Impact of Uniaxial Strain on Deterministically Generated Optical Defects in van der Waals Materials

Over the past decade, there has been significant interest in employing 2D materials (van der Waals materials) as platforms for quantum emitters with distinct quantum characteristics like single photon emission, strong spin-orbit coupling, spin polarization, and valley polarization. These emitters hold promise for diverse applications in quantum technologies such as quantum information, and quantum sensing. Unlike bulk materials, 2D materials offer exceptional flexibility, potentially enabling devices to conform to various shapes, while housing ‘surface emitters’ with enhanced resolution. Hexagonal boron nitride (h-BN) is one such promising 2D material with a wide band gap that can house various color centers and deep defects with in-gap states. In this work, site-controlled optical defects are deterministically introduced inside h-BN by utilizing a commercial plasma-focused ion beam (FIB) device to selectively expose the surface of a few tens of nm thick h-BN with variable fluence. The type of defect generated via this technique is identified as the boron vacancy complex (VB^-). This defect complex has been shown to be spin-active with a triplet ground state, which means it can directly compete with the very well-studied nitrogen-vacancy center (NV^-) in diamond for quantum sensing applications while offering the advantage of having near-surface emitters. Despite significant progress made in this field, there are still challenges that limit the effectiveness of these emitters. Unlike the NV^-centers in diamond, the VB^- defect is considerably dimmer with a lower optically detected magnetic resonance (ODMR) contrast, making it very challenging to isolate a single VB^-defect. Although VB^- ensembles have been effectively used as prototypes for quantum sensors, a large density of defects offers limited resolution and sensitivity. Despite FIB bombardment being effective in creating large defect ensembles in h-BN, precise control over the density of the defects formed is difficult to achieve since the minimum dosage that can applied by the device is finite and we can exert limited control over the formation threshold of the defects with irradiation alone. To overcome this limitation and potentially create low-density, spatially localized defects, we introduce directional strain on the h-BN crystal by transferring the h-BN flake on top of specialized nanopillars on a silicon substrate prior to irradiation. In our preliminary results, we observed localized strain-dependent photoluminescence intensity enhancement from the irradiated defects. It can be hypothesized that this observation is a consequence of several effects such as reduced defect formation energy and the induced emitter lifetime shortening due to the localized strain. Our results leads to the understanding of the impact of engineered static uniaxial strain and irradiation dose on the formation, defect density, and optical dipole orientation of room-temperature optical defects in h-BN.