⚛️ quantum physics

Observation of Shear Strain in Ion-Implanted Diamond Substrate and Diamond Nanophotonic Structures

This paper demonstrates that zero-field continuous-wave optically detected magnetic resonance (CW-ODMR) spectroscopy can detect shear strain induced by ion implantation and nanofabrication processes in diamond substrates and nanophotonic structures, evidenced by asymmetric splitting in the spectra of nitrogen-vacancy centers.

Original authors: Ayan Majumder, Vivek K Shukla, Anuj Bathla, Brajesh S. Yadav, Nanhey Singh, Padmnabh Rai, Kasturi Saha

Published 2026-04-07

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CC BY 4.0

Original authors: Ayan Majumder, Vivek K Shukla, Anuj Bathla, Brajesh S. Yadav, Nanhey Singh, Padmnabh Rai, Kasturi Saha

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Diamonds as Quantum Super-Computers

Imagine a diamond not just as a shiny jewel, but as a tiny, ultra-precise laboratory inside a crystal. Inside this laboratory, there are microscopic "workers" called Nitrogen-Vacancy (NV) centers. Think of these NV centers as atomic-scale spies. They are incredibly sensitive; they can feel the tiniest changes in magnetic fields, temperature, or even how much the diamond crystal is being squeezed or stretched (strain).

Scientists want to use these spies to build future technologies like quantum computers and super-sensitive medical sensors. To do this, they need to:

Place the spies exactly where they want them (using ion implantation).
Build tiny houses for them (nanophotonic structures like pillars) to help them talk to the outside world more efficiently.

The Problem: The Construction Crew Leaves a Mess

The paper investigates what happens to these "spies" when scientists try to build their new homes.

Ion Implantation: This is like firing tiny bullets (nitrogen ions) into the diamond to create the NV centers. It's necessary, but it's also a bit violent. It knocks atoms out of place, creating a "mess" or strain in the crystal structure.
Nanofabrication: This is like carving the diamond into tiny pillars using lasers and chemical etching. This process also scrapes and damages the surface, adding more "stress" to the crystal.

The Analogy: Imagine you are trying to set up a delicate musical instrument (the NV center) inside a glass house (the diamond). To get the instrument in, you have to smash a hole in the glass (ion implantation). Then, to make the house look nice, you carve the glass into a fancy pillar (nanofabrication). Both steps leave the glass stressed and warped.

The Discovery: Listening to the "Twang"

The scientists wanted to know: How much is the diamond warped, and does it ruin the spies?

They used a technique called ODMR (Optically Detected Magnetic Resonance).

The Metaphor: Imagine the NV center is a guitar string. In a perfect, unstressed diamond, if you pluck it, it makes a perfect, symmetrical sound (a single note or a perfectly balanced double note).
The Twist: When the diamond is strained (warped) from the construction work, the "guitar string" gets twisted. When you pluck it, the sound changes. It doesn't just get louder or quieter; the note splits unevenly. One side of the note gets higher, the other gets lower, and they aren't balanced.

The Key Finding: The researchers found that both the ion implantation and the pillar carving caused a specific type of warping called shear strain. This showed up in their data as an asymmetric split in the sound of the NV centers. It's like hearing a guitar string that has been twisted sideways—it sounds "off" in a very specific way.

Why This Matters

You might think, "So the diamond is a little damaged. Who cares?"

Here is why it's a big deal:

The "Stress Test": This paper shows that we can use the NV centers themselves as stress sensors. By listening to how their "sound" splits, we can measure exactly how much damage the manufacturing process caused. It's like using a tiny seismograph inside the diamond to feel the tremors of the construction crew.
Building Better Quantum Devices: If we want to build a quantum computer, the "spies" need to be perfectly stable. If the diamond is too warped (too much shear strain), the spies might get confused or lose their memory (coherence). By understanding this strain, scientists can tweak their manufacturing process to make the diamond houses straighter and the spies happier.
Efficiency: The paper also showed that building those tiny pillars actually helps the spies shine brighter (collect more light). Even though the pillars add some strain, the benefit of having a brighter signal outweighs the cost, provided we manage the strain correctly.

The Takeaway

Think of this research as a quality control report for the future of quantum technology.

The Goal: Build tiny, perfect diamond structures to house quantum spies.
The Reality: The tools we use to build them (firing ions and carving pillars) leave the diamond a little twisted.
The Solution: We found a way to "listen" to the diamond (using ODMR) to hear exactly how twisted it is. This "asymmetric split" is the fingerprint of the damage.

By understanding this "twist," scientists can now build better, more reliable quantum devices, ensuring that the future of quantum communication and sensing is built on a solid, well-understood foundation.

1. Problem Statement

Negatively charged nitrogen-vacancy (NV) centers in diamond are premier platforms for quantum communication, information processing, and nanoscale sensing due to their long spin coherence times and optical addressability. However, realizing scalable quantum devices requires two critical steps:

Deterministic Placement: Precisely positioning individual NV centers into nanophotonic structures (e.g., cavities, waveguides) using ion implantation.
Nanofabrication: Etching diamond to create these structures to enhance light-matter interaction and photon collection efficiency.

