Microwave-Assisted Spectroscopy Technique for Studying Charge State in Nitrogen-Vacancy Ensembles in Diamond

At a Glance

Metadata	Details
Publication Date	2020-07-06
Journal	Physical Review Applied
Authors	D. P. L. Aude Craik, P. Kehayias, A. S. Greenspon, X. Zhang, M J Turner
Institutions	Center for Astrophysics Harvard & Smithsonian, University of Maryland, College Park
Citations	23
Analysis	Full AI Review Included

Executive Summary

The research introduces a Microwave-Assisted Spectroscopy technique for the in situ determination and control of the charge state ratio ([NV^-]/[NV⁰]) in nitrogen-vacancy (NV) diamond ensembles, leading to significant improvements in quantum sensing capabilities.

In Situ Charge State Determination: The method uses a spin-state-resonant microwave drive to selectively modulate NV^- fluorescence, allowing the extraction of pure NV^- and NV⁰ spectral shapes from the total photoluminescence (PL) spectrum, accounting for local environmental variations (e.g., strain).
Enhanced ODMR Contrast: By fitting and discarding the spin-independent NV⁰ fluorescence background, the technique achieved a measured 4.8-fold enhancement in Optically-Detected Magnetic Resonance (ODMR) contrast.
Sensitivity Gains: Simulations predict contrast improvements up to 2 orders of magnitude for laser-intensity-noise-limited magnetometers, particularly those utilizing NV⁰-rich ensembles (e.g., near-surface NVs).
New Ionization Pathway: The study uncovered evidence for a previously unidentified spin-dependent ionization pathway from the NV^- singlet state (“shelf”), driven by 532 nm light, which enhances ionization when microwaves are applied.
Rate Equation Model: A 7-level rate-equation model was developed to accurately describe the observed spin-dependent ionization effects.
General Applicability: The technique was successfully applied to isolate room-temperature spectral signatures of the V2-type silicon vacancy (SiV) in 4H silicon carbide (SiC), demonstrating utility for other solid-state defects.

Technical Specifications

Parameter	Value	Unit	Context
NV Layer Thickness	10	µm	CVD-grown diamond sample
Nitrogen Concentration (¹⁴N)	10	ppm	Diamond NV layer
Carbon Isotope Purity (¹²C)	>99.95	%	Diamond substrate
Electron Irradiation Dosage	6 x 10¹⁸	e^-/cm²	Sample preparation
Annealing Temperatures	800 and 1000	°C	Sample preparation (12 hours each)
Excitation Wavelength	532	nm	Diode-pumped solid-state laser
Laser Spot Size (at sample)	~1.2	µm	Confocal setup
Microwave Drive Frequency (NV)	2.87	GHz	Resonant with NV^- m_s=0 to m_s=±1 transition
RF Drive Frequency (SiV)	70	MHz	Resonant with V2 spin transition in 4H SiC
ODMR Contrast Enhancement (Measured)	4.8	fold	Using fitting method (at 7.3 mW laser power)
ODMR Contrast Enhancement (Simulated Max)	Up to 2 orders of magnitude	N/A	For laser-intensity-noise-limited, NV⁰-rich ensembles
Fluorescence Contribution Ratio	69(1)% NV^- / 31(1)% NV⁰	%	Measured fluorescence decomposition
CCD Exposure Time (t_exp)	30	ms	Per spectrum acquisition
Shot-to-Shot Intensity Fluctuation	<0.05	%	Reduced by noise-eater circuit
Dark Ionization Rate (d_i)	100	µs^-1	Literature value used in 7-level model
Dark Recombination Rate (d_r)	300	µs^-1	Literature value used in 7-level model

Key Methodologies

The core methodology involves isolating the NV^- fluorescence component via microwave modulation and subsequent spectral fitting, followed by applying this knowledge to enhance ODMR readout.

1. Sample Preparation and Setup

Diamond Sample: A 10 µm NV layer was grown via Chemical Vapor Deposition (CVD) on a high-purity ¹²C substrate.
Defect Creation: The sample was irradiated (6 x 10¹⁸ e^-/cm²) and annealed (800 °C then 1000 °C) to create and mobilize vacancies, forming NV centers.
Optical Setup: A home-built confocal microscope was used with a 532 nm laser stabilized by a noise-eater circuit (Thorlabs NEL01). Fluorescence was collected via a spectrometer and liquid-nitrogen-cooled CCD.
Microwave Delivery: A 2.87 GHz microwave drive was delivered via an omega-loop stripline and controlled by a TTL-triggered switch.

