Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications

At a Glance

Metadata	Details
Publication Date	2022-07-15
Journal	Nanomaterials
Authors	Chanaprom Cholsuk, Sujin Suwanna, Tobias Vogl
Institutions	Fraunhofer Institute for Applied Optics and Precision Engineering, Mahidol University
Citations	35
Analysis	Full AI Review Included

Executive Summary

This study utilizes Density Functional Theory (DFT) to identify and tailor the emission wavelengths of color centers in hexagonal Boron Nitride (hBN) for quantum technology applications.

Core Achievement: Identified 92 promising radiative defects among 267 calculated defect complexes (including substitutional atoms from Groups III-VI, transition metals, and multi-defects).
Methodology: Employed spin-polarized DFT using the Heyd-Scuseria-Ernzerhof (HSE06) functional to accurately predict electronic band structures and transition energies.
Performance Metrics: hBN defects show high potential, with reported experimental Debye-Waller (DW) factors up to 82.4% and Quantum Efficiencies (QE) up to 87% at room temperature.
Wavelength Tailoring: Demonstrated theoretical feasibility of using external bi-axial strain to precisely tune defect emission wavelengths to match critical quantum technology targets (e.g., solid-state qubits, telecom bands, and quantum memories).
Target Matching: Provided a comprehensive list of hBN defects compatible with key quantum systems, including NV/SiV centers in diamond/SiC, rare-earth ion memories (Pr³⁺, Tm³⁺), and telecom O- and C-bands.
Strain Mechanism: Out-of-plane lattice deformation results in larger wavelength shifts (50 to 250 nm) compared to in-plane deformation (30 to 40 nm).

Technical Specifications

Parameter	Value	Unit	Context
Host Material Band Gap (E_g)	5.99	eV	Pristine hBN monolayer (Direct gap, HSE06 calculation)
Total Defects Investigated	267	Complexes	Substitutional, transition metal, and multi-defects (neutral and ±1 charge states)
Identified Radiative Defects	92	Defects	Neutral charge state, first-order transition
Most Common Transition Energy Range	1.6 to 2.6	eV	For most fluorescent hBN defects
DFT Functional Used	HSE06	N/A	Hybrid functional for accurate band gap prediction
Supercell Size	7 x 7 x 1	N/A	Used for defect calculations to minimize defect-defect interaction
Vacuum Layer Thickness	15	A	Added to minimize van der Waals interaction between layers
Force Convergence Criterion	0.01	eV.A^-1	Maximum force allowed during lattice structural optimization
Total Energy Convergence Criterion	10^-4	eV	Required convergence for total energy
Target Wavelength Match (PbV)	552	nm	Required strain for S_BV_B defect to match PbV center in diamond
Strain Required (S_BV_B to 552 nm)	0.10	%	Bi-axial strain application
Maximum Wavelength Shift (Out-of-plane strain)	50 to 250	nm	Observed shift range for defects like In_BV_N, Al_N, O_NS_N
Maximum Wavelength Shift (In-plane strain)	30 to 40	nm	Observed shift range for defects like Al_BV_B, Er_NN_BV_N

Compatible hBN Emitters for Quantum Applications

Target Application Wavelength (nm)	Target Quantum System	Compatible hBN Defect	Defect Transition Energy (eV)
552	PbV (diamond)	S_BV_B	2.252
637	NV (diamond)	Al_N	1.918
780	Rb-D2 (Alkali Vapor)	Er_BV_B	1.592
795	Rb-D1 (Alkali Vapor)	Al_BV_B	1.535
862	V_Si (Silicon Carbide)	Er_BV_N	1.427
1330	Telecom O-band	O_NS_N	0.946
1550	Telecom C-band	Er⁺	0.789

Key Methodologies

The study relied exclusively on computational methods using spin-polarized Density Functional Theory (DFT) to characterize defect complexes in hBN monolayers.

