Integrated Magnetometry Platform with Stackable Waveguide-Assisted Detection Channels for Sensing Arrays
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-05-26 |
| Journal | Physical Review Applied |
| Authors | Michael Hoese, Michael K. Koch, Vibhav Bharadwaj, Johannes Lang, John P. Hadden |
| Institutions | Center for Biomolecular Nanotechnologies, The University of Tokyo |
| Citations | 24 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research presents a novel, integrated diamond magnetometry platform utilizing shallow-implanted Nitrogen-Vacancy (NV-) centers coupled to femtosecond-laser-written waveguides, designed for high-sensitivity sensing arrays.
- Integrated Architecture: NV- centers are shallow-implanted a few nanometers below the diamond surface, positioned directly in the mode field maximum of internal Type-II waveguides.
- Decoupled Operation: The architecture separates optical access (excitation and detection) via the waveguide from the sensing area (diamond surface), enabling sensing of light-fragile biological samples without direct illumination.
- High Integration Potential: The platform is scalable to two-dimensional sensing arrays, facilitating spatially and temporally correlated magnetometry in material and life sciences.
- Detection Efficiency: Despite transmission losses, the large waveguide mode area (105 ”m2) addresses a large NV ensemble, theoretically improving overall sensitivity by a factor of 41 compared to conventional confocal detection (due to the âN dependency).
- Sensing Performance: The device demonstrated magnetic field resolution better than 6 ”T and achieved a continuous wave (CW) ODMR sensitivity of 62 ”T Hz-1/2 via waveguide transmission.
- Temperature Sensing: The platform successfully measured temperature dependence of the zero-field splitting (ZFS) parameters D and E, operating reliably from room temperature (294 K) up to 324 K.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Substrate | 2 x 2 x 0.3 | mm | CVD grown, electronic grade, Type II |
| Nitrogen Impurity | less than 5 | ppb | Diamond substrate specification |
| Laser Wavelength | 515 | nm | Femtosecond laser writing |
| Laser Pulse Width | 300 | fs | Femtosecond laser writing |
| Laser Repetition Rate | 500 | kHz | Femtosecond laser writing |
| Laser Power | 100 | mW | Femtosecond laser writing |
| Waveguide Length | 2 | mm | Total length |
| Waveguide Depth | 5 to 25 | ”m | Below top diamond surface |
| Waveguide Width | 15 | ”m | Center-to-center transverse spacing |
| Ion Implantation Energy | 5 | keV | Nitrogen ions (N15+) |
| Ion Implantation Dose | 5 x 1011 | cm-2 | Shallow implantation on front facet |
| Annealing Temperature | 1000 | °C | UHV annealing for NV- formation (3h) |
| MW Gyromagnetic Ratio (Îł) | 28 | GHz T-1 | NV- center property |
| Confocal Spot Size | 0.062 | ”m2 | Conventional detection reference |
| Waveguide Mode Area (1/e2) | 105 | ”m2 | Sensing area addressed by waveguide |
| Absolute Detection Efficiency (WG) | 0.05 | % | Averaged over 1/e2 area |
| NV-WG Coupling Efficiency | 0.3 | % | Estimated coupling efficiency |
| Waveguide Transmission Loss | 6.95 | dB | Outcoupling loss (79.8%) |
| Magnetic Field Resolution | better than 6 | ”T | Resolved difference between two measurements |
| CW-ODMR Sensitivity (Confocal) | 36 | ”T Hz-1/2 | Measured sensitivity |
| CW-ODMR Sensitivity (Waveguide) | 62 | ”T Hz-1/2 | Measured sensitivity via transmission |
| ODMR FWHM Linewidth (ÎÎœ) | 7.5 | MHz | Measured ODMR dip width |
| ZFS Gradient (dD/dT) | -40 ± 18 | kHz K-1 | Axial ZFS parameter temperature dependence |
| ZFS Gradient (dE/dT) | -8 ± 15 | kHz K-1 | Transverse ZFS parameter temperature dependence |
Key Methodologies
Section titled âKey MethodologiesâThe device fabrication involves three primary steps: femtosecond laser writing of waveguides, shallow ion implantation, and high-temperature annealing.
