Miniature Cavity-Enhanced Diamond Magnetometer

At a Glance

Metadata	Details
Publication Date	2017-10-27
Journal	Physical Review Applied
Authors	Georgios Chatzidrosos, Arne Wickenbrock, Lykourgos Bougas, Nathan Leefer, Teng Wu
Institutions	Centre National de la Recherche Scientifique, Helmholtz Institute Mainz
Citations	85
Analysis	Full AI Review Included

Technical Documentation and Analysis: Miniature Cavity-Enhanced Diamond Magnetometer

Source Paper: Chatzidrosos et al., Miniature cavity-enhanced diamond magnetometer (arXiv:1706.02201v1) Date: 7 June 2017 Target Application: High-Sensitivity Endoscopic and Biomedical Magnetic Field Sensing

Executive Summary

This paper successfully demonstrates a miniaturized, room-temperature magnetic field sensor leveraging Nitrogen-Vacancy (NV) centers in diamond, optimized for biomedical applications. 6CCVD’s expertise in customized Single Crystal Diamond (SCD) material is essential for replicating and advancing this design.

Core Technology: NV centers in a diamond plate are used as spin sensors, with the magnetic resonance detected via cavity-enhanced Infrared (IR) absorption (1042 nm singlet transition), providing higher collection efficiency than standard photoluminescence (PL) methods.
Achieved Sensitivity: The device exhibits a magnetic-field sensitivity noise floor of 28 pT/√Hz, operating significantly below previous IR absorption benchmarks (approx. 100 times better).
Projected Limits: The sensor approaches the projected photon shot-noise limit of 22 pT/√Hz, highlighting excellent engineering and material quality.
Miniaturization: The compact design utilizes a (111)-cut SCD plate (0.39 mm thick) acting as the input mirror, integrated into a cavity with a total optical length of $5.00 \pm 0.03$ mm, suitable for endoscopic devices.
Material Requirements: Success hinges on using high-quality SCD substrates optimized for optical clarity at 1042 nm and suitable for thin-film dielectric coatings (R₁ = 98.5%).
6CCVD Advantage: 6CCVD provides the necessary custom-cut (111) SCD wafers, precise thickness control down to 0.1 µm, and in-house polishing (Ra < 1 nm) required for high-finesse cavity integration.

Technical Specifications

Extraction of hard performance data and material parameters from the research:

Parameter	Value	Unit	Context
Measured Sensitivity	28	pT/√Hz	Noise floor (60-90 Hz region), magnetically insensitive spectrum
Photon Shot-Noise Limit	22	pT/√Hz	Projected limit, based on 4.2 mW collected IR light
Quantum Projection Limit	0.43	pT/√Hz	Estimated theoretical maximum sensitivity
Sensing Volume Dimensions	390 µm x 4500 µm²	N/A	Diamond thickness x cavity mode area
Diamond Cut Orientation	(111)	N/A	Substrate crystallographic direction
Diamond Geometric Length (L_d)	390	µm	Thickness of the diamond plate
Total Cavity Optical Length	5.00 ± 0.03	mm	Compact structure
IR Detection Wavelength	1042	nm	¹E <-> ¹A₁ singlet transition zero-phonon line
Green Pump Wavelength	532	nm	Continuous wave excitation
Cavity Finesse (F)	160 ± 4	N/A	Measured without green pump light
Input Mirror Reflectivity (R₁)	98.5 ± 0.5	%	Dielectric coating on diamond surface (for IR)
Output Mirror Reflectivity (R₂)	99.2 ± 0.8	%	Spherical mirror output coupler
ODMR Linewidth (Δν)	5.6	MHz	Full width at half maximum (outer peaks)
Estimated NV Density (Singlet)	0.68 ± 0.01	ppm	Calculated metastable singlet state population

Key Methodologies

The experiment utilized a compact Fabry-Perot cavity structure where the (111)-cut SCD served as the input mirror, enhanced by custom coatings. Key steps for the optically detected magnetic resonance (ODMR) measurement via absorption included:

Diamond Preparation: A (111)-cut SCD plate (L_d = 390 µm) was dielectrically coated for high reflectivity ($R_1 \approx 98.5%$) at the IR probe wavelength (1042 nm) and anti-reflective (AR) coating for the green pump light (532 nm) on the external surface. An AR coating for both wavelengths was applied to the internal cavity surface.
Cavity Construction: The diamond plate was glued to a holder (acting as a heat sink) and mated with a spherical mirror ($R_{C}=10 \text{mm}$, $R_2 \approx 99.2%$) using epoxy resin (Torr Seal) to form a 5 mm long cavity.
Optical Pumping: Continuous wave 532 nm green light (up to 400 mW analyzed) was used to pump the NV centers, preparing the spin population into the ³A₂ $m_s=0$ ground state and the metastable ¹E singlet state.
IR Probing & Lock: A 1042 nm external-cavity diode laser (DL-Pro) was matched to the cavity’s lowest-order longitudinal mode (TEM₀₀). The IR frequency was locked to the cavity mode using two PID controllers (fast feedback to laser current; slow feedback to cavity piezo actuator).
Microwave Manipulation (MW): MW signals were generated, amplified (16W), high-pass filtered, and applied via a mm-sized wire loop to coherently manipulate the NV spin population, inducing ODMR.
Signal Detection: Magnetic resonance (ODMR) was observed by monitoring the absorption of the 1042 nm IR light, resulting in modulation of the cavity transmission signal. The MW frequency was modulated (f_mod = 8.6 kHz) and the first harmonic of the transmission was detected using a Lock-In Amplifier (LIA).

