Single-Photon Emission in hBN

• Single-photon emission in hBN is defined by optically active point defects within a wide bandgap crystal that yield stable quantum light with narrow zero-phonon lines.
• Distinct defect classes, including carbon-based complexes and oxygen-decorated vacancies, are identified by unique spectral signatures and controlled via methods like irradiation and plasma processing.
• Integration with plasmonic resonators and nanocavities enhances emission rates and enables tunable, high-efficiency single-photon sources for quantum photonic circuits.

Single-photon emission in hexagonal boron nitride (hBN) arises from optically active point defects and their complexes within the wide bandgap ($\sim$6 eV) layered crystal. These solid-state quantum emitters exhibit robust antibunching, narrow zero-phonon lines (ZPLs), and high quantum efficiency at room temperature, making hBN a leading platform for photonic quantum technologies, including quantum communications, quantum key distribution, and nanoscale sensing. Current research has established a comprehensive picture of emitter creation, the atomistic nature of the underlying defects, photophysical properties, and pathways for device integration, although many atomic-level details remain under active investigation.

1. Atomic Structure and Identification of Single-Photon Emitters

Defect-related quantum emission in hBN is predominantly attributed to deep-level centers involving vacancies, substitutional impurities (especially carbon and oxygen), antisite complexes, and their combinations. Empirically, three main defect classes emerge based on energy, polarization, and photophysical signatures: (1) “Group 1” defects with ZPLs at 1.8–2.2 eV (570–690 nm), (2) “Group 2” defects emitting at 1.4–1.8 eV (700–880 nm), and (3) blue-wavelength centers (B-centers) with ZPLs near 436–440 nm in ultra-thin hBN (Liu et al., 3 Jul 2025).

Ab initio and vibrational spectroscopic analyses support the assignment of dominant visible-range emitters to carbon-related complexes, notably the C$_2$C$_\textrm{B}$ trimer (three carbons at two B and one N site) for yellow emission near 575 nm, and V$_\textrm{NC}_\textrm{B}$ (nitrogen vacancy adjacent to C$_\textrm{B}$) for transitions near 2.10 eV (Tieben et al., 2023, Sajid et al., 2020). Oxygen-decorated boron vacancies (e.g., V$_\textrm{B}$O$_2$) produce red-shifted emission above 700 nm, with DFT calculations showing transitions between mid-gap defect states well separated from the band edges (Xu et al., 2017). In bilayer and few-layer geometries, inter-defect coupling enables emergence of polarization-selective transitions and further spectral tuning (Mehta et al., 14 Nov 2024).

A minority of reported SPEs in hBN (centers with strong heterogeneity near 2 eV) originate from polycyclic aromatic hydrocarbon (PAH) molecules trapped at the hBN/substrate interface, rather than intrinsic lattice defects (Neumann et al., 2023).

2. Photophysical Properties and Quantum Metrics

Single-photon emission from hBN defects is characterized by the following photophysical observables:

• Zero-phonon line (ZPL): Bright, sharp emission with fitted center energies from 1.7–2.2 eV (blue to red). Linewidths are $\sim$5–30 meV at room temperature (resolving to $\sim$230 $\mu$eV at low $T$ in encapsulated ultra-thin flakes) (Liu et al., 3 Jul 2025).
• Debye-Waller factor (DW): Fraction of emission in the ZPL, typically 0.22–0.52, with higher DW ($\sim$0.45–0.52) reported for low-Huang–Rhys centers and carbon trimer CBC defects (Bhunia et al., 2023, Chatterjee et al., 14 May 2025).
• Huang–Rhys factor ($S$): Electron–phonon coupling $S \sim 0.6$ for ultra-coherent SPEs, $S \sim 1$–$1.5$ for typical defects, and $S \sim 0.26$–$0.37$ for best-performing carbon trimer defects (Bhunia et al., 2023, Chatterjee et al., 14 May 2025, Tieben et al., 2023).
• Excited-state lifetime: $2.5$–$3$ ns for bright, low-$S$ centers; Purcell-enhanced lifetimes down to $0.48$ ns in plasmonic nanocavities; ultrafast (25 ps) regimes achievable with metallic nanocavities (Sakib et al., 3 May 2024, Dowran et al., 15 May 2024).
• Antibunching ($g^{(2)}(0)$): Firm single-photon emission is confirmed by room-temperature $g^{(2)}(0)<0.4$, with the lowest reported values $g^{(2)}(0)=0.015$ in carbon-doped thin films (Chatterjee et al., 14 May 2025). Blue-emitting B-centers in few-layer hBN consistently reach $g^{(2)}(0)=0.36$ (Liu et al., 3 Jul 2025).

