Multi-wavelength Spin Dynamics of Defects in Hexagonal Boron Nitride

Optically addressable solid-state spin defects are essential platforms for quantum sensing and information processing. Recently, single spin defects with combined S = 1 and S = 1/2 spin transitions were discovered in hexagonal boron nitride (hBN). In this work we unveil their excitation dynamics. In particular, we study the effects of the excitation wavelength on the spin-dependent fluorescence and the spin dynamics of these peculiar quantum spin defects. We find that changing the excitation wavelength leads to a threefold enhancement in both the optically detected magnetic resonance (ODMR) contrast and the corresponding magnetic field sensitivity. In addition, we find that the excitation wavelength has a strong impact on the photodynamics of spin complex emitters. Our work presents valuable insights to the mechanistic understanding of spin complex emitters in hBN and highlights the importance of excitation wavelength for optimising their performance in quantum sensing and quantum technologies.

💡 Research Summary

In this work the authors investigate how the excitation wavelength influences the spin‑photon dynamics of a recently discovered spin‑complex defect in hexagonal boron nitride (hBN). The defect is unusual because it exhibits both S = 1 and S = ½‑like transitions, which are attributed to a strongly coupled electron pair (forming a triplet) and a weakly coupled, more delocalised pair, respectively. Using a confocal microscope they compare continuous‑wave optically detected magnetic resonance (CW‑ODMR), time‑resolved photoluminescence (PL), and long‑timescale second‑order autocorrelation measurements under two excitation wavelengths: 532 nm (green) and 633 nm (red).

Key findings:

1. ODMR Contrast: The red (633 nm) excitation yields a three‑fold increase in ODMR contrast compared with green (532 nm). For the S = ½ transition the contrast rises from ~36 % to ~98 %, and a similar enhancement is observed for the S = 1 transitions (−1↔0, 0↔+1, −1↔+1).

2. Power Dependence: Saturation powers differ markedly (P_sat ≈ 151 µW for 532 nm, 79 µW for 633 nm). Contrast increases with excitation power up to a normalized power of ≈1, then declines, mirroring behavior previously reported for NV‑ centers.

3. Photodynamics: Under 532 nm excitation the PL is stable over minutes, whereas 633 nm excitation induces pronounced blinking, with the emitter switching between bright and dark states. Long‑timescale g²(τ) measurements reveal stronger photon bunching for the green excitation, indicating that the blinking dynamics contribute additional decay channels.

4. Proposed Energy‑Level Model: The authors suggest that the two wavelengths populate distinct singlet excited states (e₁ for 633 nm, e₂ for 532 nm). e₁ couples efficiently to the metastable (MS) regime, enhancing intersystem crossing (ISC) into the triplet manifold and thus boosting ODMR contrast, but also increasing the probability of occupying non‑radiative MS states, which manifests as blinking. e₂ couples more weakly to the MS, resulting in lower contrast but more stable emission.

5. Rate‑Equation Analysis: By adapting a five‑level rate model previously used for hBN spin complexes, they fit the power‑dependent contrast data, confirming that a higher ISC rate from e₁ to the MS state reproduces the observed trends. The model also captures the power‑induced reduction of contrast due to spin‑polarisation dynamics.

6. Co‑Excitation Experiments: Simultaneous illumination with both wavelengths shows that adding a small amount of 532 nm light (as little as 2 µW) dramatically suppresses the blinking seen under pure 633 nm excitation, while also increasing the overall PL intensity until saturation. Histogram analysis of the photon counts allows quantitative extraction of the dark‑state fraction, which drops from ~11 % (pure 633 nm) to <1 % when the green component dominates.

Overall, the study demonstrates that the excitation wavelength is a powerful knob for tailoring the spin‑dependent optical response of hBN spin‑complex defects. Red excitation maximises ODMR contrast and magnetic‑field sensitivity but introduces instability due to frequent MS cycling. Green excitation provides stable emission with lower contrast. By judiciously combining the two, one can achieve both high contrast and stable fluorescence, a crucial requirement for practical quantum‑sensing applications. The findings highlight the importance of wavelength engineering in the design of hBN‑based quantum devices and provide a mechanistic framework for interpreting the complex photophysics of multi‑spin defects in two‑dimensional materials.