Parabolic Diamond Scanning Probes for Single-Spin Magnetic Field Imaging
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-12-02 |
| Journal | Physical Review Applied |
| Authors | Natascha Hedrich, Dominik Rohner, Marietta Batzer, Patrick Maletinsky, Brendan J. Shields |
| Institutions | University of Basel |
| Citations | 40 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the engineering and fabrication of high-performance diamond scanning probes utilizing a truncated parabolic geometry to optimize single Nitrogen-Vacancy (NV) center photon collection for nanoscale magnetometry.
- Performance Breakthrough: Achieved a median saturated photoluminescence (PL) count rate of 2.1 ± 0.2 MHz, representing the highest reported flux for single NVs in scanning probes to date and a 5-fold improvement over state-of-the-art cylindrical designs.
- Enhanced Efficiency: The parabolic reflector geometry yields a high device collection efficiency (ηdev) of 0.57, effectively collimating NV emission into a narrow numerical aperture (NAeff = 0.46).
- Scalable Fabrication: A robust, two-stage ICP-RIE dry etching process was developed, utilizing a flowable oxide (FOX-16) mask with controlled erosion to reliably produce the required parabolic curvature.
- Nanoscale Resolution: The truncated tip design minimizes the NV-sample separation to an effective distance of 40 ± 5 nm, enabling magnetic imaging with a spatial resolution better than 50 nm.
- Broadband Operation: The devices operate efficiently across the full NV photoluminescence spectrum (630 nm to 800 nm).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Median Saturated PL Rate (IPL) | 2.1 ± 0.2 | MHz | Highest reported for single NVs in scanning probes. |
| Median Saturation Power (Psat) | 27 | ”W | CW 532 nm excitation power. |
| Device Collection Efficiency (ηdev) | 0.57 | Dimensionless | Efficiency of the parabolic structure alone. |
| Overall Detection Efficiency (η) | 0.12 | Dimensionless | Including setup losses (ηsetup = 0.21). |
| Effective NV-Sample Separation | 40 ± 5 | nm | Measured during CoFeB stripe imaging (NV to Ta surface). |
| Spatial Resolution | < 50 | nm | Demonstrated imaging resolution. |
| Median NV Excited State Lifetime (ms= | 0>) | 22 | ns |
| Median Steady-State NV Population | 0.79 | Dimensionless | Measured at saturation power (Psat). |
| Parabolic Tip End Facet Diameter | ~200 | nm | Minimal diameter supporting strong optical mode confinement. |
| Emission Numerical Aperture (NAeff) | 0.46 | Dimensionless | Effective NA referenced to the objective center. |
| Diamond Material | Type-IIa | Dimensionless | High-purity host material. |
| Diamond Refractive Index (n) | 2.4 | Dimensionless | Host material property. |
Key Methodologies
Section titled âKey MethodologiesâThe parabolic diamond scanning probes are fabricated using a scalable, two-stage Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) process with a flowable oxide mask (FOX-16) for precise curvature control.
- Diamond Preparation:
- High-purity Type-IIa diamond is pre-patterned with cantilever structures etched to a depth of 2 ”m.
- NV centers are created via ion implantation.
- Mask Patterning:
- A ~300 nm thick layer of FOX-16 (Flowable Oxide) is applied.
- 1 ”m diameter discs are patterned onto the cantilevers using electron beam lithography (EBL).
- Stage 1 Etch (Tapered Waveguide):
- Goal: Etch a ~6 ”m tapered pillar waveguide section.
- Chemistry: O2 etch (50 sccm O2 flow) alternating with short O2/CF4 steps (4 s) to clean resputtered material.
- Parameters: Pressure: 0.5 Pa; ICP Power: 500 W; Bias Voltage: 110 V; Duration: 240 s steps (repeated 9 times).
- Result: The mask is eroded at the edges, resulting in a trapezoidal cross-section (base diameter ~900 nm).
- Stage 2 Etch (Parabolic Tip Formation):
- Goal: Controlled mask erosion to achieve parabolic curvature.
- Chemistry: CF4 is introduced at increasing flow rates (2 sccm to 10 sccm) in successive steps.
- Parameters: O2 Flow: 50 sccm; ICP Power: 500 W; Bias Voltage: 40 V; Pressure: 0.5 Pa.
- Mechanism: Varying the CF4 concentration controls the relative etch rate between the FOX mask and the diamond, precisely tuning the angle of the pillar walls to form the parabolic profile.
- Device Release: A deep etch is performed from the back side of the diamond to release the cantilevers for assembly into scanning probes.
Commercial Applications
Section titled âCommercial ApplicationsâThe high-sensitivity, high-resolution NV scanning probes developed here are critical enabling technology for advanced quantum and nanoscale applications.
- Quantum Sensing and Metrology:
- Quantitative imaging of magnetic phenomena in condensed matter systems (e.g., skyrmions, antiferromagnets, 2D magnetic materials).
- Detection of weak magnetic signals from nuclear spins in single proteins or 2D materials, requiring high sensitivity.
- Nanoscale Materials Characterization:
- Scanning probe sensing of electric fields, temperature, and strain at the nanoscale.
- Imaging electron transport properties in novel materials like graphene.
- Quantum Information Technology:
- The high collection efficiency and narrow emission NA are foundational for integrating NV centers into scalable photonic circuits and quantum memories.
- Advanced Scanning Probe Microscopy (SPM):
- Integration into Atomic Force Microscopy (AFM) systems to provide quantitative magnetic field maps alongside topography, enhancing materials analysis capabilities.
- Cryogenic Applications: The low saturation power (27 ”W) is advantageous for use in cryogenic environments, minimizing laser-induced heating.
View Original Abstract
Enhancing the measurement signal from solid state quantum sensors such as the\nnitrogen-vacancy (NV) center in diamond is an important problem for sensing and\nimaging of condensed matter systems. Here we engineer diamond scanning probes\nwith a truncated parabolic profile that optimizes the photonic signal from\nsingle embedded NV centers, forming a high-sensitivity probe for nanoscale\nmagnetic field imaging. We develop a scalable fabrication procedure based on\ndry etching with a flowable oxide mask to reliably produce a controlled tip\ncurvature. The resulting parabolic tip shape yields a median saturation count\nrate of 2.1 $\pm$ 0.2 MHz, the highest reported for single NVs in scanning\nprobes to date. Furthermore, the structures operate across the full NV\nphotoluminescence spectrum, emitting into a numerical aperture of 0.46 and the\nend-facet of the truncated tip, located near the focus of the parabola, allows\nfor small NV-sample spacings and nanoscale imaging. We demonstrate the\nexcellent properties of these diamond scanning probes by imaging ferromagnetic\nstripes with a spatial resolution better than 50 nm. Our results mark a 5-fold\nimprovement in measurement signal over the state-of-the art in scanning-probe\nbased NV sensors.\n