Electron-Induced State Conversion in Diamond NV Centers Measured with Pump–Probe Cathodoluminescence Spectroscopy
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2019-12-02 |
| Journal | ACS Photonics |
| Authors | Magdalena Solà-Garcia, Sophie Meuret, Toon Coenen, Albert Polman |
| Institutions | Institute for Atomic and Molecular Physics |
| Citations | 53 |
| Analysis | Full AI Review Included |
Technical Analysis and Documentation: Electron-Induced State Conversion in Diamond NV Centers
Section titled “Technical Analysis and Documentation: Electron-Induced State Conversion in Diamond NV Centers”Executive Summary
Section titled “Executive Summary”This research successfully demonstrates the use of ultrafast pump-probe Cathodoluminescence (CL) spectroscopy to analyze charge state dynamics in Nitrogen-Vacancy (NV) centers in single-crystal diamond (SCD). This technique is crucial for developing robust quantum optical systems.
- Novel Technique: First-time application of ultrafast pump-probe CL spectroscopy, using picosecond electron pulses (pump) and laser pulses (probe), enabling nanoscale characterization of NV dynamics.
- Core Finding: High-energy electron irradiation (5 keV) induces rapid and reversible conversion of the negatively charged NV center ($\text{NV}^{-}$) to the neutral state ($\text{NV}^{0}$).
- Material Requirement: The experiment utilized a high-purity, low-nitrogen SCD sample (NV concentration $\approx$ 1.2 ppb) to minimize background noise and maximize defect control.
- Dynamic Timescales: Key carrier dynamics were precisely measured: carrier diffusion lifetime ($\tau_{R}$) of $\approx$ 0.8 ns, $\text{NV}^{0}$ spontaneous emission decay ($\tau_{0}$) of $\approx$ 20 ns, and the critical $\text{NV}^{0} \rightarrow \text{NV}^{-}$ back transfer time ($\tau_{back}$) of $\approx$ 500 ms.
- Mechanism Insight: The results confirm that electron-induced carrier generation and recombination are the driving forces behind the $\text{NV}^{-} \rightarrow \text{NV}^{0}$ conversion, explaining why $\text{NV}^{-}$ emission is typically absent in CL measurements.
- Device Relevance: The ability to control NV charge states using electron beams provides a pathway for integrating NV centers into scalable, electrically driven quantum devices and sensors.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the experimental results and modeling presented in the paper:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Sample Material | Single-Crystal Diamond (SCD) | N/A | CVD grown, 300 µm thick |
| Nitrogen Concentration | <1 | ppm | Low impurity level |
| Boron Concentration | <0.05 | ppm | Low impurity level |
| Total NV Concentration | $\approx$ 1.2 | ppb | Equivalent to 200 $\mu$m⁻³ |
| Electron Beam Energy | 5 | keV | Primary excitation source (pump) |
| Electron Pulse Rate | 5.04 | MHz | Repetition rate for pump-probe |
| Electrons per Pulse (Max) | 400 | e⁻/pulse | Used for steady-state measurements |
| Conversion Saturation Point | $\approx$ 20 | e⁻/pulse | Saturation of $\text{NV}^{-} \rightarrow \text{NV}^{0}$ conversion |
| Laser Excitation Wavelength | 517 | nm | Second harmonic (probe) |
| Laser Pulse Energy | 0.9 | nJ/pulse | Used for PL measurements |
| Carrier Diffusion Lifetime ($\tau_{R}$) | 0.8 | ns | Derived from CL intensity fit |
| $\text{NV}^{0}$ Spontaneous Decay ($\tau_{0}$) | $\approx$ 20 | ns | Radiative decay time |
| $\text{NV}^{0} \rightarrow \text{NV}^{-}$ Back Transfer ($\tau_{back}$) | $\approx$ 500 | ms | Critical timescale for reversibility |
| Initial $\text{NV}^{-}$ Fraction | $\approx$ 0.4 | N/A | Before electron irradiation |
| Modeled CL Collection Depth | 23 | µm | Fit parameter for confocal geometry |
| Diamond Bandgap | 5.5 | eV | Energy required for electron-hole pair generation |
Key Methodologies
Section titled “Key Methodologies”The experiment relied on precise control over material properties and synchronized ultrafast excitation sources:
- Material Selection: A 300 µm thick, high-purity SCD sample (low N and B concentration) was used to ensure high-quality NV centers and minimize competing defects.
- Charge Dissipation Layer: The sample was coated with a thin charge dissipation layer (E-spacer 300) to prevent charging effects during high-flux electron excitation.
