Electron spin contrast of Purcell-enhanced nitrogen-vacancy ensembles in nanodiamonds

At a Glance

Metadata	Details
Publication Date	2017-07-25
Journal	Physical review. B./Physical review. B
Authors	Simeon Bogdanov, Mikhail Y. Shalaginov, А. В. Акимов, Alexei Lagutchev, Polina Kapitanova
Institutions	Texas A&M University, Russian Quantum Center
Citations	26
Analysis	Full AI Review Included

6CCVD Technical Analysis: Electron Spin Contrast in Purcell-Enhanced NV Ensembles

This technical documentation analyzes the research paper detailing the performance of dense Nitrogen-Vacancy (NV) ensembles in nanodiamonds when coupled with plasmonic structures, focusing on the resulting trade-off between fluorescence lifetime ($\tau_{av}$) and spin readout contrast ($C_{T1}$). This analysis highlights key material requirements and experimental techniques relevant to advanced quantum sensing and nanophotonics, aligning directly with 6CCVD’s specialized MPCVD diamond capabilities.

Executive Summary

This high-density summary outlines the key experimental findings, the novel methodology, and the critical implications for designing robust NV-based quantum sensors.

Core Research Question: Quantitative investigation into how fluorescence lifetime shortening (via the Purcell effect in plasmonic environments) affects the optical spin contrast ($C_{T1}$) in dense NV ensembles (NVEs).
Key Finding on Contrast: Significant reduction in spin contrast was observed in environments with high Photonic Density of States (PDOS). $C_{T1}$ dropped sharply from up to 18% (dielectric environment, $\tau_{av}$ ≈ 24 ns) to below 5% (plasmonic TiN environment, $\tau_{av}$ ≈ 7.5 ns).
Mechanism Identified: The reduction in spin contrast is attributed to the relative decrease in the probability of non-radiative decay pathways when the radiative lifetime is drastically shortened, particularly critical when NVEs are operated below optical saturation.
Novel Methodology: Introduced a non-conventional, microwave-free measurement technique for $C_{T1}$ in large NV ensembles based on the process of thermal spin relaxation ($T_{1}$ measurement).
Engineering Conclusion: For optimal spin readout Signal-to-Noise Ratio (SNR) in NVEs operating at low optical excitation rates ($k_{opt}$ ≈ 1.5 MHz), researchers should focus on methods that modify the far-field radiation pattern (e.g., solid immersion lenses, bulk waveguides) rather than those that induce a high PDOS and significantly shorten the fluorescence lifetime (Purcell enhancement).
Material Implication: Purcell-enhanced schemes only become effective for SNR improvement when utilizing low-defect concentration diamond operated at high (saturating) optical powers, decoupling contrast from lifetime effects.

Technical Specifications

Parameter	Value	Unit	Context
Nanodiamond Size (Average)	76 ± 20	nm	Used for NV ensemble integration (NVEs)
NV Centers per Ensemble	400	NVs	Average density in each nanodiamond
Inter-Defect Separation Distance (IDSD)	≈ 8	nm	Characterizes the density of the NVE
Plasmonic Material	Titanium Nitride (TiN)	N/A	Formed islands (200 nm thick, 0.5 mm diameter)
Substrate Material	C-Sapphire	N/A	Used as the dielectric reference environment
Optical Excitation Rate ($k_{opt}$)	≈ 1.5	MHz	Used for all primary spin contrast measurements
Fluorescence Lifetime ($\tau_{av}$) - Sapphire	15 to 24	ns	Range observed in the dielectric environment
Fluorescence Lifetime ($\tau_{av}$) - TiN	7.5 to 12.5	ns	Range observed in the high PDOS plasmonic environment
Spin Contrast ($C_{T1}$) - Sapphire	Up to 18	%	Maximum observed contrast (longer lifetime)
Spin Contrast ($C_{T1}$) - TiN	Down to 4	%	Minimum observed contrast (shortest lifetime)
Spin Relaxation Time ($T_{1}$) - Sapphire	345 ± 40	µs	Measured using the thermal relaxation technique
Spin Relaxation Time ($T_{1}$) - TiN	445 ± 60	µs	Measured using the thermal relaxation technique
Read Pulse Detection Time ($t_{det}$)	300	ns	Initial window for fluorescence collection
Singlet Deshelving Rate ($k_{s}$)	7	MHz	Rate fitted in the kinetic model
Intersystem Crossing Rate $k^{(0)}_{cross}$ (m_s = 0)	5	MHz	Spin-conserving non-radiative rate
Intersystem Crossing Rate $k^{(1)}_{cross}$ (m_s = ±1)	30	MHz	Spin-conserving non-radiative rate

Key Methodologies

The experiment introduced a specialized non-coherent technique suitable for dense NV ensembles (NVEs) and employed standard optical characterization methods to link material environment to quantum performance.