Table 1 summarizes key properties for representative SPE types:

Defect Type	ZPL (nm/eV)	DW	S	Lifetime (ns)	$g^{(2)}(0)$
Carbon trimer (C$_2$C$_\textrm{B}$)	575/2.15	0.35–0.45	1	3–5	0.36–0.015
V$_\textrm{NC}_\textrm{B}$ (C$_\textrm{B}$ + V$_\textrm{N}$)	590–630/2.1	$\sim$0.11–0.22	1.5	3	$<$0.4
V$_\textrm{B}$O$_2$	711–718/1.73	—	—	2.4	0.1
B-centers (ultra-thin)	436/2.84	—	—	3–5	0.36
Organic PAH	590–620/2.0	—	—	5–9	0.2–0.4

High quantum efficiency (up to $87\pm7\,\%$ at 580 nm) is achieved for these defect centers (Nikolay et al., 2019).

3. Fabrication, Deterministic Creation, and Tuning

SPEs in hBN can be activated and engineered by multiple methods:

• Direct irradiation: Low-energy electron irradiation (20–30 keV) plus high-temperature annealing (850 °C, Ar/H$_2$) enables site-specific, stable SPE creation with low $S$ (Bhunia et al., 2023).
• Plasma processing: Ar-plasma etching followed by inert or oxidative annealing enhances emitter yield by %%%%39$\sim$40%%%% and enables scalable patterning (Xu et al., 2017).
• Remote electron activation: In sub-5 nm hBN, “remote” electron-beam irradiation avoids catastrophic etching, generating stable B-centers (Liu et al., 3 Jul 2025).
• Direct growth and doping: Pulsed laser deposition (PLD) with in-situ carbon source enables large-area, uniform films hosting CBC-type emitters with record-high purity ($g^{(2)}(0)=0.015$) (Chatterjee et al., 14 May 2025).
• Strain and pressure engineering: Nanoscale “blisters” induce local strain fields (up to $\varepsilon\sim1\%$) shifting ZPL energies by $\sim-30$ meV/%, isolating single, ultrastable emitters (Liu et al., 2019). Pressure cycling across a 0.1–0.01 atm range can reversibly switch between single- and ensemble-defect regimes.

Extensions include determination of atomic structure via photoluminescence excitation (PLE) spectroscopy (Tieben et al., 2023), FP cavity or metasurface embedding for rate enhancement (Sakib et al., 3 May 2024, Cao et al., 2020, Wang et al., 2021), and quantification of defect–environment interactions through encapsulation (Liu et al., 3 Jul 2025).

4. Electronic Structure, Vibronic Coupling, and Emission Mechanisms

The optical transitions responsible for SPEs are intragap transitions between molecular orbitals localized on the defect, strongly coupled to high-energy optical phonons ($\hbar\Omega\sim$165–285 meV). The emission spectrum is described by a vibronic ladder with intensity-weighted ZPL and phonon sidebands, following a Franck–Condon (Poisson) distribution:

$$I(\omega) = I_\textrm{ZPL} e^{-S} \sum_{n=0}^\infty \frac{S^n}{n!} \delta[\omega - (\omega_\textrm{ZPL} - n\Omega)]$$

Multiplier PLE peaks and RIXS measurements detect regular harmonics at $E_n = n E_0$, $E_0 = 285$ meV, with up to $n=10$ observed in defective hBN, assigning the main elementary excitation to N $\pi^*$ antibonding orbitals of defect complexes (Pelliciari et al., 15 Feb 2024).

In most defect species (such as V$_\textrm{NC}_\textrm{B}$, C$_2$C$_\textrm{B}$), the optical dipole lies in the hBN basal plane, granting strong linear polarization and compatibility with on-chip planar photonic coupling (Sajid et al., 2020, Tieben et al., 2023). Oxygen-decorated boron vacancies yield emission dipoles likewise in-plane.

Theoretical Stark and strain tuning analyses indicate ZPL shifts of $\sim$5.4 nm/GV·m$^{-1}$ (12 meV/GV·m$^{-1}$) in vertical electric fields, consistent with out-of-plane dipoles $|\boldsymbol{\mu}|=0.19$–$0.9$ D and polarizabilities up to 150 Å$^3$; DFT models confirm field response for asymmetric configurations such as V$_\textrm{N}$X$_\textrm{B}$ (X=C, N, O) (Noh et al., 2018).

