Near-Infrared Quantum Emission from Oxygen-Related Defects in hBN

TL;DR

This work demonstrates a scalable oxygen-plasma method to engineer oxygen-related defects in hBN that function as near-infrared single-photon emitters spanning 700–960 nm, with blinking-free ZPLs and cryogenic linewidths down to 2.7 GHz, plus room-temperature operation. Optical characterization reveals two ZPL families responsive to pump wavelength, strong single-photon emission, and high dipole visibility, while finite-temperature phonon analysis shows weak vibronic coupling. First-principles calculations identify ON_VN and ON_VN_H centers as the most plausible defects, predicting ZPLs in the NIR and paramagnetic spin-doublet ground states, which supports potential spin–photon interfaces. Overall, the oxygen-related NIR SQEs in hBN offer a promising platform for indistinguishable NIR photons and spin-photon networking in van der Waals materials.

Abstract

Color centers hosted in hexagonal boron nitride (hBN) have emerged as a promising platform for single-photon emission and coherent spin-photon interfaces that underpin quantum communication and quantum networking technologies. As a wide-bandgap van der Waals material, hBN can host individual optically active quantum defects emitting across the ultraviolet to visible spectrum, but existing color centers often show broad phonon sidebands (PSBs), unstable emission, or inconvenient wavelengths. Here, we show a simple, scalable oxygen-plasma process that reproducibly creates oxygen-related single quantum emitters in hBN with blinking-free zero-phonon lines spanning the near-infrared (NIR) spectrum from 700-960 nanometers. These emitters demonstrate room-temperature operation, high brightness, and ultra-sharp cryogenic linewidths in the few-gigahertz range under non-resonant excitation. Analysis of the PSBs shows weak electron-phonon coupling and predominant zero-phonon-line emission, while first-principles calculations identify plausible oxygen-related defect configurations. These emitters provide a promising platform for indistinguishable NIR single photons towards free-space quantum networking.

Figures (5)

