Optically Active Spin Defects in Few-Layer Thick Hexagonal Boron Nitride

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 ($V_{\mathrm{B}}^{-}$) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of $V_{\mathrm{B}}^{-}$ centers with the hBN thickness with a focus on (i) the zero-field splitting parameters, (ii) the optically induced spin polarization rate and (iii) the longitudinal spin relaxation time. This Letter provides important insights into the properties of $V_{\mathrm{B}}^{-}$ centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

Figure 1

(a) PL raster scan of hBN flakes of different thicknesses doped with $V_{\mathrm{B}}^{-}$ centers. Inset: line profile of the AFM topography recorded across the flake labeled ②, corresponding to few-layer hBN. (b)–(d) Optically detected ESR spectra recorded at zero external magnetic field (b) for the 15-nm-thick hBN flake labeled ① and (c),(d) for few-layer hBN labeled ② and ③. The decreased ESR contrast in few-layer hBN results from a reduced signal to background ratio. (e),(f) Sketch showing a central $V_{\mathrm{B}}^{-}$ electronic spin (blue arrow) surrounded by charged defects and the resulting energy level structure of its triplet ground state (e) for a bulklike hBN flake and (f) for few-layer hBN. In panels (b)–(d), the red solid line shows the result of a simulation considering the interaction of the $V_{\mathrm{B}}^{-}$ electronic spin with an electric field produced by surrounding charged defects (see the Supplemental Material [26]).

Figure 2

(a) PL time traces measured for $V_{\mathrm{B}}^{-}$ centers hosted in a 15-nm-thick hBN flake during a $50\text{−}\mathrm{\mu }\mathrm{s}$-long laser pulse at three different laser powers. The data are vertically shifted for clarity and the red solid lines are data fitting with an exponential function. (b) Spin polarisation rate ${\mathcal{R}}_p$ as a function of the laser power $\mathcal{P}$ for hBN flakes with different thicknesses. The dashed lines are linear fits from which the slope ${\mathcal{S}}_p=d{\mathcal{R}}_p/d\mathcal{P}$ is extracted. (c) Parameter ${\mathcal{S}}_p$ as a function of the hBN thickness. The red dashed line correspond to a simulation of the thickness-dependent absorption of $V_{\mathrm{B}}^{-}$ centers for a 532 nm excitation wavelength.

Figure 3

(a) Spin relaxation curves recorded at room temperature for $V_{\mathrm{B}}^{-}$ centers hosted in a 85-nm-thick hBN flake (triangles) and in few-layer-thick hBN (squares). The inset shows the experimental pulse sequence. The spin-dependent PL signal is integrated at the beginning of the readout pulse (500 ns window) and normalized using a reference PL value obtained at the end of the first laser pulse. (b) Measurements performed on the same hBN flakes at cryogenic temperature (4 K). In (a) and (b), the solid lines are data fitting with an exponential decay. (c) Evolution of the $T_1$ time with the hBN thickness at room temperature (red points) and at 4 K (blue points). The dashed lines are guides for the eye.