5. Integration, Photonic Enhancement, and Device Architectures

Efficient realization of hBN-based single-photon sources for quantum networks and photonic circuits has driven the integration of defects into microcavities, nanostructures, and heterostructures:

• Plasmonic resonators: Gold/Al$_2$O$_3$ plasmonic nanoresonators enable local %%%%62$T$63%%%% field enhancement, $F_p\approx4.9$ Purcell factors, and sub-nanosecond emission lifetimes across large arrays, with deterministic site activation (Sakib et al., 3 May 2024).
• Gap nanocavities: Ag nanocube/hBN/Au architectures yield lifetime reduction from $5.3$ ns to $25$ ps (over $200\times$), and $7\times$ brightness enhancement, supporting fast, on-chip, high-fidelity single-photon output (Dowran et al., 15 May 2024).
• Monolithic metasurfaces: hBN-based quasi-bound states in the continuum (Q $>$ 10$^5$) metasurfaces deliver modeled Purcell factors up to $3.3 \times 10^4$ (Cao et al., 2020).
• Cavity-QED designs: hBN microdisk resonators (r = 1–3 μm, h = 100–500 nm) achieve Purcell factors $\gtrsim$1 for modest $Q$ and approach strong coupling ($g/\max(\kappa,\gamma_\mathrm{deph})$ up to $500$) in optimized configurations (Wang et al., 2021).
• Van der Waals heterostructures: Integration of few-layer or ultra-thin hBN with graphene gates yields field-tunable and addressable SPEs; encapsulation restores optical properties lost through surface proximity (Liu et al., 3 Jul 2025, Noh et al., 2018).
• Scalability: Patterned emitter arrays, mask lithography, and direct growth/doping protocols enable device-level determinism and repeatability.

These enhancements boost photon collection, radiative efficiency, and allow for integration with waveguides and photonic circuits necessary for high-speed quantum networks. Table 2 summarizes emission rates and integration enhancements:

Structure	Lifetime (ns/ps)	Peak Count Rate (Mcps)	Purcell Factor
hBN/SiO$_2$	5.3 ns	0.25	1
hBN/Au (no cube)	0.58 ns	0.98	9
hBN/Au + Ag SNC	0.025 ns (25 ps)	1.76	212
Plasmonic PNR/Al$_2$O$_3$	0.48 ns	3.8+	4.9

6. Strain, Electric Field, and Environmental Tuning

hBN SPEs display pronounced sensitivity to mechanical and electric fields, enabling spectral alignment, charge-state control, and local field probing:

• Stark effect: Vertical field modulation shifts ZPLs by up to 5.4 nm GV$^{-1}$ m, sufficient for wavelength-matching multiple emitters and addressing with nm-resolved gates (Noh et al., 2018).
• Strain tuning: Nanoscale blisters and purpose-built microdevices yield reversible spectral tuning of $-30$ meV per % strain, modulating activation, photostability, and emission brightness (Liu et al., 2019).
• Photochromic and charge-state switching: Some defect centers show field-induced intensity steps, reversible modulation tied to defect charging and local photodoping, and correlated changes in multi-ZPL emission (Noh et al., 2018, Feldman et al., 2020).
• Environmental encapsulation: Encapsulating ultra-thin hBN with bulk-like capping layers restores ZPL linewidth, suppresses photobleaching, and stabilizes emission, underlying the importance of the dielectric and chemical environment (Liu et al., 3 Jul 2025).

These tunabilities enable quantum sensor deployment with sub-10 nm spatial resolution and open control channels for spectral multiplexing and synchronized emitter operation.

7. Challenges, Open Questions, and Prospects

Despite advances, the precise chemical identity and charge state of some emitters remain unresolved. Notably, the contribution of extrinsic organics, the full catalog of intrinsic defect configurations (especially for group 2 emitters), and environmental quenching mechanisms are active areas for research (Neumann et al., 2023, Sajid et al., 2019). Strategies for deterministic placement, reliable charge-state stabilization, optical linewidth narrowing, and integration into complex quantum architectures are under constant development.

The broad accessibility of hBN SPEs—spanning from deep ultraviolet to near-infrared, with room-temperature stability and high quantum efficiency—positions this material as a cornerstone for quantum photonics, scalable quantum networks, and nanoscale quantum sensing. Ongoing work leverages advanced spectroscopy (RIXS, PLE), first-principles simulation, and hybrid integration to refine both fundamental understanding and device utility (Pelliciari et al., 15 Feb 2024, Tieben et al., 2023, Wang et al., 2021).