2. Charge State Determination via Microwave Modulation

Resonant Modulation: The microwave frequency was set resonant with the NV^- m_s=0 to m_s=±1 transition (2.87 GHz).
Spectral Acquisition: Alternating PL spectra were acquired with microwaves ON (S_MWon) and OFF (S_MWoff).
Difference Spectrum (S_diff): S_diff = S_MWoff - S_MWon was calculated, isolating the modulated NV^- spectral shape (plus a small NV⁰ signature due to spin-dependent ionization).
Scale Factor Fitting: A scale factor (k) was applied to S_diff. The correct factor (k₀) was determined by minimizing the residual NV^- Zero-Phonon Line (ZPL) feature (at 637 nm) in the extracted trial NV⁰ spectrum (S^trial_NV0).
Ionization Correction: The S^trial_NV0 spectrum was corrected for the effect of spin-dependent ionization (which alters the steady-state NV⁰ population when microwaves are ON) to yield the final, pure NV⁰ spectrum (S_NV0).
Decomposition: The total S_MWoff spectrum was decomposed into the corrected S_NV- and S_NV0 components to determine the in situ charge-state ratio R.

3. High-Contrast ODMR (Fitting Method)

Data Acquisition: Full PL spectra were acquired at each frequency point during an ODMR scan.
Spectral Fitting: Each spectrum was fitted using the previously established pure S_NV- and S_NV0 spectral shapes.
Background Suppression: The NV⁰ contribution was mathematically discarded.
Contrast Calculation: ODMR contrast was calculated based only on the fitted NV^- fluorescence signal, effectively suppressing the spin-independent background noise and increasing the signal-to-noise ratio (SNR).

Commercial Applications

The methodologies developed here are critical for advancing quantum sensing and solid-state defect engineering, particularly in areas requiring high sensitivity and spatial resolution.

Industry/Application	Relevance and Value Proposition
Quantum Sensing & Magnetometry	High-Sensitivity DC Magnetometry: The 4.8-fold contrast enhancement directly translates to increased magnetic field sensitivity (reduced minimum detectable field), crucial for next-generation pT/√Hz sensors.
Near-Surface Sensing	High-Resolution Imaging: By suppressing the NV⁰ background, the method significantly improves the sensitivity of near-surface NV ensembles, enabling high-resolution magnetic imaging of biological samples (e.g., live imaging) and condensed matter systems.
Diamond Material Engineering	Charge State Optimization: Provides a quantitative, in situ tool to measure and tune the steady-state NV^- population under various illumination and material conditions, guiding the fabrication of NV^--rich diamonds needed for high-performance magnetometers.
Spin-to-Charge Conversion	Readout Mechanism Development: Understanding the newly identified spin-dependent ionization pathway from the singlet state is vital for optimizing and scaling up readout techniques that rely on spin-to-charge conversion.
Solid-State Defect Characterization	SiC and Other Defects: The technique can be adapted to isolate spectral features of other fluorescent defects (e.g., Silicon Vacancies in SiC) that exhibit spin-dependent fluorescence, facilitating the study of their optical and spin properties for use in quantum computing or sensing platforms.
Picolitre NMR/MRI	Microscale Analysis: Increased sensitivity enables the use of NV ensembles for nuclear magnetic resonance and magnetic resonance imaging at extremely small volumes (picolitre scale).

View Original Abstract

We introduce a microwave-assisted spectroscopy technique to determine the\nrelative concentrations of nitrogen vacancy (NV) centers in diamond that are\nnegatively-charged (NV${}^-$) and neutrally-charged (NV${}^0$), and present its\napplication to studying spin-dependent ionization in NV ensembles and enhancing\nNV-magnetometer sensitivity. Our technique is based on selectively modulating\nthe NV${}^-$ fluorescence with a spin-state-resonant microwave drive to\nisolate, in-situ, the spectral shape of the NV${}^-$ and NV${}^0$ contributions\nto an NV-ensemble sample’s fluorescence. As well as serving as a reliable means\nto characterize charge state ratio, the method can be used as a tool to study\nspin-dependent ionization in NV ensembles. As an example, we applied the\nmicrowave technique to a high-NV-density diamond sample and found evidence for\na new spin-dependent ionization pathway, which we present here alongside a\nrate-equation model of the data. We further show that our method can be used to\nenhance the contrast of optically-detected magnetic resonance (ODMR) on NV\nensembles and may lead to significant sensitivity gains in NV magnetometers\ndominated by technical noise sources, especially where the NV${}^0$ population\nis large. With the high-NV-density diamond sample investigated here, we\ndemonstrate up to a 4.8-fold enhancement in ODMR contrast. The techniques\npresented here may also be applied to other solid-state defects whose\nfluorescence can be selectively modulated by means of a microwave drive. We\ndemonstrate this utility by applying our method to isolate room-temperature\nspectral signatures of the V2-type silicon vacancy from an ensemble of V1 and\nV2 silicon vacancies in 4H silicon carbide.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.14.014009

References

2017 - High Sensitivity Magnetometers, Smart Sensors, Measurement and Instrumentation