Spin measurements of NV centers coupled to a photonic crystal cavity

At a Glance

Metadata	Details
Publication Date	2019-12-01
Journal	APL Photonics
Authors	T. Jung, J. Görlitz, B. Kambs, C. Pauly, N. Raatz
Institutions	Leipzig University, Element Six (United Kingdom)
Citations	24
Analysis	Full AI Review Included

Technical Documentation: Enhanced Optical Spin Read-Out SNR for NV Centers via Photonic Crystal Cavity Coupling

Executive Summary

This document analyzes a breakthrough application demonstrating significant enhancement of Nitrogen-Vacancy (NV) center spin read-out reliability by coupling the emitter to a two-dimensional Photonic Crystal (PhC) cavity fabricated in ultra-high purity Single Crystal Diamond (SCD).

Core Achievement: Successful demonstration of an almost tripled (≈ 3x) Signal-to-Noise Ratio (SNR) for optical spin read-out in cavity-coupled NV centers.
Material Foundation: The architecture relies on high-quality, ultrapure, CVD-grown (001)-oriented Single Crystal Diamond membranes with nitrogen concentrations below 5 ppb.
Nanoscale Fabrication: PhC cavities were precisely fabricated via Focused Ion Beam (FIB) milling in Reactive Ion Etched (RIE) thinned diamond membranes, achieving high optical Quality factors (Q-factors up to 8250).
NV Center Integration: NV centers were deterministically positioned into the cavity mode maximum using a novel high-resolution Atomic Force Microscope (AFM) implantation technique (5 keV N⁺ ions).
Spectral Control: A combination of thermal oxidation (for blue shift) and light-assisted gas adsorption (for reversible red shift) was employed to precisely tune the cavity mode into resonance with the NV Zero Phonon Line (ZPL).
Performance Metrics: On resonance, an enhancement of ZPL emission by almost one order of magnitude was observed, accompanied by a reduction in spontaneous emission lifetime (from 9.0 ns to 8.0 ns).

Technical Specifications

The following table summarizes key operational parameters and measured performance metrics extracted from the research.

Parameter	Value	Unit	Context
SNR Enhancement (ZPL Read-Out)	2.8 (Almost tripled)	Factor	Calculated total enhancement on resonance
Maximum Q-factor Achieved	8250	Dimensionless	Observed at 644.8 nm (Room Temperature)
SCD Crystal Orientation	(001)	Crystal Plane	Starting material, CVD-grown
SCD Purity (Nitrogen)	< 5	ppb	Ultrapure Electronic Grade
Final Membrane Thickness	Few hundred	nm	Post-RIE thinning
Photonic Crystal Lattice Constant	250	nm	Array periodicity ($a$)
PhC Hole Radius	68	nm	Optimized for M0-cavity
N Ion Implantation Energy	5	keV	Via AFM-tip aperture
Lateral Implantation Accuracy	74	nm	Total accuracy (aperture + straggle)
Emitter Lifetime (Off-Resonance)	9.0	ns	Recorded lifetime trace
Emitter Lifetime (On-Resonance)	8.0	ns	Observed shortening due to Purcell effect
Post-FIB Annealing Temperature	1000	°C	Vacuum annealing for crystal restoration

Key Methodologies

The robust fabrication and tuning of the NV-PhC system required highly controlled processes for material preparation, nanostructuring, and post-treatment.

