We investigate the distribution and temperature-dependent optical properties of sharp, zero-phonon emission from defect-based single photon sources in multilayer hexagonal boron nitride (h-BN) flakes. We observe sharp emission lines from optically active defects distributed across an energy range that exceeds 500 meV. Spectrally resolved photon-correlation measurements verify single photon emission, even when multiple emission lines are simultaneously excited within the same h-BN flake. We also present a detailed study of the temperature-dependent line width, spectral energy shift, and intensity for two different zero-phonon lines centered at 575 and 682 nm, which reveals a nearly identical temperature dependence despite a large difference in transition energy. Our temperature-dependent results are well described by a lattice vibration model that considers piezoelectric coupling to in-plane phonons. Finally, polarization spectroscopy measurements suggest that whereas the 575 nm emission line is directly excited by 532 nm excitation, the 682 nm line is excited indirectly.

We demonstrate direct coupling between phonons and diamond nitrogen-vacancy (NV) center spins by driving spin transitions with mechanically generated harmonic strain at room temperature. The amplitude of the mechanically driven spin signal varies with the spatial periodicity of the stress standing wave within the diamond substrate, verifying that we drive NV center spins mechanically. These spin-phonon interactions could offer a route to quantum spin control of magnetically forbidden transitions, which would enhance NV-based quantum metrology, grant access to direct transitions between all of the spin-1 quantum states of the NV center, and provide a platform to study spin-phonon interactions at the level of a few interacting spins.

Abstract:We investigate the polarization selection rules of sharp zero-phonon lines (ZPLs) from isolated defects in hexagonal boron nitride (h-BN) and compare our findings with the predictions of a configuration coordinate model involving two electronic states. Our survey, which spans the spectral range ~550-740 nm, reveals that, in disagreement with a two-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle ( ) to the ZPL Stokes shift ( ) and find that ≈ 0° when corresponds to an allowed h-BN phonon frequency and that 0°≤ ≤ 90° when exceeds the maximum allowed h-BN phonon frequency. Consequently, a two-level configuration coordinate model succeeds at describing excitations mediated by the creation of one optical phonon but fails at describing excitations that require the creation of multiple phonons. We propose that direct excitations requiring the creation of multiple phonons are inefficient due to the low Huang-Rhys factors in In the Frank-Condon approximation, where the fast electronic rearrangement precedes the slower lattice relaxation, the transition rate from to ( * ,is proportional to where * − * are the associated Laguerre polynomials and is a measure of defect-lattice coupling called the Huang-Rhys factor. In natural units = To investigate the polarization properties of absorption, we rotate the polarization of the exciting light and monitor the total fluorescence intensity. The result of this absorption measurement is shown as the green triangles in Fig. 2b. Fixing the polarization of the exciting light to maximize the fluorescence, we determine the polarization of the emitted photons using a polarization analyzer placed in the collection path of the microscope. The result of this emission measurement is shown as the red circles in Fig. 2b. The solid lines are best fits to the data using the equationwhere 〈 〉 in each fit is the orientation of the absorption or emission dipole spectrally averaged over the collection window. As predicted by Equation 1, we find that the maxima of absorption and emission are aligned for this defect.Additionally, we have shown previously that the temperature dependence of the ZPL intensity in h-BN is well-modeled by the temperature-dependent DebyeWaller factor [33]. Thus, we conclude that the configuration coordinate model is a good description of the observed properties for the defect shown in Fig. 2.A survey of defect ZPLs that span an appreciable energy range reveals that, in contrast to the data shown in Fig. 2 For the emission measurement we fix the polarization angle of the exciting light to ( ) and record an emission spectrum for a series of positions of the polarization analyzer in the collection path. In an analogous fashion to the absorption case we obtain ( , ) and ( ) for the emitted light. For the case of emission we apply a calibration to ( ) to correct for wavelength-and polarization-dependent retardances (see Supporting Information) introduced by the collection path of the confocal microscope. To be...