Computational Platform: All calculations were performed using QuantumATK (version S-2021.06).
Functional Selection: The Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional was employed to ensure accurate prediction of the electronic band gap and defect states, avoiding underestimation common with non-hybrid functionals.
Supercell Construction: Point-like defects were modeled within a 7 x 7 x 1 hBN supercell. A 15 A vacuum layer was added perpendicular to the hBN plane to eliminate spurious interactions between periodic layers.
Structural Optimization: Lattice structural optimization was performed, allowing only internal coordinates to relax. Convergence criteria required forces to be below 0.01 eV.A^-1 and total energy convergence to reach 10^-4 eV.
K-Point Sampling: A 5 x 5 x 1 Monkhorst-Pack reciprocal space grid was used for structural optimization, and a dense 11 x 11 x 1 k-point grid centered at the Gamma point was implemented for electronic structure calculations.
Defect Screening Criteria: Defects were classified as promising based on three criteria:
- Transition Type: Must form a two-level system with a radiative transition (identified via the imaginary part of the dielectric function).
- Localization: Must be deep-level defects, well isolated from band edges (VB/CB) to ensure high room-temperature QE.
- Transition Energy: Must possess an optical emission wavelength useful for quantum technology applications.
Strain Tuning Simulation: Bi-axial strain was simulated by applying a 3 x 3 stress tensor (identical values for xx and yy components) to the optimized crystal structure, followed by geometry relaxation. Strain (s) was calculated as the fractional change in lattice parameter (AL/L₀).

Commercial Applications

The ability to precisely tailor the emission wavelength of robust, room-temperature single-photon emitters in 2D materials directly supports several critical quantum and photonics industries.

Quantum Communication:
- Long-Distance Networks: Tailoring emission to the telecom O-band (1330 nm, O_NS_N defect) and C-band (1550 nm, Er⁺ defect) enables efficient coupling into existing fiber infrastructure for quantum key distribution (QKD).
- Integrated Photonics: The 2D nature of hBN allows for easy integration of emitters with waveguides and fiber networks, crucial for compact quantum devices.
Quantum Computing and Qubits:
- Solid-State Qubit Coupling: Tuning hBN emitters to match the resonance wavelengths of established solid-state qubits (e.g., NV centers in diamond at 637 nm, V_Si centers in SiC at 862 nm) facilitates efficient optical coupling between different quantum systems.
Quantum Memory:
- Rare-Earth Ion Systems: Matching emission to quantum memory wavelengths, such as those used by Pr³⁺:Y₂SiO₅ or Tm³⁺:Y₂SiO₅, allows hBN single-photon sources to efficiently load or read out quantum states from memory devices.
- Alkali Vapor Systems: Tuning to alkali vapor transitions (e.g., Rb-D1/D2 lines) supports vapor-based quantum memories and sensors.
Quantum Sensing and Metrology:
- High-Repetition Sources: The fast radiative decay lifetime of hBN emitters supports high repetition rates, beneficial for high-throughput quantum sensing applications.
Random Number Generation:
- The bright and pure single-photon emission from hBN defects has already been demonstrated for use in quantum random number generation (QRNG).

View Original Abstract

Optical quantum technologies promise to revolutionize today’s information processing and sensors. Crucial to many quantum applications are efficient sources of pure single photons. For a quantum emitter to be used in such application, or for different quantum systems to be coupled to each other, the optical emission wavelength of the quantum emitter needs to be tailored. Here, we use density functional theory to calculate and manipulate the transition energy of fluorescent defects in the two-dimensional material hexagonal boron nitride. Our calculations feature the HSE06 functional which allows us to accurately predict the electronic band structures of 267 different defects. Moreover, using strain-tuning we can tailor the optical transition energy of suitable quantum emitters to match precisely that of quantum technology applications. We therefore not only provide a guide to make emitters for a specific application, but also have a promising pathway of tailoring quantum emitters that can couple to other solid-state qubit systems such as color centers in diamond.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12142427

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