Pump-Enhanced Continuous-Wave Magnetometry Using Nitrogen-Vacancy Ensembles

At a Glance

Metadata	Details
Publication Date	2017-09-05
Journal	Physical Review Applied
Authors	Sepehr Ahmadi, Haitham A. R. El-Ella, J. Bindslev Hansen, Alexander Huck, Ulrik L. Andersen
Institutions	Technical University of Denmark
Citations	44
Analysis	Full AI Review Included

6CCVD Technical Documentation: Pump-Enhanced Continuous-Wave Magnetometry

Reference: Ahmadi et al., “Pump-Enhanced Continuous-Wave Magnetometry Using Nitrogen-Vacancy Ensembles,” Phys. Rev. Applied 8, 034001 (2017).

Executive Summary

This paper presents a robust methodology for high-sensitivity, continuous-wave (CW) magnetometry leveraging nitrogen-vacancy (NV) ensembles within CVD Single-Crystal Diamond (SCD). The results achieved near shot-noise limited performance despite highly unoptimized material and collection parameters, demonstrating significant potential for industrial and biological sensing applications.

Near Shot-Noise Performance: A magnetic noise floor of $\sim 200 \text{ pT}/\sqrt{\text{Hz}}$ was measured, closely approaching the projected shot-noise limit of $\sim 160 \text{ pT}/\sqrt{\text{Hz}}$ for the current setup.
Cavity-Enhanced ODMR: The use of a 532 $\text{nm}$ resonant confocal optical cavity effectively amplified the input pump power, enabling the high excitation rates necessary to reach the linewidth-narrowing regime.
Material Efficiency: High sensitivity ($\sim 3 \text{ nT}/\sqrt{\text{Hz}}$) was achieved using an off-the-shelf SCD diamond with an intrinsically low ¹⁴N-V concentration ($\sim 0.2 \text{ ppb}$) and poor fluorescence collection efficiency ($\lt 2%$).
Linewidth Narrowing: Optical enhancement provided the necessary $\Gamma_{\text{p}}/\Omega$ ratio to counteract power broadening, crucial for maximizing the ODMR signal slope (d$\omega_{\text{c}}\text{S}_{\text{LI}}$).
CW Applicability: The method utilizes CW ODMR, which is applicable to both DC and AC magnetic field measurements, unlike more complex pulsed spin-echo sequences.
Scalability Insight: The research suggests that simply increasing NV ensemble density may require impractical increases in excitation power to maintain optimal sensitivity, underscoring the necessity of optimizing optical coupling and material purity.

Technical Specifications

The following hard parameters define the operational characteristics and performance metrics achieved in the referenced study.

Parameter	Value	Unit	Context
Achieved Noise Floor	200 $\text{pT}/\sqrt{\text{Hz}}$	$\text{pT}/\sqrt{\text{Hz}}$	Across $0.1$ to $159$ $\text{Hz}$ bandwidth
Extracted Sensitivity	$\sim 3 \text{ nT}/\sqrt{\text{Hz}}$	$\text{nT}/\sqrt{\text{Hz}}$	Measured using an applied $60$ $\text{Hz}$ AC field
Projected Shot-Noise Limit	$\sim 160 \text{ pT}/\sqrt{\text{Hz}}$	$\text{pT}/\sqrt{\text{Hz}}$	Calculated optimum for $\Omega=5.7 \text{ MHz}$ and $\Gamma_{\text{p}}=6 \text{ MHz}$
Diamond Material	SCD (CVD Growth)	N/A	Polished, untreated, $6 \times 6 \times 1.2 \text{ mm}$
NV Concentration	$\sim 0.2 \text{ ppb}$	$\text{ppb}$	Estimated intrinsic ¹⁴N-V^- density
Excitation Volume NV Count	$\sim 10^9$	Centers	Order of magnitude estimate
Optical Pump Wavelength	$532$	$\text{nm}$	P-polarized laser (Coherent Verdi SLM)
Maximum Intracavity Power ($P_{\text{cav}}$)	$\sim 9$	$\text{W}$	Corresponding to saturation power $P_{\text{sat}}$
Microwave Drive Frequency ($\omega_{\text{c}}/2\pi$)	$2.884$	$\text{GHz}$	Split-ring resonator resonance
Microwave Rabi Frequency ($\Omega$)	$\sim 0.55$	$\text{MHz}$	Optimized experimental value used for noise floor measurement
Optical Excitation Rate ($\Gamma_{\text{p}}$)	$\sim 0.3$	$\text{MHz}$	Optimized experimental value for noise floor measurement
Collection Efficiency	$\lt 2$	$%$	Estimated due to high refractive index
Surface Quality	Polished	N/A	Used in a cavity system ($\mathcal{F}=45$ loaded)

Key Methodologies

The experimental approach focused on optimizing excitation uniformity and efficiency across the NV ensemble volume by combining a confocal optical cavity with a microwave resonator.

1. Diamond Material Preparation and Placement

A CVD-grown Single-Crystal Diamond ($6 \times 6 \times 1.2 \text{ mm}$) with an intrinsic substitutional nitrogen concentration of $\lt 1 \text{ ppm}$ was used.
The diamond was mounted on an apertured circuit board patterned with a microwave (MW) split-ring resonator antenna.
The crystal was placed vertically between two confocal-cavity mirrors (radius of curvature $10 \text{ cm}$) at a Brewster angle ($\theta_{\text{B}} = 67^\circ \pm 0.4^\circ$) relative to the cavity axis.