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Waveguide Fabrication (Femtosecond Laser Writing):
- A Yb:KGW Fiber Laser (515 nm, 300 fs pulse width, 500 kHz repetition rate) was used.
- The laser beam was focused into the Type II diamond slab using a high-NA objective (1.25 NA).
- Type-II waveguides (two nearby lines of reduced refractive index) were written at depths ranging from 5 ”m to 25 ”m below the surface, creating a stressed region that acts as the waveguide core.
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NV- Center Creation (Shallow Ion Implantation):
- Nitrogen ions (N15+) were implanted into the front facet of the diamond substrate using a low-energy ion implanter.
- Implantation parameters were set to 5 keV energy and a dose of 5 x 1011 cm-2, targeting a shallow depth (a few nanometers) to maximize coupling to the surface sensing environment.
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Defect Activation (Annealing):
- The implanted substrate was annealed in an ultra-high vacuum (UHV) environment.
- The annealing sequence included an intermediate soak for 1 hour at 500 °C.
- The final annealing step was performed at 1000 °C for 3 hours to mobilize vacancies and form stable NV- centers.
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Optical Detection (ODMR):
- The NV- ensemble was excited off-resonantly (532 nm laser) coupled from the back side into the waveguide.
- The NV- signal was read out through the waveguide (transmission) using a low-NA objective (0.25 NA) to keep the front sensing facet accessible.
- CW-ODMR measurements were performed by applying a microwave (MW) field via a 20 ”m diameter wire placed close to the NV ensemble.
Commercial Applications
Section titled âCommercial ApplicationsâThis integrated diamond photonics platform is highly relevant for advanced quantum sensing and integrated device manufacturing.
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Biomagnetometry and Life Sciences:
- Sensing magnetic fields and temperature in biological systems (e.g., cells, tissues) where light exposure must be minimized (decoupled optical access is critical).
- Characterization of paramagnetic labels and biomolecules that are photo-fragile.
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Material Science and Condensed Matter Physics:
- Spatially and temporally correlated magnetometry, enabling imaging of magnetic field distributions and dynamics (e.g., probing magnetism in 2D materials or antiferromagnetic order).
- Studying superconductivity and other phenomena requiring cryogenic temperatures (the device is compatible with a large temperature range).
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Integrated Quantum Sensors:
- Development of compact, robust, and scalable two-dimensional sensor arrays (large filling factors up to 0.2) for on-chip quantum sensing.
- Integration with microfluidic channels for on-chip quantum sensing of liquids.
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Advanced Photonics and Optoelectronics:
- Utilization of femtosecond laser writing for 3D integrated photonics in diamond, a host material offering superior mechanical and thermal properties.
- High-efficiency interfacing of solid-state quantum emitters (NV- centers) with photonic integrated circuits.
View Original Abstract
The negatively charged nitrogen vacancy (N-Vâ) center in diamond has shown great success in nanoscale, high-sensitivity magnetometry. Efficient fluorescence detection is crucial for improving the sensitivity. Furthermore, integrated devices enable practicable sensors. Here, we present an integrated architecture which allows us to create N-Vâ centers a few nanometers below the diamond surface, and at the same time covering the entire mode field of femtosecond-laser-written type-II waveguides. We experimentally verify the coupling efficiency, showcase the detection of magnetic resonance signals through the waveguides and perform proof-of-principle experiments in magnetic field and temperature sensing. The sensing task can be operated via the waveguide without direct light illumination through the sample, which is important for magnetometry in biological systems that are sensitive to light. In the future, our approach will enable the development of two-dimensional sensing arrays facilitating spatially and temporally correlated magnetometry.