Laser threshold magnetometry using green-light absorption by diamond nitrogen vacancies in an external cavity laser
At a Glance
Section titled âAt a Glanceâ| 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 BergâSĂžrensen |
| Institutions | Technical University of Denmark, Leipzig University |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research proposes and models a novel approachâLaser Threshold Sensing (LTS)âfor highly sensitive magnetic field detection using Nitrogen Vacancy (NV) centers in diamond, aiming to overcome the limitations of conventional fluorescence-based methods.
- Core Value Proposition: LTS eliminates the bright red fluorescence background noise that severely limits the sensitivity of standard NV magnetometry, enabling operation in a zero-background regime.
- Methodology: The NV-doped diamond is integrated into a semiconductor External Cavity Laser (ECL). Magnetic field changes modulate the green pump light absorption, which, in turn, shifts the laserâs threshold current (Ith).
- Predicted Sensitivity: Theoretical modeling predicts magnetic field sensitivity reaching the sub-picotesla range (as low as 0.02 pT/âHz) under optimal conditions (high T2* and high NV density).
- Optimal Material Requirements: Achieving high sensitivity requires high NV- density (approximately 10 ppm) and long ensemble dephasing times (T2* greater than 1 ”s).
- Feasibility Constraints: Practical operation is constrained by the maximum feasible drive current (Ith less than 300 mA) and the need for the change in Ith to exceed the drive current shot noise.
- Critical Limiting Factor: Amplified Spontaneous Emission (ASE), characterized by the spontaneous emission factor (ÎČ), severely degrades sensitivity by blurring the sharp lasing transition, necessitating specialized low-ÎČ gain chips.
- Implementation: The scheme is highly suitable for miniaturization, utilizing a standard current-driven laser diode or gain chip (e.g., InGaN for green light).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Predicted Best Sensitivity | 0.02 | pT/âHz | Optical shot noise limited (T2* = 10 ”s, NNV = 104 ppb) |
| Feasible Sensitivity (D3) | 50 | pT/âHz | Limited by drive current shot noise |
| Diamond Thickness (Modeled) | 500 | ”m | Representative of single crystal plates |
| Optimal NV Density | 104 | ppb (10 ppm) | Maximum sensitivity achieved before excessive absorption limits output |
| Required T2* for Sub-pT | > 1 | ”s | Necessary for high sensitivity operation |
| ODMR Linewidth (D3) | 3.2 | MHz | Calculated from T2* = 0.1 ”s |
| Maximum Absorption Contrast (D3) | 0.22 | % | Maximum change in green absorption (on/off resonance) |
| External Cavity Length (Lr) | 10 | mm | Sufficient for diamond and necessary optics |
| Output Mirror Reflectivity (R1) | 0.9 | - | Optimized for reasonable threshold current |
| Threshold Current (Feasibility Limit) | 300 | mA | Limit imposed for thermal stability/practical heatsinking |
| Transparency Carrier Density (Ntr) | 1 x 1025 | m-3 | Typical for InGaN laser structures |
| Carrier Lifetime (ÏN) | 4 | ns | Typical III-V semiconductor parameter |
| Differential Gain Factor (a) | 5 x 10-20 | m2 | Typical value used for threshold current calculation |
| Confinement Factor (Î) | 0.02 | - | Typical for thin active layer laser diodes |
Key Methodologies
Section titled âKey MethodologiesâThe work is based on theoretical modeling using coupled rate equations for the semiconductor laser and the NV defect system.
-
External Cavity Laser (ECL) Modeling:
- The physical setup (Fabry-Perot laser diode, external cavity, diamond) is simplified using a three-mirror model, treating the entire structure as a single cavity with an effective reflectivity (Re).
- The total cavity loss (at) includes intrinsic gain medium loss (ac), mirror losses (am), and diamond absorption loss (ad).
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Semiconductor Rate Equations:
- Standard rate equations for photon density (S) and carrier density (N) are solved to determine the steady-state lasing threshold current (Ith).
- Ith is defined by material parameters (Ntr, ÏN, a, Î) and cavity parameters (at, L).
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NV Absorption Rate Model:
- A detailed rate model for the NV energy levels (triplet ground state 3A2, excited state 3E, singlet states) is used to calculate the green absorption coefficient (α) as a function of microwave resonance (ODMR).
- The absorption contrast (C) is calculated as the relative change in light intensity absorbed by the diamond between on- and off-microwave resonance states.
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Simulated ODMR Spectrum:
- The change in absorption (C) is translated into a change in the threshold current (ÎIth).
- The ODMR spectrum is simulated as a peak in the external cavity laser output power (Pout) versus microwave frequency, using a Lorentzian lineshape defined by the ensemble dephasing time (T2*).
-
Sensitivity Calculation and Optimization:
- Magnetic field sensitivity is calculated by dividing the total background noise level by the maximum ODMR slope.
- Primary noise sources considered are optical shot noise (from Pout) and drive current shot noise (Ish).
- Optimization (using gradient descent) was performed on key parameters (R1, Î, T2*, a, NNV) to maximize sensitivity while adhering to practical limits (Ith less than 300 mA).
Commercial Applications
Section titled âCommercial ApplicationsâThe Laser Threshold Sensing (LTS) technique, particularly when combined with diamond NV centers, offers significant advantages for high-performance, miniaturized sensing systems.
- Quantum Magnetometry:
- Development of highly sensitive, compact magnetometers for applications requiring picotesla-level detection (e.g., geological surveys, non-invasive medical diagnostics like magnetoencephalography).
- Miniaturized Sensor Systems:
- The use of a semiconductor gain chip in an external cavity configuration is highly compatible with integration and miniaturization, suitable for chip-scale quantum sensors.
- General Quantum Sensing Platform:
- LTS is broadly applicable to any material defect (not just NV centers) where a controllable difference in optical absorption can be generated, including defects in Silicon Carbide (SiC) and 2D materials.
- Temperature and Strain Sensing:
- Since NV center absorption is also sensitive to temperature and strain, the LTS method provides a zero-background readout scheme for these parameters, potentially improving sensor resolution in harsh environments.
- Semiconductor Engineering:
- The requirement for specialized low spontaneous emission factor (ÎČ) gain chips drives development in quantum well structures with flatter logarithmic gain-carrier density relations, beneficial for high-precision laser control.
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