Laser threshold magnetometry using green-light absorption by diamond nitrogen vacancies in an external cavity laser

At a Glance

Metadata	Details
Publication Date	2021-06-04
Journal	Physical review. A/Physical review, A
Authors	James L. Webb, Andreas F. L. Poulsen, Robert Staacke, Jan Meijer, Kirstine Berg‐Sørensen
Institutions	Technical University of Denmark, Leipzig University
Citations	13
Analysis	Full AI Review Included

Technical Documentation & Analysis: Laser Threshold Magnetometry using NV- Diamond

Executive Summary

This research proposes a novel Laser Threshold Sensing (LTS) scheme utilizing Nitrogen Vacancy (NV-) centers in diamond for high-sensitivity magnetometry, offering a significant advantage over conventional methods.

Core Innovation: LTS eliminates the high optical shot noise background inherent in conventional red fluorescence optically detected magnetic resonance (ODMR) by monitoring changes in green pump light absorption near the lasing threshold of an External Cavity Laser (ECL).
Predicted Performance: Theoretical modeling predicts sub-picotesla level magnetic field sensitivity, reaching as low as 0.02 pT/√Hz under optimized conditions.
Material Requirement: Achieving high sensitivity requires high-purity Single Crystal Diamond (SCD) with precise control over defect concentration and long ensemble dephasing times ($T_{2}$).
Optimal Specifications: The highest sensitivity is achieved with an optimal NV density of approximately 0.01 ppm ($10^{4}$ ppb) and a dephasing time $T_{2}$ > 1 µs.
6CCVD Value Proposition: 6CCVD specializes in MPCVD growth with the necessary isotopic purity (¹²C enrichment) and nitrogen incorporation control to manufacture the high-performance SCD plates required to replicate and exceed these theoretical results.

Technical Specifications

The following table summarizes the critical parameters and performance metrics extracted from the theoretical modeling and experimental validation discussed in the paper.

Parameter	Value	Unit	Context
Predicted Sensitivity (Best Case)	0.02 - 0.3	pT/√Hz	Requires T₂ = 1-10 µs
Optimal NV Density (Modeled)	0.01 (10⁴)	ppm (ppb)	Maximizes sensitivity; requires precise N control
Required Dephasing Time (T₂)	> 1	µs	Necessary for sub-pT/√Hz operation
Diamond Thickness (Modeled)	500	µm	Representative of commercial SCD plates
Experimental Sample Thickness	1	mm	HPHT diamond used for validation
Maximum Absorption Contrast (D3)	0.22	%	For 10 ppm NV density
Feasible Threshold Current Limit	< 300	mA	Constraint for thermal stability
External Cavity Length (L_r)	10	mm	Used for practical implementation modeling
Output Mirror Reflectivity (R₁)	0.9	-	Optimized for low threshold current

Key Methodologies

The proposed magnetometry scheme relies on precise control over the diamond material properties and integration into a specialized external cavity laser setup.

External Cavity Laser (ECL) Setup: A Fabry-Perot semiconductor laser diode/gain chip is coupled to an external cavity of length $L_{r}$ (10 mm), which contains the diamond plate of thickness $d$.
Absorption Modulation: Green pump light absorption by the NV- centers is modulated by applying resonant microwaves, causing population transfer between spin states.
Cavity Loss Integration: The change in diamond absorption ($\alpha_{d}$) is integrated into the total cavity loss ($\alpha_{t}$), which directly influences the lasing threshold current ($I_{th}$).
Laser Threshold Sensing (LTS): The laser is driven at a current equal to the off-resonance threshold ($I_{off}^{th}$). When microwaves are applied (on resonance), the reduced absorption shifts the threshold current ($I_{on}^{th}$), causing the laser to emit light (P_out).
Rate Equation Modeling: Standard rate equations for photon and carrier density are used to model the relationship between absorption contrast (C), differential gain factor ($\alpha$), confinement factor ($\Gamma$), and the resulting magnetic field sensitivity.
Noise Analysis: Sensitivity is calculated by considering the fundamental limits imposed by optical shot noise and laser drive current shot noise.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials necessary to realize the predicted sub-picotesla sensitivity in this cutting-edge quantum sensing application.

Applicable Materials for Replication and Extension

To achieve the required long dephasing times ($T_{2} > 1$ µs) and precise NV density (optimal 0.01 ppm), the research requires high-quality, isotopically pure Single Crystal Diamond (SCD).

Material Requirement	6CCVD Solution	Technical Advantage
High Purity / Long T₂	Isotopically Purified Optical Grade SCD	¹²C enrichment minimizes spin bath noise, enabling $T_{2}$ values > 1 µs, critical for high sensitivity.
Precise NV Density	Controlled Nitrogen Doping SCD	6CCVD offers precise nitrogen incorporation during MPCVD growth to hit the optimal $10^{4}$ ppb (0.01 ppm) NV concentration, maximizing absorption contrast while maintaining high $T_{2}$.
High Absorption (D3 Regime)	Standard Optical Grade SCD	For applications requiring high absorption contrast (e.g., Diamond D3, 10 ppm NV), 6CCVD supplies standard SCD with higher native nitrogen content.

Customization Potential for ECL Integration

The success of the LTS scheme relies on minimizing optical losses and ensuring robust integration into the external cavity. 6CCVD offers comprehensive customization services to meet these engineering demands:

Custom Dimensions and Thickness: The paper models a 500 µm thick plate. 6CCVD provides SCD plates with custom dimensions (up to 125 mm PCD) and precise thickness control from 0.1 µm up to 500 µm (and substrates up to 10 mm), ensuring optimal cavity length matching.
Ultra-Low Roughness Polishing: To minimize scattering losses ($\alpha_{c}$) within the external cavity, 6CCVD guarantees Ra < 1 nm polishing on SCD surfaces, significantly improving photon lifetime ($T_{p}$) and overall sensitivity.
Integrated Metalization Services: While not explicitly modeled here, practical implementation requires microwave antennas (for Rabi frequency $\Omega_{R}$ control) and thermal management. 6CCVD offers in-house deposition of standard metals (Au, Pt, Ti, W, Cu) for integrated antenna structures or heat sinks directly onto the diamond surface.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on material selection and optimization for advanced quantum sensing projects. We can assist researchers in defining the precise nitrogen concentration and isotopic purity required to balance the trade-offs between absorption contrast (C) and dephasing time ($T_{2}$) for similar Laser Threshold Magnetometry applications.

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

View Original Abstract

Nitrogen vacancy (NV) centers in diamond have attracted considerable recent\ninterest for use in quantum sensing, promising increased sensitivity for\napplications ranging from geophysics to biomedicine. Conventional sensing\nschemes involve monitoring the change in red fluorescence from the NV center\nunder green laser and microwave illumination. Due to the strong fluorescence\nbackground from emission in the NV triplet state and low relative contrast of\nany change in output, sensitivity is severely restricted by a high optical shot\nnoise level. Here, we propose a means to avoid this issue, by using the change\nin green pump absorption through the diamond as part of a semiconductor\nexternal cavity laser run close to lasing threshold. We show theoretical\nsensitivity to magnetic field on the pT/sqrt(Hz) level is possible using a\ndiamond with an optimal density of NV centers. We discuss the physical\nrequirements and limitations of the method, particularly the role of amplified\nspontaneous emission near threshold and explore realistic implementations using\ncurrent technology.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.103.062603