The Core Issue: Both ion implantation and subsequent nanofabrication (etching) introduce lattice damage and crystal strain. This strain alters the electronic spin levels of the NV centers, potentially degrading qubit performance (coherence, fidelity). While axial strain is well-characterized, the specific impact of shear strain induced by these fabrication processes on the zero-field splitting of NV centers requires further investigation and quantification.

2. Methodology

The authors investigated two distinct sample types to isolate strain effects from different fabrication stages:

Sample 1: Ion-Implanted Bulk Diamond
- Material: Chemical Vapor Deposition (CVD) grown single-crystal diamond (Type IIa).
- Process: Nitrogen ion implantation ( $^{14}\text{N}^+$ , 130 keV) at three different doses ( $1\times10^{13}$ , $1\times10^{14}$ , and $2\times10^{16}$ ions/cm $^2$ ).
- Post-Processing: Annealing at 1000°C to repair lattice damage and activate NV centers.
- Simulation: SRIM (Stopping and Range of Ions in Matter) simulations were used to model ion trajectories and vacancy distributions.
Sample 2: Diamond Nanopillars
- Material: CVD diamond with low intrinsic NV concentration.
- Process: Fabrication of truncated conical nanopillars using Electron Beam Lithography (EBL), Titanium masking, and Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE).
- Simulation: 3D Finite-Difference Time-Domain (3D-FDTD) simulations (Ansys Lumerical) were performed to optimize nanopillar dimensions (height ~576 nm, top/bottom diameters ~146/304 nm) for maximum photon collection efficiency.
Characterization Techniques:
- Raman Spectroscopy: To assess lattice damage, graphitization thresholds, and defect density.
- Zero-Field Continuous-Wave Optically Detected Magnetic Resonance (CW-ODMR): The primary tool for probing local crystal strain. The system utilized a home-built laser-scanning confocal microscope integrated with microwave excitation.
- UV-Vis Absorption: To quantify substitutional nitrogen concentration.

3. Key Contributions

Identification of Shear Strain Signatures: The paper demonstrates that asymmetric splitting in the zero-field CW-ODMR spectrum is a direct signature of in-plane shear strain ( $\epsilon_{xy}$ ) induced by both ion implantation and nanofabrication.
Quantification of Strain: The authors successfully quantified the transverse strain components in both bulk implanted samples and nanostructured pillars, distinguishing them from axial strain effects.
Validation of Nanofabrication Impact: The study confirms that the ICP-RIE etching process, essential for creating photonic structures, introduces significant shear strain that manifests in the spin properties of the NV centers.
Theoretical Correlation: The experimental observations were correlated with a theoretical model (based on M. Sahnawaz Alam et al.) describing how shear strain mixes the $|m_s = \pm 1\rangle$ spin states, leading to asymmetric ODMR peaks.

4. Key Results

SRIM Simulations: Confirmed that nitrogen ions create a Gaussian distribution of vacancies peaking at a depth of ~150 nm. High-dose implantation ( $2\times10^{16}$ ions/cm $^2$ ) caused significant lattice damage (amorphization), evidenced by weakened Raman peaks and the appearance of a graphite G-peak.
Photon Collection Enhancement: 3D-FDTD simulations and experimental PL measurements showed that diamond nanopillars significantly enhance photon collection. The PL signal from nanopillars was 5 times higher than the background, consistent with simulation predictions.
ODMR Spectroscopy Findings:
- Bulk Samples: Nitrogen-ion-implanted samples exhibited asymmetric splitting in their zero-field ODMR spectra.
- Nanopillar Samples: Diamond nanopillars also displayed asymmetric splitting, confirming that the etching process introduces shear strain.
- Quantitative Strain Values:
  - Transverse Strain: Estimated at 1.9875 MHz for the ion-implanted sample and 1.8835 MHz for the nanopillar sample.
  - Spectral Imbalance: Measured at 0.058 (implanted) and 0.068 (nanopillar), indicating the degree of asymmetry caused by the shear strain component ( $\epsilon_{xy}$ ).
- Strain Tensor Analysis: The absence of a center frequency shift indicated that $\epsilon_{xx} \approx 0$ , while the asymmetry confirmed non-zero $\epsilon_{xy}$ (shear) and $\epsilon_{yy}$ components.

5. Significance

Qubit Performance Optimization: Understanding and quantifying shear strain is critical for calibrating quantum gates and maintaining qubit coherence in diamond-based quantum networks. Strain-induced splitting can lead to frequency shifts that complicate spin control.
Fabrication Guidelines: The results provide a feedback loop for nanofabrication processes. Engineers can now use zero-field ODMR asymmetry as a sensitive, non-destructive probe to monitor and minimize shear strain during the etching of diamond photonic circuits.
Scalability: By characterizing the strain introduced by standard fabrication techniques (implantation and etching), this work supports the development of scalable, high-fidelity quantum devices where deterministic placement of emitters is required without compromising their quantum properties.
Diagnostic Tool: The study reinforces zero-field CW-ODMR as a powerful, atomic-scale sensor for mapping local crystal strain, offering insights beyond what is possible with standard structural characterization techniques.