We report optically detected magnetic resonance (ODMR) measurements of an ensemble of spin-1 negatively charged boron vacancies in hexagonal boron nitride. The photoluminescence decay rates are spin-dependent, with intersystem crossing rates of 1.02 ns–1 and 2.03 ns–1 for the m S = 0 and m S = ±1 states, respectively. Time gating the photoluminescence enhances the ODMR contrast by discriminating between different decay rates. This is particularly effective for detecting the spin of the optically excited state, where a zero-field splitting of |D ES | = 2.09 GHz is measured. The magnetic field dependence of the photoluminescence exhibits dips corresponding to the ground (GSLAC) and excited-state (ESLAC) anticrossings and additional anticrossings due to coupling with nearby spin-1/2 parasitic impurities. Comparison to a model suggests that the anticrossings are mediated by the interaction with nuclear spins and allows an estimate of the ratio of the singlet to triplet spin-dependent relaxation rates of κ0/κ1 = 0.34.

We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2 eV. Resonant, or quasi-resonant, excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect-dependent threshold between 2.25 and 2.6 eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4 eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center.

We demonstrate the use of Stimulated Emission Depletion (STED) spectroscopy to map the electron-optical-phonon sideband of the ground state of the radiative transition of color centers in hexagonal boron nitride emitting at 2.0−2.2 eV, with in-plane linear polarization. The measurements are compared to photoluminescence of excitation (PLE) spectra that maps the electron-optical-phonon sideband of the excited state. The main qualitative difference is a red-shift in the longitudinal optical phonon peak associated with E 1u symmetry at the zone center. We compare our results to theoretical work on different defect species in hBN and find they are consistent with a carbon-based defect.

We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2eV. Resonant, or quasi-resonant excitation also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Photoluminescence excitation spectroscopy reveals a defect dependent energy threshold for repumping the color center into the bright state of between 2.2 and 2.6eV. Photoionization and photocharging of the defect is the most plausible explanation for this behaviour, with the negative and neutral charge states of the boron vacancy potential candidates for the bright and dark states,

Stimulated emission depletion, or STED microscopy, is a well-established super-resolution technique, but is ultimately limited by the chosen fluorophore. Here we demonstrate STED microscopy with color centers in nanoscale flakes of hexagonal boron nitride using time-gated, continuous-wave STED. For color centers with zero phonon line emission around 580 nm, we measure a STED cross section of (5.5 ± 3.2) × 10 −17 cm 2 , achieve a resolution of ∼50 nm, and resolve two color centers separated by 250 nm, which is less than the diffraction limit. The achieved resolution is limited by the numerical aperture of the objective lens (0.8) and the available laser power, and we predict that a resolution of sub-10 nm can be achieved with an oil immersion objective lens, similar to the state-of-the-art resolution obtained with nitrogen vacancy centers in diamond.