Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride

At a Glance

Metadata	Details
Publication Date	2022-05-27
Journal	Physical Review Letters
Authors	Zhao Mu, Hongbing Cai, Disheng Chen, Jonathan Kenny, Zhengzhi Jiang
Institutions	National University of Singapore, ARC Centre of Excellence for Transformative Meta-Optical Systems
Citations	56
Analysis	Full AI Review Included

Technical Documentation: Excited-State ODMR in hBN V_B^- Centers

Executive Summary

This research investigates the excited-state (ES) spin properties of negatively charged Boron Vacancy (V_B^-) centers in hexagonal Boron Nitride (hBN), a critical step toward realizing dynamic nuclear polarization (DNP) and advancing 2D quantum sensing.

Core Finding: The zero-field splitting (ZFS) of the V_B^- ES ($D_{ES}$) was determined to be approximately 2160 MHz at cryogenic temperatures (7 K).
Methodology: ES-ODMR (Optically Detected Magnetic Resonance) was performed using pulsed and continuous-wave (CW) microwave excitation at cryogenic temperatures (7 K) and room temperature (293 K).
Key Contrast Mechanism: Unlike Nitrogen Vacancy (NV) centers in diamond, the ODMR contrast for V_B^- is more prominent at cryo-temperature (up to 12%) due to significantly longer ES fluorescence lifetimes (up to 2.32 ns at 4 K).
Quantum Application: The existence of the Excited-State Level Anti-Crossing (ESLAC) point (~800 G) at cryogenic temperatures provides a viable route for nuclear spin manipulation and DNP in hBN spin defects.
Material Analogy: The study directly compares V_B^- in hBN to NV^- centers in diamond, underscoring the ongoing need for high-purity, wide-bandgap materials for solid-state spin qubits.

Technical Specifications

Parameter	Value	Unit	Context
Ground State ZFS ($D_{GS}$)	3460	MHz	Room Temperature (RT)
Ground State ZFS ($D_{GS}$)	3684	MHz	Cryogenic Temperature (7 K)
Excited State ZFS ($D_{ES}$)	~2160	MHz	Cryogenic Temperature (7 K)
Excited State ZFS ($D_{ES}$)	~2117	MHz	Room Temperature (293 K)
GS-ODMR Contrast (Max)	12	%	7 K
ES-ODMR Contrast (Max)	12	%	7 K
ES Lifetime (RT)	0.67	ns	Room Temperature
ES Lifetime (Cryo)	2.32	ns	4 K
ESLAC Magnetic Field	~800	G	Excited State Level Anti-Crossing
GSLAC Magnetic Field	~1330	G	Ground State Level Anti-Crossing
Excitation Wavelength	532	nm	Laser Source
Stripline Width	50	µm	Gold stripline for MW delivery

Key Methodologies

The investigation relied on precise material preparation and advanced spectroscopic techniques to characterize the spin defects:

Sample Preparation: A 50 µm wide straight gold stripline was deposited onto a Si/SiO₂ substrate. Exfoliated hBN flakes were transferred onto the stripline to ensure a homogeneous in-plane magnetic field for spin manipulation.
Defect Generation: V_B^- centers were generated by bombarding the hBN with Ga⁺ ions at the center and edge of the gold line.
Optical Setup: Emission was collected via a home-built confocal microscope and directed into a spectrometer or photon counting device. Excitation was performed using a 532 nm laser.
Microwave (MW) Delivery: The gold stripline was wire-bonded to a chip carrier for feeding MW signals, enabling both CW-ODMR and pulsed ODMR measurements.
Pulsed ODMR Sequence: Spins were initialized to the $m_s = 0$ ground state (5 µs laser pulse), followed by a 500 ns dwell time, a 1 µs MW pulse, and a final 5 µs laser readout pulse. This sequence confirmed the ES nature of the 2351 MHz dip.
Magnetic Field Dependence: GS-ODMR and ES-ODMR spectra were measured under varying external magnetic fields (up to 2000 G) applied nearly perpendicular to the hBN surface to determine g-factors and LAC points.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality, wide-bandgap materials in advancing solid-state quantum technology. While the paper focuses on hBN, the techniques and applications (quantum sensing, DNP) are directly analogous to those utilizing diamond spin qubits (NV^-, SiV^-). 6CCVD is the world-leading supplier of MPCVD diamond engineered specifically for these demanding applications.

Applicable Materials

To replicate or extend this research using the established, robust platform of diamond spin qubits, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Essential for hosting high-coherence spin defects like NV^- or SiV^-. Our SCD material offers extremely low nitrogen and silicon impurity concentrations, minimizing decoherence and maximizing spin contrast, crucial for achieving long coherence times necessary for DNP.
- Recommendation: High-Purity SCD, < 1 ppb Nitrogen concentration.
Polycrystalline Diamond (PCD) Substrates: For large-area sensing applications or integration into complex microwave circuitry, 6CCVD offers PCD wafers up to 125mm in diameter, providing a robust platform for scaling up quantum devices.
Boron-Doped Diamond (BDD): While the paper focuses on V_B^-, BDD is available for researchers requiring controlled p-type doping for charge state stabilization or electrochemical applications.

Customization Potential

The experimental setup required precise material engineering, including specific dimensions and metalization for MW delivery. 6CCVD’s in-house capabilities meet and exceed these requirements:

Requirement in Paper	6CCVD Custom Capability	Technical Specification
Substrate Dimensions	Custom plates and wafers	Plates/wafers up to 125mm (PCD)
Thickness Control	SCD/PCD layers grown to specification	SCD (0.1µm - 500µm)
Microwave Stripline	Custom Metalization Services	Au, Pt, Pd, Ti, W, Cu deposition
Surface Quality	Ultra-low roughness polishing	Ra < 1nm (SCD), Ra < 5nm (Inch-size PCD)
Defect Engineering	High-purity growth for intrinsic defects (e.g., SiV) or post-growth implantation support	SCD substrates optimized for low strain and high yield of engineered defects.

Engineering Support

The challenges noted in the paper—such as the unknown hyperfine interaction in the ES and the need for resonant optical addressing—are common hurdles in solid-state spin physics. 6CCVD’s in-house PhD team specializes in the material science of quantum defects.

Material Selection for Quantum Sensing: Our experts can assist researchers transitioning from hBN to diamond or silicon carbide platforms, ensuring optimal material selection (e.g., specific SCD orientation, doping levels, and surface termination) for similar Dynamic Nuclear Polarization (DNP) and Quantum Sensing projects.
Global Logistics: We provide reliable global shipping (DDU default, DDP available) to ensure your time-sensitive research materials arrive safely and promptly, whether you are working at room temperature or 7 K cryostats.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevlett.128.216402