Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity

Abstract

The negatively charged boron vacancy (VB –) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, VB – preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of VB – defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of VB – with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled VB – defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single VB – defects.

Detection of Paramagnetic Spins with an Ultrathin van der Waals Quantum Sensor

“…In a photon-shot-noise-limited T 1 -relaxometry experiment, the signal-to-noise ratio scales as , SNR ∝ normalΓ 1 ext normalΓ 1 int scriptC I PL where Γ 1 int = 1/ T 1 int is the intrinsic relaxation rate in the absence of the target spin (assumed to satisfy Γ 1 int ≫Γ 1 ext ), C is the relative spin contrast, and I PL is the PL signal from one readout pulse. Currently, V B – defect ensembles exhibit inferior contrast and PL output compared to typical NV ensembles and so will need to be improved through material optimization (e.g., to increase defect creation yield, as currently only a small fraction of boron vacancies are in the desired negatively charged state) or photonics engineering to enhance collection. ,, On the other hand, the intrinsic relaxation time observed in this work for V B – in hBN nanopowders ( T 1 int ≈ 15–20 μs) is similar to that of NVs in nanodiamonds of comparable size of order 10 nm, which do not reach the longer relaxation times exhibited by NVs in bulk diamond but nevertheless have found widespread interest. Considering all these factors together, we believe hBN nanopowders can be improved as a sensing platform to detect submillimolar concentrations of paramagnetic ions and emerge as a viable alternative to nanodiamonds for T 1 relaxometry in this concentration regime with the potential of a lower production cost, making it appealing for applications such as high-sensitivity point-of-care diagnostics. , Note that for these applications the development of robust surface functionalization methods will be necessary …”

“…producing trilayer flakes with stable V B − defects in the middle layer is a plausible scenario. While there has been no report of V B − defects in trilayer hBN to date, V B − defects have been successfully detected in flakes as thin as 4 nm 53 while still exhibiting ODMR, implying the defects are within 2 nm (i.e., 3 atomic layers) of the closest surface. Compared to an NV center at d = 5 nm (typical minimum depth observed 26−28 Nevertheless, a potential gain in Γ 1 ext due to better proximity does not guarantee improved sensitivity to external spins.…”

Extending Plasmonic Enhancement Limit with Blocked Electron Tunneling by Monolayer Hexagonal Boron Nitride

“…This active control of electron tunneling effect provides important insights for advanced applications that rely on optimized plasmonic enhancement, thereby providing important guidance for further understanding quantum mechanical effects and photon-phonon interactions in plasmonic enhancement, such as quantum plasmonics and nanogap photodynamics. Moreover, coupled with the single defects in h-BN, this NPoM system is expected to improve the emission intensity of single-photon sources, 47 which is crucial for quantum photonic devices.…”

Spin Defects in hBN assisted by Metallic Nanotrenches for Quantum Sensing

“…Several reports have revealed that SPEs in hBN are spin manipulable and that, in particular, negatively charged boron vacancy ( V B – ) defects can be easily generated and their spin population can be controlled at room temperature. − Although V B – defects have been utilized in proof-of-principle quantum sensors over the last three years, their broader application necessitates boosting their emission intensities. Recently, a few groups have developed different methods to enhance the V B – emission in layered hBN, all of them demonstrating an emission enhancement with respect to the bare substrate. − However, all these methods ignored the effect of the hBN thickness on the resonance behavior of the photonic structures, which can dramatically affect the actual emission enhancement and, potentially, also the emitter’s emission stability.…”