2. Optical Cavity Setup and Locking

The cavity used mirrors with unequal reflectivity: $R_1 = 94.8%$ (Input) and $R_2 = 99.8%$ (Output), designed to approach impedance matching conditions when the diamond was included.
The cavity was pumped by a phase-modulated, p-polarized $532 \text{ nm}$ laser.
Pound-Drever-Hall (PDH) technique was used, locking the laser frequency to the cavity resonance via a piezoelectric actuator attached to the $R_1$ mirror.
The effective excitation volume, governed by the $\text{LG}_{00}$ mode profile, was estimated to be $\sim 3.5 \times 10^{-2} \text{ mm}^{3}$.

3. Microwave (MW) Excitation and Detection

The MW drive was delivered using the split-ring resonator antenna, providing spatially uniform power delivery across the ensemble. The resonator showed a resonance at $2.884 \text{ GHz}$ ($\sim 91 \text{ MHz}$ bandwidth).
Hyperfine Line Excitation: To maximize contrast, simultaneous excitation of all three ¹⁴N hyperfine lines was achieved by mixing the modulated MW drive frequency ($\omega_{\text{c}}/2\pi$) with a sine wave oscillating at the axial hyperfine splitting frequency ($A_{||} = 2.16 \text{ MHz}$).
Signal Readout: Optically Detected Magnetic Resonance (ODMR) fluorescence ($\gt 600 \text{ nm}$) was collected using a high-NA objective ($\ge 0.7$), filtered, and focused onto a Si-biased detector.
Lock-in Amplification: The detected signal was noise filtered and amplified using a lock-in amplifier (Stanford Research Systems SR510) with a $1 \text{ ms}$ time constant, setting a low-pass filter corner frequency at $159 \text{ Hz}$.

6CCVD Solutions & Capabilities

The findings in this paper confirm the technical viability of CVD diamond for high-performance quantum sensing. The primary limitation identified—low intrinsic NV concentration and non-optimized collection—can be directly mitigated by leveraging 6CCVD’s expertise in custom material engineering and fabrication.

Applicable Materials for Replication and Enhancement

To replicate the achieved sensitivity or significantly extend the research toward femtotesla (fT) noise floors, 6CCVD recommends tailored diamond materials:

Research Requirement	6CCVD Material Solution	Rationale & Advantage
Ultra-Low Intrinsic N	Optical Grade SCD ($[N] < 5 \text{ ppb}$)	Provides the lowest native impurity levels for pristine optical/electronic properties, minimizing background absorption and broadening.
Controlled High NV Density	Engineered SCD (Controlled ¹⁵N Doping)	Replicate the NV concentration used (0.2 $\text{ppb}$) or increase to highly optimized densities ($\gt 100 \text{ ppb}$) necessary for scaling sensitivity ($\propto 1/\sqrt{N}$). Using ¹⁵N simplifies hyperfine structure, reducing spectral overlap complexities seen in this work.
Mitigating Inhomogeneous Broadening	High Purity Isotope-Enriched SCD (99.999% ¹²C)	Eliminates the primary source of inhomogeneous spin broadening (¹³C nuclear spins), crucial for maintaining collective coherence and maximizing ODMR contrast, especially at higher NV densities.

Customization Potential for Cavity Integration

The study relies heavily on precise geometry and high surface quality for successful cavity integration (loaded finesse $\mathcal{F}=45$). 6CCVD provides the necessary fabrication precision:

Custom Dimensions and Geometry: The paper used a $6 \times 6 \times 1.2 \text{ mm}$ geometry. 6CCVD offers custom laser cutting and shaping services to create specific diamond prisms, wedges, or wafers up to $125 \text{ mm}$ for optimized Brewster angle coupling and integration into specific cavity or waveguide designs.
Ultra-High Polishing: The success of the confocal cavity requires minimal scattering losses. 6CCVD guarantees Ra $\lt 1 \text{ nm}$ polishing for Single-Crystal Diamond surfaces, minimizing absorption and increasing effective cavity finesse ($\mathcal{F}$).
Integrated Microwave Structures: The MW split-ring resonator integration suggests the need for durable electrical contacts. 6CCVD offers in-house metalization services (Ti/Pt/Au, Au/Pd, Cu, W) for direct deposition of contact pads or microstrip lines onto the diamond surface, facilitating robust integration of antenna structures.

Engineering Support

6CCVD’s in-house PhD team provides consultative support for complex quantum sensing projects. We specialize in tailoring material properties (N concentration, isotopic purity, thickness control, metalization schemes) to meet specific requirements for continuous-wave (CW) ODMR and NV magnetometry setups, ensuring optimal $\Gamma_{\text{p}}/\Omega$ ratios and coherence times.

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

View Original Abstract

Ensembles of nitrogen-vacancy centers in diamond are a highly promising\nplatform for high-sensitivity magnetometry, whose efficacy is often based on\nefficiently generating and monitoring magnetic-field dependent infrared\nfluorescence. Here we report on an increased sensing efficiency with the use of\na 532-nm resonant confocal cavity and a microwave resonator antenna for\nmeasuring the local magnetic noise density using the intrinsic nitrogen-vacancy\nconcentration of a chemical-vapor deposited single-crystal diamond. We measure\na near-shot-noise-limited magnetic noise floor of 200 pT/$\sqrt{\text{Hz}}$\nspanning a bandwidth up to 159 Hz, and an extracted sensitivity of\napproximately 3 nT/$\sqrt{\text{Hz}}$, with further enhancement limited by the\nnoise floor of the lock-in amplifier and the laser damage threshold of the\noptical components. Exploration of the microwave and optical pump-rate\nparameter space demonstrates a linewidth-narrowing regime reached by virtue of\nusing the optical cavity, allowing an enhanced sensitivity to be achieved,\ndespite an unoptimized collection efficiency of <2 %, and a low\nnitrogen-vacancy concentration of about 0.2 ppb.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.8.034001