In the reporting period, we focused on the (i) enhancement of VB- emission and spin resonance contrast by systematically varying the defect density at different irradiation doses.We investigated an hBN flake that was irradiated with a 30 keV nitrogen ion beam in square areas of 4x4 µm with different irradiation fluences ranging from 1.5x 10^11 to 1.1x 10^15 ions/cm². To investigate the sample, a home-built room temperature pulsed ODMR setup equipped with confocal microscope with a single-photon counting module was used together with laser excitation of 473 nm to initialise and read out the defect spin state. We found a correlation between the spin-relaxation times and the fluence of the ion irradiation. The T2* time increased by a factor of two with decreasing fluence. The highest observed T2* was 111 ns, and the data suggest even higher values with less intense irradiation. Considering the brightness as a function of fluence, we find that the generation of VB– with a fluence of 1.5x 10^14 nitrogen ions/cm² gives the best sensitivity in our magnetic field sensor. In summary, although the creation of a single defect has not yet been achieved, we present an irradiation protocol for optimized quantum sensing. For increased irradiation fluence, we observe that PL and thus defect density increases, while spin coherence times decrease, pointing at an optimum intermediate fluence for the desired sensing applications.
Another aspect we focused on was (ii) identifying the sources of spin decoherence of these defects and how to bypass them. The main source of spin decoherence of VB- defect is clearly the magnetic environment, which means that the electronic spin center is surrounded by 100% magnetic nuclei (boron and nitrogen). Without isotopic purification of the crystal, which is probably very expensive if it is realistic at all, it seems difficult to reach the level of spin-coherence times known for carbon-based materials. In order to utilise the main advantages of hBN, namely the two-dimensionality, and avoiding isotope purification of the crystal, we investigated the dynamics of the intermediate state (IS), also called metastable state or shelf state, because it can trap electrons for a certain time. This state plays a significant role in the overall utility of the spin defect and possibly mitigates the requirements for spin coherence times. We investigated the PL dynamics using a simple optical pumping scheme with variable and short delay time between pulses. Our experiments have shown that the IS delays the relaxation of electrons to the ground state with a characteristic lifetime of 24 ns. This lifetime is at least doubled if the temperature is lowered to 4 Kelvin. This has a clear impact on the subsequent sensing protocol and must be considered when designing the pulsed optically detected magnetic resonance (ODMR) experiment. We demonstrate that by doing so, the efficiency of spin manipulation (Rabi oscillation amplitude and ODMR contrast) is increased. We believe that understanding the role of IS in the pump cycle and the degree of spin polarization of the ground state can partially mitigate the requirements of a long spin-spin relaxation time and still ensure the sensitivity of the magnetic field sensor.
