggests a pathway to employ high-index dielectric metasurfaces as hybrid structures for enhancement of TMD nonlinearities with applications in nonlinear microscopy, optoelectronics, and signal processing.

Two-dimensional hexagonal boron nitride offers intriguing opportunities for advanced studies of light–matter interaction at the nanoscale, specifically for realizations in quantum nanophotonics. Here, we demonstrate the generation of optically addressable spin defects based on the negatively charged boron vacancy (VB –) center. We show that these centers can be created in exfoliated hexagonal boron nitride using a variety of focused ion beams (nitrogen, xenon, and argon). Using a combination of laser and resonant microwave excitation, we carry out optically detected magnetic resonance spectroscopy measurements, which reveal a zero-field ground state splitting for the defect of ∼3.46 GHz. We also perform photoluminescence excitation spectroscopy and temperature-dependent photoluminescence measurements to elucidate the photophysical properties of the VB – centers. Our results are important for advanced quantum and nanophotonics realizations involving manipulation and readout of spin defects in hexagonal boron nitride.

Quantum emitters in hexagonal boron nitride (hBN) are emerging as bright and robust sources of single photons for applications in quantum optics. In this work we present detailed studies on the limiting factors to achieve Fourier Transform limited spectral lines. Specifically, we study phonon dephasing and spectral diffusion of quantum emitters in hBN viaresonant excitation spectroscopy at cryogenic temperatures. We show that the linewidths of hBN quantum emitters are phonon broadened, even at 5K, with typical values of the order of ~ one GHz. While spectral diffusion dominates at increasing pump powers, it can be minimized by working well below saturation excitation power. Our results are important for future utilization of quantum emitters in hBN for quantum interference experiments.

Quantum emitters in hexagonal boron nitride (hBN) flakes have recently emerged as a promising platform for nanophotonic and quantum applications. The solvent-exfoliation process of these flakes has, however, remained largely unexplored. In this work, we demonstrate a surfactant-assisted exfoliation technique in an aqueous solution to exfoliate a variety of commercially available hBN powders into hBN nanoflakes. We show that the selection of hBN powder greatly impacts the optical properties of the resultant quantum emitters embedded in exfoliated hBN nanoflakes. We find that the sample with the best optical performance also shows the lowest impurity levels in its starting hBN powder. Our study provides further insight into quantum emitter fabrication in hBN and tailoring of their optical properties.

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.