Tip-enhanced quantum-sensing spectroscopy for bright and reconfigurable solid-state single-photon emitters

Abstract

Atom-like defects in hexagonal boron nitride (hBN) provide room-temperature single-photon emission and coherent spin states, making them attractive for quantum-computing and -sensing applications. However, their random spatial and spectral characteristics hamper deterministic coupling with nano-optical cavities, limiting their use as bright single-photon sources and sensitive quantum sensors. Here, we present tip-enhanced quantum-sensing spectroscopy of single-photon emitters in hBN. Through precise spatial positioning of individual emitters within tip-cavities with different plasmon resonances, we adaptively control the enhancement rates of both excitation and emission, as well as the single-photon purity. In this way, optimal selection of their relative contributions can effectively reconfigure solid-state single-photon sources, with simultaneous nano-spectroscopic space- and time-resolved analyses. Furthermore, we demonstrate tip-enhanced quantum-sensing with single spin defects through optically detected magnetic resonance (ODMR) experiments in tip-coupled hBN nanoflakes. Our approach provides a unique pathway toward highly-sensitive and deterministic quantum-sensing with room-temperature single-photon emitters.

Figures (4)

• Figure 1: Tip-enhanced quantum-sensing spectroscopy for hBN single-photon emitters. (a) Schematic illustration of deterministic emitter-cavity coupling and tip-enhanced single-photon emission. (b) Energy level diagrams of the defect-state transitions in hBN for an uncoupled single-photon emitter (left) and the same single-photon emitter when coupled to the tip-cavity (right). The overlaid contour map (red) in the right panel highlights the calculated spatial distribution of the spontaneous emission rate enhancement in the tip-cavity. (c) Top: Representative normalized photoluminescence spectra from multiple hBN single-photon emitters, each exhibiting a distinct ZPL. Bottom: Normalized tip-plasmon spectra, showing a tunable spectral overlap with the ZPL energies of the hBN single-photon emitters. Abbreviations: single-photon emitter (SPE); photoluminescence intensity (I$_\textnormal{PL}$); scattering intensity (I$_\textnormal{scatt}$).
• Figure 2: Distance-dependent spectroscopic sensing of hBN single-photon emitters. (a) Evolution of the single-photon emission spectra as a function of tip–sample distance d. (b) Schematic of three regimes defined by d-dependence. (c) Systematic tuning of the single-photon emission intensity for regime I (d = 10 nm), regime II (d = 5 nm), and regime III (d = 2 nm). (d) Representative single-photon emission spectra showing the evolution of emission brightness from the far-field (black) to regime I (orange) and regime III (red). (e) Normalized time-resolved single-photon emission decay traces of hBN single-photon emitter uncoupled (black) and coupled (red) to the plasmonic cavity. (f) Single-photon emission intensity as a function of excitation power for an uncoupled hBN single-photon emitter (black) and the same single-photon emitter coupled to the plasmonic cavity (red).
• Figure 3: Reconfigurable brightness and single-photon purity of hBN single-photon emitters. (a) Conceptual illustration of the interplay among the single-photon emission (SPE), the excitation laser (Exc), and the plasmon of tip-cavity. By tuning the spectral overlap of these resonances (indicated by colored regions), either the excitation rate (${\gamma}_{\textnormal{exc}}^{}$/${\gamma}_{\textnormal{exc}}^{0}$) or spontaneous emission rate (${\gamma}_{\textnormal{sp}}^{}$/${\gamma}_{\textnormal{sp}}^{0}$) can be selectively enhanced. (b) Peak values of F$_\textnormal{P}$ (blue, left axis) are evaluated at the emitter position (650 nm, $\sim$1.91 eV) and peak excitation-rate enhancement ${\gamma}_{\textnormal{exc}}^{}$/${\gamma}_{\textnormal{exc}}^{0}$ (red, right axis) are evaluated at the laser excitation wavelength (594 nm, $\sim$2.09 eV), both plotted against the plasmon energy. Measured second-order correlation functions g$^{(2)}$($\uptau$) of uncoupled (black) and cavity-coupled (red) emission under two coupling conditions: ${\gamma}_{\textnormal{sp}}^{}$/${\gamma}_{\textnormal{sp}}^{0}$$>$${\gamma}_{\textnormal{exc}}^{}$/${\gamma}_{\textnormal{exc}}^{0}$ (c) and ${\gamma}_{\textnormal{sp}}^{}$/${\gamma}_{\textnormal{sp}}^{0}$$<$${\gamma}_{\textnormal{exc}}^{}$/${\gamma}_{\textnormal{exc}}^{0}$ (d).
• Figure 4: Optically detected magnetic resonance of a hBN single defect integrated to the plasmonic tip. (a) Schematic of tip-enhanced quantum-sensing with a hBN single defect on the Au tip. (b) Microscope images of the plasmonic tip mounted on the quartz tuning fork resonator. (c) SPE intensity map of a single defect embedded within the hBN nanoflake at the tip apex. (d) Simulated plasmonic field distributions under s-polarized (left) and p-polarized (right) excitation, showing distinct field enhancements in the plasmonic nanocavity. (e) Single-photon emission spectra as a function of excitation polarization angle (P$_\textnormal{exc}$). An excitation polarization of 0$^\circ$ corresponds to alignment parallel to the tip axis, while 90$^\circ$ corresponds to alignment perpendicular to the tip axis. (f) ODMR spectra of a single defect at the tip apex under 45$^\circ$ (blue) and 0$^\circ$ (red) excitation.