- Electron Pulse Generation (Pump): Picosecond electron pulses (0-400 e⁻/pulse) were generated by focusing the 4th harmonic ($\lambda$=258 nm) of an Yb-doped fiber fs-laser onto a laser-driven cathode (ZrO coated W tip) inside a Scanning Electron Microscope (SEM).
- Optical Pulse Generation (Probe): Synchronous laser pulses (2nd harmonic, $\lambda$=517 nm) were focused onto the sample, delayed by 1.3 ns relative to the electron pulse, to optically probe the NV charge state.
- Luminescence Collection: Cathodoluminescence (CL) and Photoluminescence (PL) signals were collected using an Al parabolic mirror and directed to a spectrometer or a Time-Correlated Single Photon Counting (TCSPC) module for spectral and temporal analysis.
- Dynamics Modeling: A three-dimensional rate equation model was developed, incorporating Monte Carlo simulations (Casino software) for initial carrier distribution, carrier diffusion ($\tau_{R}$), and the integrated effect of subsequent pulses, to accurately describe the $\text{NV}^{-} \rightarrow \text{NV}^{0}$ conversion and back transfer dynamics.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD provides the foundational MPCVD diamond materials and advanced processing required to replicate, optimize, and extend this critical research into NV center charge state control for quantum applications.
Applicable Materials
Section titled “Applicable Materials”The study requires high-purity, low-strain SCD with controlled nitrogen incorporation. 6CCVD specializes in delivering materials tailored for NV center research:
| 6CCVD Material Solution | Specification & Relevance to Research |
|---|---|
| Optical Grade Single Crystal Diamond (SCD) | SCD wafers with ultra-low nitrogen (<1 ppm) and boron (<0.05 ppm) background, essential for creating isolated, high-coherence NV centers. |
| Controlled Nitrogen Doping | We offer precise in-situ nitrogen doping during CVD growth to achieve specific NV concentrations (e.g., the 1.2 ppb used in this study) or higher densities for ensemble measurements. |
| High-Purity Substrates | SCD substrates available up to 10 mm thickness, providing robust thermal and mechanical stability for complex pump-probe setups and high-energy electron beam experiments. |
| Boron-Doped Diamond (BDD) | For experiments requiring Fermi level shifting via external voltage (as referenced in the paper), 6CCVD provides BDD films for integrated device architectures. |
Customization Potential
Section titled “Customization Potential”The success of ultrafast CL experiments depends heavily on precise material geometry and surface preparation. 6CCVD’s capabilities directly address these needs:
| Research Requirement | 6CCVD Customization Capability |
|---|---|
| Custom Thickness | The paper used a 300 µm thick sample. We offer SCD plates from 0.1 µm up to 500 µm, allowing researchers to optimize interaction volume and collection depth (e.g., the 23 µm modeled depth). |
| Surface Quality | NV center experiments require minimal surface defects. 6CCVD guarantees ultra-low roughness polishing: Ra < 1 nm for SCD, ensuring high optical collection efficiency and minimizing surface-induced decoherence. |
| Charge Dissipation & Device Integration | The paper used a temporary charge dissipation layer. 6CCVD offers in-house custom metalization (Au, Pt, Pd, Ti, W, Cu) for permanent, robust electrical contacts or conductive layers, crucial for applying external voltages to manipulate charge states in future device iterations. |
| Large Area Wafers | For scaling up quantum systems, we provide PCD wafers up to 125 mm in diameter, enabling high-throughput device fabrication. |
Engineering Support
Section titled “Engineering Support”Understanding complex carrier dynamics (0.8 ns diffusion, 500 ms back transfer) is vital for designing functional NV-based quantum sensors and emitters.
6CCVD’s in-house PhD team provides expert consultation on material selection and optimization for similar Ultrafast Carrier Dynamics and Charge State Control projects. We assist engineers in selecting the optimal nitrogen concentration, crystal orientation, and surface termination to maximize the yield of the desired NV charge state ($\text{NV}^{-}$ for quantum computing or $\text{NV}^{0}$ for specific sensing 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 are reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers, NV<sup>0</sup> and NV<sup>-</sup>, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, an understanding of the dynamics between the different states still remains challenging. Here, conversion from NV<sup>-</sup> to NV<sup>0</sup> due to electron-induced carrier generation is shown. Ultrafast pump-probe cathodoluminescence spectroscopy is presented for the first time, with electron pulses as pump and laser pulses as probe, to prepare and read out the NV states. The experimental data are explained with a model considering carrier dynamics (0.8 ns), NV<sup>0</sup> spontaneous emission (20 ns), and NV<sup>0</sup> → NV<sup>-</sup> back transfer (500 ms). The results provide new insights into the NV<sup>-</sup> → NV<sup>0</sup> conversion dynamics and into the use of pump-probe cathodoluminescence as a nanoscale NV characterization tool.