6CCVD Solutions & Capabilities

The development of high-performance, miniaturized quantum sensors like this cavity-enhanced magnetometer depends critically on the quality and customization of the MPCVD diamond material. 6CCVD is uniquely positioned to supply the materials required to replicate, optimize, and scale this research.

Applicable Materials

To achieve the low-loss, high-coherence, and high-quality optical surface necessary for a high-finesse cavity ($F=160$), researchers must use premium Single Crystal Diamond (SCD) material.

Required Material Specification	6CCVD Material Solution	Engineering Rationale & Sales Driver
High-Purity NV Host	Quantum Grade SCD	Essential for maximizing coherence time and maintaining NV concentration (0.68 ppm) uniformity for optimal spin projection noise performance (0.43 pT/√Hz limit).
Crystallographic Orientation	Custom (111)-Cut SCD	The experiment requires a (111) substrate to align the magnetic field optimally relative to the NV orientation features visible in Fig. 3. 6CCVD offers custom crystallographic orientation plates up to 125 mm.
Precise Thickness Control	SCD Wafers (0.1 µm to 500 µm)	The paper uses a $390 \text{ µm}$ plate ($L_d$), a critical parameter for cavity length optimization. 6CCVD guarantees thickness tolerance for thin SCD wafers, crucial for high-finesse structures.
Surface Finish	Ultra-Polished SCD (Ra < 1 nm)	Low surface roughness is mandatory for high-finesse optical coatings and minimizing scattering losses in the cavity (which would reduce the overall finesse). 6CCVD provides industry-leading polishing on SCD.

Customization Potential for Enhanced Performance

The reported device used external dielectric coatings. Future performance improvements cited in the paper—such as increasing IR light power via smaller mode volumes or implementing a “critically matched cavity” ($R_1 = R_2 + A$)—require unparalleled dimensional control and custom fabrication features, services 6CCVD provides in-house.

Dimensional Flexibility: 6CCVD provides custom laser cutting and precise machining to create miniaturized components (up to 125mm) required for complex endoscopic probes.
Metalization Services: While this study relied on dielectric coatings, future prototypes requiring electrical manipulation or integration (e.g., microwave control structures, integrated micro-coils) can utilize 6CCVD’s internal metalization capabilities (Ti/Pt/Au, Au, W, Cu), minimizing interface losses and ensuring high adhesion to the diamond substrate.
Boron Doping for Electrodes: For advanced sensor designs involving integrated electrical detection or thermal management, 6CCVD offers Boron-Doped Diamond (BDD) thin films, available in custom thicknesses and doping concentrations.

Engineering Support

6CCVD’s in-house PhD engineering team possesses deep expertise in the requirements for high-performance quantum sensing and optical systems. We can assist researchers in material selection for similar NV-center magnetometry and biomedical/endoscopic sensing projects, specifically advising on:

Selecting the optimal NV density (ppm) and defect control (NV to NV^- ratio) for minimizing spin projection noise.
Defining precise thickness and cut specifications to integrate the diamond seamlessly into high-finesse optical systems.
Developing custom metalization stacks for high-power microwave transmission structures.

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

View Original Abstract

We present a highly sensitive miniaturized cavity-enhanced room-temperature\nmagnetic-field sensor based on nitrogen-vacancy (NV) centers in diamond. The\nmagnetic resonance signal is detected by probing absorption on the 1042\,nm\nspin-singlet transition. To improve the absorptive signal the diamond is placed\nin an optical resonator. The device has a magnetic-field sensitivity of 28\npT/$\sqrt{\rm{Hz}}$, a projected photon shot-noise-limited sensitivity of 22\npT/$\sqrt{\rm{Hz}}$ and an estimated quantum projection-noise-limited\nsensitivity of 0.43 pT/$\sqrt{\rm{Hz}}$ with the sensing volume of $\sim$ 390\n$\mu$m $\times$ 4500 $\mu$m$^{2}$. The presented miniaturized device is the\nbasis for an endoscopic magnetic field sensor for biomedical applications.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.8.044019