Enhanced Quantum Properties of Single Photon Emitters in Hexagonal Boron Nitride Flakes Using Plasmonic Nanocavities

Recently, single-photon emitters (SPEs) in solid-state materials have drawn a huge interest for different usages in quantum sensing and quantum nanophotonics [1]. Significant developments led to the finding of a variety of atom-like SPEs in low-dimensional and two-dimensional (2D) materials, such as hexagonal boron nitride (hBN), which exhibit favorable quantum properties at room temperature, making them highly desirable element for integrated quantum photonic circuits [2]. One major challenge of using these SPE host 2D materials in such applications is their low quantum efficiency with a fluorescence emission rate way below 0.5 Mc s⁻¹.<br/>In this work, we use composite nanophotonic structures based on gap plasmonic modes, integrated with hBN multilayered flakes, to achieve a speedup and enhancement in the quantum interaction processes at room temperature. We first created a high density (&gt; 0.5 SPE/1 µm²) of stable SPE in thin (thickness ≤ 35 nm) hBN flakes deposited on a Si/SiO₂substrate by using a high-temperature (1100 °C) annealing method under O₂ flow. We then transferred the annealed hBN flakes with SPEs to epitaxial gold (Au) film (thickness ~ 100 nm), grown on Si substrate, and characterized their quantum properties using a home-built confocal fluorescence microscope [3]. We demonstrated a plasmonic enhancement of SPE properties from the nanograins (size 50- 65 nm), found in the Au film, manifested by a decrease of quantum emitter lifetime by one order of magnitude and an increase of the emitter fluorescence by ~ 392% [4].<br/>To enhance further the quantum properties of SPEs on a selected thin hBN flake we spin coated commercial silver nanocubes (SNCs) with size of 100 nm on the annealed Au/Si substrate. We optimized the spin coating parameters to increase the chance of getting the SNCs coupled spatially with the SPEs [3]. We observed plasmon nanocavity enhancement of SPE properties near the SNCs on top of the hBN flake manifested by a decrease of emitter lifetime by 100%, and a fluorescence enhancement of 200% [4]. The overall saturation counts is &gt; 2 Mc/s for some of the SPEs in the plasmonic (metallic) Au/hBN/SNC nanocavities in comparison to ~ 0.25 Mc/s for SPEs in just hBN flakes deposited on SiO₂/Si substrates. The presented two order of magnitude enhancement of SPE quantum properties at room temperature is further investigated by using COMSOL numerical simulations. The hBN flakes are integrated into the fabricated nanophotonic cavities that are characterized by an optical frequency resonance response where light-matter interaction is particularly enhanced [4]. [1] M. Atatüre et <i>al.</i>, Nat Rev Mater 3, 38–51 (2018). [2] J. D. Caldwell, et <i>al.</i>, Nat. Rev. Mat. 4, 552-567 (2019). [3] M. Dowran, et <i>al.</i>, Adv. Opt. Mat. 11 (16), 2370062 (2023). [4] M. Dowran et <i>al.</i>, under preparation.<br/>Acknowledgement: This material is based upon work supported by the NSF/EPSCoR RII Track-1: Emergent Quantum Materials and Technologies (EQUATE), Award OIA-2044049. The research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience (and/or NERCF), which are supported by NSF under Award ECCS: 2025298, and the Nebraska Research Initiative. Christos Argyropoulos acknowledges partial support from NSF Award DMR: 2224456.