Fabrication of oriented NV center arrays in diamond via femtosecond laser writing and reorientation

At a Glance

Metadata	Details
Publication Date	2025-09-23
Journal	Frontiers in Quantum Science and Technology
Authors	Kai Klink, Andrew Kirkpatrick, Yukihiro Tadokoro, Jonas N. Becker, Shannon Singer Nicley
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel, all-optical method for the deterministic fabrication and orientation control of nitrogen-vacancy (NV) center arrays in diamond, addressing a critical limitation in solid-state quantum technology.

Core Achievement: Successful fabrication and reorientation of NV centers along a specific crystallographic axis using a two-step femtosecond laser process: writing (creation) followed by annealing (reorientation).
Scalable Alignment: Achieved deterministic alignment of all NV centers in a 9-site array on a (111)-oriented diamond substrate, parallel to the optical axis.
Enhanced Performance: Alignment significantly improves light collection efficiency (37% to 44% relative difference compared to misaligned centers at 1.3 NA), crucial for maximizing signal contrast and sensitivity.
Sensitivity Gain: Oriented arrays enable coherent addressing of all emitters simultaneously, potentially leading to a factor of four increase in magnetic sensitivity compared to randomly oriented arrays.
Process Control: The method maintains the spatial ordering of the laser-written array while achieving uniform orientation across the sites.
Substrate Versatility: Demonstrated orientation control in both (111)-oriented diamond (full alignment) and (100)-oriented diamond (selection between two orientation classes).

Technical Specifications

Parameter	Value	Unit	Context
Fabrication Laser Wavelength	515	nm	Yb:KYW ultrafast laser source
Confocal Excitation Wavelength	532	nm	Continuous Wave (CW) laser
Objective Numerical Aperture (NA)	1.45	-	Oil immersion, 100x magnification
Seed Pulse Duration	270	fs	Initial pulse for vacancy generation
Seed Pulse Energy	1.47	nJ	At 515 nm
Diffusion Pulse Repetition Rate	200	kHz	Used for vacancy mobilization/annealing
Diffusion Pulse Energy	1.19	nJ	At 515 nm
(111) Substrate Nitrogen Content	10	ppb	HPHT grown diamond
(100) Substrate Nitrogen Content	80	ppb	CVD grown diamond
(111) Array Pitch	10	µm	Spacing between NV centers
(111) Array Depth	20	µm	Depth below diamond surface
Background-Corrected g⁽²⁾(0)	0.1	-	Confirms single NV center emission (Site 3f, 0.45 mW excitation)
NV1 Collection Efficiency (1.3 NA)	11.2	%	NV center aligned parallel to optical axis
Relative Efficiency Difference (1.3 NA)	37.2	%	Difference between aligned (NV1) and 109.5° rotated (NV2)

Key Methodologies

The fabrication and reorientation process relies on an integrated ultrafast laser writing system with an in situ confocal microscope for real-time monitoring and feedback.

Substrate Selection and Preparation:
- Experiments used both HPHT (111)-oriented diamond (10 ppb N) and CVD (100)-oriented diamond (80 ppb N).
- Surfaces were polished to low roughness (5 nm for (111), 1 nm for (100)).
NV Center Fabrication (Laser Writing):
- Vacancy Creation (Seed Pulse): A 270 fs, 1.47 nJ pulse (515 nm) was applied to generate vacancies and interstitial carbon atoms via multiphoton ionization.
- NV Formation (Diffusion Pulse Train): A 200 kHz train of 1.19 nJ diffusion pulses was applied. These pulses mobilize vacancies until one combines with a substitutional nitrogen atom (N_s) to form an NV^- center. The pulse train was terminated upon detection of an NV signal.
Orientation Determination (Polarization Analysis):
- The NV fluorescence was collected and analyzed using a 1/2 waveplate and a polarizing beam splitter.
- The full polarization profile was measured by rotating the half-wave plate, allowing identification of the NV center’s emission dipole orientation relative to the crystallographic axes.
Deterministic Reorientation (Femtosecond Laser Annealing):
- If the measured orientation did not match the desired target (e.g., [111] parallel to the optical axis), an additional annealing pulse train (diffusion pulses) was applied.
- The reorientation process was monitored by observing fluctuations in fluorescence intensity, consistent with a change in polarization and transmission through the analyzer.
- The annealing and subsequent polarization analysis were repeated iteratively until the desired orientation was confirmed via RMS error minimization against theoretical polarization patterns.

Commercial Applications

The ability to deterministically align NV centers in scalable arrays opens pathways for manufacturing high-performance quantum devices.

Quantum Sensing and Metrology: Fabrication of highly sensitive magnetometers and thermometers, leveraging the factor of four potential sensitivity increase from coherently addressed, aligned arrays.
Quantum Information Processing (QIP): Enables the scalable creation of solid-state spin registers necessary for quantum computing and quantum communication technologies.
Integrated Photonics: Aligned NV centers, which emit more efficiently into a smaller numerical aperture (NA), are ideal for integration into diamond-based photonic devices and waveguides, minimizing light loss due to total internal reflection.
Nanoscale Imaging: Provides the foundation for high-resolution imaging systems (e.g., biological or material science imaging) that require atomic-scale spatial resolution and exceptional sensitivity under ambient conditions.
Defect Engineering: The all-optical, high-spatial-resolution technique is valuable for fundamental research and manufacturing of other optimized color centers and defects in diamond and other wide-bandgap semiconductors.

View Original Abstract

Introduction Nitrogen-vacancy (NV) centers in diamond are widely recognized as highly promising solid-state quantum sensors due to their long room temperature coherence times and atomic-scale size, which enable exceptional sensitivity and nanoscale spatial resolution under ambient conditions. Ultrafast laser writing has demonstrated the deterministic spatial control of individual NV − centers, however, the resulting random orientation of the defect axis limits the magnetic field sensitivity and signal contrast. Methods We developed an all-optical approach for reorienting laser-written NV − centers to lie along a specific crystallographic axis using femtosecond laser annealing. The orientation is determined by polarization analysis, and the annealing and subsequent polarization analysis are repeated until the desired orientation is observed. Results Our method achieves deterministic alignment of NV − centers along the optical axis in (111)-oriented diamond substrates and allows selection between two observable orientation classes in (100)-oriented substrates. The reorientation preserves spatial ordering while producing uniform orientation across arrays of NV − centers. Discussion This approach enables scalable fabrication of orientation-controlled NV − arrays, and paves the way for scalable, high performance quantum devices based on orientation-controlled NV − centers.

Tech Support

Original Source

DOI: https://doi.org/10.3389/frqst.2025.1667545

References

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