Low-Charge-Noise Nitrogen-Vacancy Centers in Diamond Created Using Laser Writing with a Solid-Immersion Lens
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-06-03 |
| Journal | ACS Photonics |
| Authors | Viktoria Yurgens, Josh A. Zuber, Sigurd FlÄgan, Marta De Luca, Brendan Shields |
| Institutions | University of Basel, University of Warsaw |
| Citations | 42 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details a novel, low-damage method for creating high-quality Nitrogen-Vacancy (NV) centers in bulk diamond using femtosecond laser writing facilitated by a Solid-Immersion Lens (t-SIL).
- Low-Energy NV Creation: The use of a truncated SIL (t-SIL) significantly reduced the required laser pulse energy to 5.8 nJ, enabling vacancy creation via tunneling breakdown (Keldysh parameter γ « 1) rather than high-damage multi-photon ionization.
- Record-Low Charge Noise: The resulting NV centers exhibit exceptionally low charge noise, demonstrated by narrow Zero-Phonon Line (ZPL) linewidths. Mean linewidths ranged from 62.1 MHz to 74.5 MHz across three samples.
- Full Inhomogeneous Broadening Measured: These linewidths include the full effect of long-term spectral diffusion caused by the 532 nm charge-state repump laser, confirming the low-noise environment under operational conditions.
- High Yield: The method achieved a high probability (up to 92.3%) of NV centers having ZPL linewidths below the critical 100 MHz threshold required for high-fidelity quantum interference applications.
- Surface Proximity: The t-SIL geometry minimizes spherical aberration, allowing NV centers to be reliably created close to the diamond surface (as shallow as ~2 ”m) without inducing surface graphitization.
- Methodological Advancement: A model was successfully applied to disentangle power broadening from inhomogeneous broadening (Îin) in the ZPL linewidth measurements.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Material | Bulk electronic-grade | - | 40 ”m thick, [N] < 5 ppb (Element Six) |
| Laser Wavelength (Writing) | 800 | nm | Femtosecond pulsed laser |
| Laser Pulse Duration | ~35 | fs | Writing source |
| Minimum Pulse Energy (Writing) | 5.8 | nJ/pulse | Threshold for visible GR1 PL |
| NV Creation Regime | Tunneling breakdown | - | Keldysh parameter γ « 1 |
| Annealing Temperature | 1100 | °C | In vacuum, 3 hours duration |
| Solid-Immersion Lens (t-SIL) | Cubic zirconia (n=2.14) | - | Truncated hemispherical |
| Objective Numerical Aperture (NA) | 0.85 to 0.9 | - | Standard air objective |
| Effective NA (with t-SIL) | ~1.8 | - | Resultant NA for focusing |
| Shallowest NV Depth | 2.0 | ”m | Below diamond surface (Sample A) |
| Mean ZPL Linewidth (Lowest) | 62.1 | MHz | Sample A and B (measured with repump) |
| Mean ZPL Linewidth (Highest) | 74.5 | MHz | Sample C (measured with repump) |
| Linewidth Yield (< 100 MHz) | 90.9 to 92.3 | % | Probability based on ECDF (Samples A & B) |
| Homogeneous Linewidth Limit (Bulk) | ~13 | MHz | Lifetime limit (Îł) |
| Charge Repump Laser | 532 | nm | Nd:YAG laser (0.6 mW average power) |
| Effective Coupling Strength (c) | 1.0 ± 0.1 x 105 | MHz2/”W | Derived from power broadening fit |
Key Methodologies
Section titled âKey MethodologiesâThe NV centers were created using a controlled femtosecond laser writing process followed by high-temperature annealing:
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Optical Setup and SIL Integration:
- A truncated hemispherical cubic zirconia Solid-Immersion Lens (t-SIL, n=2.14) was optically coupled to the bulk diamond sample (40 ”m thick).
- A standard air objective (NA=0.85 to 0.9) was used to focus the writing laser through the t-SIL, achieving a high effective NA (~1.8) and minimizing spherical aberration.
-
Laser Writing Parameters:
- A femtosecond ytterbium-doped fiber laser (800 nm, ~35 fs pulse duration) was used for irradiation.
- Pulse energies were tuned between 3.8 nJ and 35.8 nJ, with vacancy creation observed starting at 5.8 nJ/pulse.
- The low energy threshold ensures operation in the tunneling breakdown regime (γ « 1), which minimizes extended lattice damage.
-
Vacancy Placement:
- Arrays of vacancies were written at depths ranging from 2.0 ”m to 40 ”m below the top surface.
- An inverse geometry was used for Sample C to create NV centers as close as 1 ”m from the bottom surface without inducing graphitization.
-
Annealing:
- Samples were annealed in vacuum at 1100 °C for three hours to mobilize the laser-created vacancies, allowing them to combine with native nitrogen impurities to form NV centers.
-
Optical Characterization (PLE):
- Photoluminescence Excitation (PLE) spectroscopy was performed in a liquid helium bath cryostat to measure the ZPL linewidths.
- A 532 nm Nd:YAG laser (0.6 mW average power) was pulsed (100 kHz frequency) for charge-state reinitialization (repump) during the PLE sequence to ensure the full effect of spectral diffusion was included in the measurement.
-
Linewidth Analysis:
- Linewidths were measured as a function of resonant excitation power (P).
- The data was fitted using a convolution model (Lorentzian spectral diffusion L with P22 occupation probability) to separately extract the inhomogeneous broadening (Îin) and the power broadening contribution (Rabi coupling Ω).
Commercial Applications
Section titled âCommercial ApplicationsâThe creation of NV centers with exceptionally low charge noise and high optical coherence is critical for advancing solid-state quantum technologies.
- Quantum Communication and Networks:
- High-Fidelity Entanglement: The narrow, stable linewidths (up to 92.3% < 100 MHz) are essential for achieving high-rate two-photon quantum interference, a prerequisite for long-distance spin-spin entanglement between remote NV qubits.
- Integrated Quantum Photonics:
- Diamond Microcavities: This low-damage laser writing technique is ideal for integrating NV centers into structured diamond devices (e.g., Fabry-Perot microcavities) where traditional etching methods often degrade optical quality and increase charge noise.
- Quantum Sensing (Low-Noise Probes):
- Nanoscale Magnetometry: NV centers created close to the surface (tens of nanometers) with minimal surrounding lattice damage offer superior spin and optical coherence, leading to higher sensitivity and stability for nanoscale magnetic and electric field sensing applications.
- Solid-State Qubits:
- Charge-State Stable Emitters: The method yields color centers characterized by a stable charge state and a low charge-noise environment, making them highly reliable candidates for robust solid-state quantum bits (qubits).
View Original Abstract
We report on pulsed-laser-induced generation of nitrogen-vacancy (NV) centers in diamond facilitated by a solid-immersion lens (SIL). The SIL enables laser writing at energies as low as 5.8 nJ per pulse and allows vacancies to be formed close to a diamond surface without inducing surface graphitization. We operate in the previously unexplored regime, where lattice vacancies are created following tunneling breakdown rather than multiphoton ionization. We present three samples in which NV center arrays were laser-written at distances between similar to 1 and 40 mu m from a diamond surface, all presenting narrow distributions of optical linewidths with means between 62.1 and 74.5 MHz. The linewidths include the effect of long-term spectral diffusion induced by a 532 nm repump laser for charge-state stabilization, thereby emphasizing the particularly low-charge-noise environment of the created color centers. Such high-quality NV centers are excellent candidates for practical applications employing two-photon quantum interference with separate NV centers. Finally, we propose a model for disentangling power broadening from inhomogeneous broadening in the NV center optical linewidth.