Two-dimensional hexagonal boron nitride (h-BN) is a wide bandgap material which has promising mechanical and optical properties. Here we report the realization of an initial nucleation density of h-BN <1 per mm using low-pressure chemical vapor deposition (CVD) on polycrystalline copper. This enabled wafer-scale CVD growth of single-crystal monolayer h-BN with a lateral size up to ∼300 μm, bilayer h-BN with a lateral size up to ∼60 μm, and trilayer h-BN with a lateral size up to ∼35 μm. Based on the large single-crystal monolayer h-BN domain, the sizes of the as-grown bi- and trilayer h-BN grains are 2 orders of magnitude larger than typical h-BN multilayer domains. In addition, we achieved coalesced h-BN films with an average grain size ∼100 μm. Various flake morphologies and their interlayer stacking configurations of bi- and trilayer h-BN domains were studied. Raman signatures of mono- and multilayer h-BN were investigated side by side in the same film. It was found that the Raman peak intensity can be used as a marker for the number of layers.

Coherent control of the nitrogen-vacancy (NV) center in diamond's triplet spin state has traditionally been accomplished with resonant ac magnetic fields under the constraint of the magnetic dipole selection rule, which forbids direct control of the |−1 ↔ |+1 spin transition. We show that high-frequency stress resonant with the spin state splitting can coherently control NV center spins within this subspace. Using a bulk-mode mechanical microresonator fabricated from single-crystal diamond, we apply intense ac stress to the diamond substrate and observe mechanically driven Rabi oscillations between the |−1 and |+1 states of an NV center spin ensemble. Additionally, we measure the inhomogeneous spin dephasing time (T * 2 ) of the spin ensemble using a mechanical Ramsey sequence and compare it to the dephasing times measured with a magnetic Ramsey sequence for each of the three spin qubit combinations available within the NV center ground state.These results demonstrate coherent spin driving with a mechanical resonator and could enable the creation of a phase-sensitive ∆-system within the NV center ground state. Here we use a mechanical microresonator to apply a large amplitude ac stress to a single crystal diamond. Building on recent spectroscopy experiments [8], we tune the frequency of this stress wave into resonance with the |(m s =) − 1 ↔ |+1 spin transition to mechanically drive Rabi oscillations of an NV center spin ensemble. Using this capability, we measure the inhomogeneous dephasing time for an ensemble of mechanically controlled NV center spin qubits to be T * 2 = 0.45±0.05 µs and compare this result to T * 2 for magnetically driven qubits constructed from the same NV center ensemble. We find that the mechanically driven {−1, +1} qubit coherence is similar to that of a magnetically driven {−1, +1} qubit, and these {−1, +1} qubits dephase twice as quickly as magnetically driven {0, −1} or {+1, 0} qubits.NV centers couple to mechanical stress (σ ⊥ and σ ) and magnetic fields (B ⊥ and B ) 2 through their ground-state spin Hamiltonian (shown schematically in Fig. 1a)where D 0 /2π = 2.87 GHz is the zero-field splitting, γ N V /2π = 2.8 MHz/G is the gyromagnetic ratio, ⊥ /2π = 0.015 MHz/MPa and /2π = 0.012 MHz/MPa are the perpendicular and axial stress coupling constants [10,14], P/2π = −4.945 MHz and A /2π = −2.166 MHz are the hyperfine parameters [15][16][17], and S x , S y , S z (I x , I y , I z ) are the x, y, and z components of the electronic (nuclear) spin-1 operator. The NV center symmetry axis defines the z-axis of our coordinate system as depicted in Fig. 1b In this work, we use two devices, both fabricated from type IIa, 100 "optical grade" diamonds purchased from Element Six. These samples are specified to contain fewer than 1 ppm nitrogen impurities, and each contained a native NV ensemble as received. The first sample, Sample A, has an NV center density of ∼ 110 NVs/µm 3 , while Sample B has a density of ∼ 120 NVs/µm 3 . To generate the large amplitude, high-frequency stress waves neede...