• Figure 1: Fabrication process and overview of generated NIR SQE's.(a) Schematic illustration of the fabrication process for generating NIR SQEs. Panel I denotes a mechanical exfoliation method to create sub-100 nm hBN samples. Panel II denotes an oxygen plasma treatment to introduce oxygen-related defects into the hBN lattice. Panel III denotes a thermal anneal process to stabilize the SQE within the hBN. (b) An optical micrograph of a processed hBN sample. The dashed line indicates the area of a photoluminescence (PL) confocal map. (c) A confocal hyperspectral PL map of the processed hBN sample showcasing spatially resolved single-emitters across the sample. (d) An atomic force microscopy (AFM) image is shown, highlighting the smooth surface topography of the hBN remains intact after the plasma treatment and processing. The inset shows a line scan across the dashed line and indicates a sample thickness of 50 nm. (e) Representative PL spectra of the generated spatially isolated NIR SQEs. Spectra of SQEs 1-5 in panel (c) are shown from left to right, respectively. SQEs are observed over a wide range of wavelengths spanning from 700 nm to 960 nm.
• Figure 2: Characterization of NIR SQEs.(a) A confocal hyperspectral PL map at 4 K of a processed hBN sample. Local hotspots indicate spatially isolated NIR SQEs. (b) A select SQE is highlighted and indicated by the dashed circle in panel (a). The second-order autocorrelation function of the selected NIR SQE demonstrates photon anti-bunching with a raw $g^2(0)$ value of 0.23 $\pm$ 0.09. (c) A histogram of spatially isolated single emitters from processed hBN sample in (a) and (b) displaying the span of ZPL wavelengths observed. The orange (blue) bins indicate the span of all isolated single emitters found under 660 nm (765 nm) pump excitation. A Gaussian fit indicates a center wavelength of 770 $\pm$ 33 and 856 $\pm$ 38 nm for 660 nm and 765 nm pump excitation, respectively. The dashed line represents the spectral location of a 800 nm longpass filter to reject the 765 nm pump laser (d) Representative PL spectra of spatially isolated NIR single emitters under 765 nm pump excitation. SQEs display spectral inhomogeneity spanning over a range of wavelengths spanning from 800 nm to 960 nm. (e) Quasi-resonant excitation of an NIR SQE with a ZPL of 801 nm indicated by arrow in (d) under 795 nm pump. A spectrometer-limited FWHM of 55.1 $\mu$eV (13.3 GHz) under pump power of 148 $\mu$W is extracted. The inset displays the linewidth dependence of the SQE on laser excitation wavelength from 765 nm to 795 nm under 550 $\mu$W pump power.
• Figure 3: Electron-Phonon Coupling(a) Normalized lineshape $L(\Delta E)$ of a SQE excited under 660 nm pump with a fitted ZPL at 791.3 nm (1.567 eV). Bottom inset shows the zoom-in of the PSB with the $n$-phonon spectral distribution decomposition with an extracted $S_{HR}$ of 2.14 $\pm$ 0.40. Top inset shows the 1-phonon vibronic coupling probability distribution function. (b) Normalized lineshape $L(\Delta E)$ of a SQE excited under 765 nm pump with a fitted ZPL at 859.9 nm (1.442 eV). Bottom inset shows the zoom-in of the PSB with the $n$-phonon spectral distribution decomposition with an extracted $S_{HR}$ of 0.72 $\pm$ 0.12. Top inset shows the 1-phonon vibronic coupling probability distribution function. (c) Temperature dependent Huang-Rhys analysis of the SQE under 765 nm pump in (b). Extracted $S_{HR}$ at each temperature fall within one confidence interval of each other indicating temperature independent electron-phonon coupling strength over 4-40 K.
• Figure 4: Non-resonant excitation linewidth characterization.(a) A representative linewidth of an NIR SQE at 4 K under 40 $\mu W$ of pump power with a spectrometer-limited FWHM of 148 $\mu$eV compared to the same SQE under 3.4 mW of pump power with a spectrometer-limited FWHM of 279 $\mu$eV both under non-resonant pump excitation at 660 nm. (b) Linewidth power-broadening dependence of multiple NIR SQEs at 4 K. A two-level system power broadening model is fit to the experimental data points. (c) PL spectra of the representative SQE shown at both 4 K and 300 K. (d) Temperature-dependence of linewidth of a specific SQE taken under 500 $\mu W$ excitation power (indicated by arrow in panel (b)). The experimental data is fit to a model based on phonon-assisted broadening mechanisms coupled to deep and isolated electronic states of the defect. Shaded regions indicate contributions from different phonon-broadening mechanisms, instrument response, and homogeneous broadening.
• Figure 5: Properties of the O$_\mathrm{N}$V$_\mathrm{N}$ and the O$_\mathrm{N}$V$_\mathrm{N}$H centers. (a) Structure of (O$_\mathrm{N}$V$_\mathrm{N}$)$^+$. (b) Left: Kohn-Sham states for the ground state $^2A$ of (O$_\mathrm{N}$V$_\mathrm{N}$)$^+$. Right: charge-density isosurfaces of the defect states of (O$_\mathrm{N}$V$_\mathrm{N}$)$^+$. (c) Calculated CCD for the $^2A \leftrightarrow~^2A$ transition. (d) Structure of (O$_\mathrm{N}$V$_\mathrm{N}$H)$^0$. (e) Left: Kohn-Sham states for the ground state $^2A$ of (O$_\mathrm{N}$V$_\mathrm{N}$H)$^0$. Right: charge-density isosurfaces of the defect states of (O$_\mathrm{N}$V$_\mathrm{N}$H)$^0$. (f) Calculated CCD for the $^2A \leftrightarrow~^2A$ transition. The isosurface absolute value is set to $4.0 \times 10^{-8}~e/a_0^3$.