SCD Membrane Preparation:
- 30 µm thick, ultrapure (001) SCD was etched via RIE (Ar/O₂ plasma) to remove 5 µm of polishing damage.
- The remaining film was bonded to a silicon substrate via a 50 nm Hydrogen Silsesquioxane (HSQ) layer, cured at 600°C.
- The final air-suspended membrane thickness was achieved by further RIE thinning from the topside to a few hundred nanometers.
Photonic Crystal Fabrication (FIB Milling):
- PhC arrays and M0-cavities were defined using Focused Ion Beam (FIB) milling, targeting defect-free spots of suitable membrane thickness.
- Optimizations (overmilling, drift control, metal protection layer deposition) maintained conical hole inclination angles below 4°, essential for preserving high Q-factors.
Deterministic NV Implantation:
- NV centers were created using a low-energy ion source and high-resolution AFM-tip implantation technique (5 keV N⁺ ions).
- A FIB-milled hole (70 nm aperture) in the AFM tip apex allowed alignment accuracy of approximately 1 nm, ensuring precise positioning at the mode field maximum.
Post-Processing and Activation:
- Samples underwent two main steps to activate NV centers and restore crystal quality:
  - Annealing in vacuum (p ≤ 10^-6 mbar) at 900°C for 10 hours.
  - Subsequent oxidation (450°C in air for 3 hours) and tri-acid cleaning to remove graphitic residuals and ensure oxygen termination for negatively charged NV centers (NV^-).
Spectral Tuning to Resonance:
- The cavity mode was first blue-shifted irreversibly using thermal oxidation (525°C).
- Fine-tuning and reversible red-shifting were achieved by light-assisted adsorption of residual gas in the cryostat chamber, allowing continuous optical control to match the 637 nm NV-ZPL.

6CCVD Solutions & Capabilities

This research validates the critical role of high-purity, custom-specified Single Crystal Diamond in achieving high-performance quantum systems. 6CCVD stands ready as an expert partner to replicate, extend, and industrialize this methodology.

Applicable Materials

To replicate and advance the reported SNR enhancement, 6CCVD recommends materials optimized for quantum applications and advanced nanofabrication:

Optical Grade SCD Wafers: We supply ultra-high purity, low-strain SCD with nitrogen concentrations well below 5 ppb, ensuring minimal spectral diffusion and long spin coherence times required for high-fidelity quantum sensing and networking applications.
Custom Crystal Orientation (Critical Optimization): The paper identifies that utilizing (111)-oriented SCD is necessary for optimal NV dipole alignment, potentially increasing the total Purcell factor and the resultant SNR enhancement by a factor of > 6. 6CCVD specializes in supplying custom-oriented SCD plates up to inch size to meet this advanced requirement.

Customization Potential

The fabrication requirements demonstrated in this paper—ultra-thin membranes, precise geometries, and high surface quality—fall directly within 6CCVD’s core production competencies.

Research Requirement	6CCVD Material Service	Engineering Value Proposition
Thin Membrane Precursors: SCD wafers must be prepared for RIE thinning to reach a thickness of a few hundred nanometers.	Precision Thickness Control: SCD wafers can be ordered in custom thicknesses from 0.1 µm to 500 µm, allowing engineers to tailor RIE recipes for superior uniformity.	Maximizes the yield of viable thin membranes, ensuring homogenous etching and predictable cavity Q-factors.
High Surface Quality: Polishing uniformity is crucial for subsequent RIE and FIB steps.	Ultra-Low Roughness Polishing: We guarantee SCD substrates with surface roughness Ra < 1 nm, far superior to the requirements for high-fidelity optical systems.	Minimizes scattering losses in PhC structures and prevents defects that compromise spin coherence properties.
Integrated Device Architecture: External magnetic fields (2 mT) and microwave pulses (Fig 6) imply the need for integrated electrical components.	Custom Metalization Services: 6CCVD offers in-house deposition of thin films including Au, Pt, Pd, Ti, W, and Cu, allowing researchers to integrate microwave antennas and contacts directly onto the SCD membranes.	Accelerates the development cycle for functional quantum devices, moving quickly from material science to integrated system testing.

Engineering Support

Successfully executing complex recipes like RIE thinning, FIB milling, and AFM implantation requires deep domain expertise in diamond material science.

6CCVD’s in-house PhD team provides specialized consultation to assist researchers and technical engineers in optimizing material specifications for quantum sensing, quantum computing, and integrated nanophotonics projects. We ensure that your starting SCD material—whether Single Crystal (SCD), Polycrystalline (PCD), or Boron-Doped (BDD)—is perfectly matched to your intended fabrication and application workflow.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Nitrogen-vacancy (NV) centers feature outstanding properties such as a spin coherence time of up to 1 s as well as a level structure offering the possibility to initialize, coherently manipulate, and optically read-out the spin degree of freedom of the ground state. However, only about 3% of their photon emission is channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC) cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam milling in thin reactive ion etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope implantation technique. On cavity resonance, we observe a lifetime shortening from 9.0 ns to 8.0 ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.5120120

References

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