Material Preparation:
- Nanodiamonds containing dense NVEs were dispersed onto a C-Sapphire substrate.
- Plasmonic Titanium Nitride (TiN) islands (200 nm thick, 0.5 mm diameter) were formed on the substrate to create areas of varying local Photonic Density of States (PDOS).
Fluorescence Lifetime Measurement (TCSPC):
- Time-Correlated Single-Photon Counting (TCSPC) was used to measure the fluorescence decay curves.
- Decay data was fitted assuming gamma-distributed lifetimes for ensembles of two-level systems, yielding $\tau_{av}$ values ranging from 7.5 ns (TiN) to 24 ns (Sapphire).
Novel Spin Contrast Measurement (Thermal Spin Relaxation):
- A method based on thermal $T_{1}$ spin relaxation was devised, avoiding the complexities of conventional coherent spin population inversion (Rabi oscillations) in dense ensembles.
- Process Steps:
  1. Initialization Pulse: An optical pulse projects the spin population into the $m_{s}=0$ state.
  2. Delay: A controlled time delay ($\Delta t$) allows part of the population to relax thermally back into the $m_{s}=\pm 1$ states.
  3. Read Pulse: A read pulse is applied, and fluorescence is collected during the first $t_{det} = 300$ ns.
- Result: The $T_{1}$ spin contrast ($C_{T1}$) is derived by comparing detected photon numbers ($N_{\infty} - N_{0}$) between fully thermalized and initialized spins.
Kinetic Modeling and Correlation:
- Observed data was modeled using a linear rate equation-based master equation to describe the NV center transitions and kinetics.
- The model successfully correlated the measured decrease in spin contrast with the shortening of the radiative decay rate ($k_{rad}$, which depends on the local PDOS environment).

6CCVD Solutions & Capabilities

The findings of this research—that achieving high SNR requires optimizing the surrounding diamond structure (waveguides, solid immersion lenses) and potentially operating at high excitation rates in ultra-pure material—directly reinforce the demand for high-quality, engineered MPCVD diamond components. 6CCVD is uniquely positioned to supply the materials required to transition this research from nanodiamonds to integrated, high-performance sensing platforms.

Applicable Materials

The core conclusion demands high-purity, low-defect diamond suitable for bulk integration and high-power operation.

Material Grade	6CCVD Offering	Application Alignment
Optical Grade Single Crystal Diamond (SCD)	Ultra-High Purity (low N concentration), Plates/Wafers up to 10mm thickness.	Essential for replicating the low-defect environment necessary to utilize Purcell enhancement schemes (i.e., operating at saturating powers where $C_{T1}$ is lifetime-independent). High purity minimizes ensemble effects (e.g., Auger-type effects) observed in dense nanodiamonds.
Polycrystalline Diamond (PCD)	Wafers up to 125 mm diameter; thickness up to 500 µm.	Ideal for creating large-scale, integrated photonic structures (e.g., bulk diamond waveguides or photonic crystals) designed to modify the far-field radiation pattern (the preferred method for SNR improvement highlighted by the study).
Boron-Doped Diamond (BDD)	Custom BDD films/wafers.	While not explicitly used for NV $C_{T1}$ enhancement in this paper, BDD is vital for creating conductive electrodes for microwave delivery or high-speed electronic integration alongside NV quantum devices.

Customization Potential

The experimental use of a complex multi-layer substrate (Sapphire + TiN islands) and the proposed use of integrated photonic structures (waveguides, SILs) are capabilities 6CCVD excels at supporting.

Required Service	6CCVD Customization Capability	Relevance to NV Research
Nanophotonic Integration	Custom thicknesses of SCD/PCD up to 500 µm (wafers) or 10 mm (substrates).	Allows engineering teams to fabricate structures like bulk diamond waveguides (mentioned as superior for SNR) or membranes for membrane-coupled resonators.
Advanced Metalization	Full in-house capability for standard metal stacks: Au, Pt, Pd, Ti, W, Cu.	Essential for fabricating the required plasmonic structures (like the TiN islands used here) or creating necessary microwave strip-lines or RF antennae for future ODMR-based applications. Ti is available as an adhesion layer or film.
Precision Shaping & Polishing	Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD.	Critical for creating highly efficient optical interfaces, such as the curved surfaces needed for Solid Immersion Lenses (SILs), one of the recommended approaches for high collection efficiency without reducing $C_{T1}$.
Custom Dimensions	Plates/wafers up to 125 mm (PCD).	Supports scalability from laboratory experiments (nanodiamonds) to full-scale integrated quantum devices on large substrates.

Engineering Support

The kinetic model presented in the paper (governing NV center kinetics and transitions) demonstrates the highly specific engineering required for advanced NV applications. 6CCVD’s expertise in controlling the diamond growth parameters is crucial for tuning the material properties that feed these models.

Defect Control: 6CCVD’s in-house PhD materials science team is experienced in controlling nitrogen incorporation during MPCVD growth, crucial for achieving the desired NV concentration, whether requiring dense NVEs (IDSD ≈ 8 nm) or isolated single NV centers for high-saturation operation.
Application Guidance: Our team can provide consultative support on material selection for projects seeking to maximize electron spin readout sensitivity, advising clients on whether their application (e.g., magnetometry, quantum computing) dictates the use of far-field modification techniques (requiring thicker, highly polished SCD/PCD) or if the high-saturation/low-defect approach is feasible.

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

View Original Abstract

Nitrogen-vacancy centers in diamond allow for coherent spin state\nmanipulation at room temperature, which could bring dramatic advances to\nnanoscale sensing and quantum information technology. We introduce a novel\nmethod for the optical measurement of the spin contrast in dense\nnitrogen-vacancy (NV) ensembles. This method brings a new insight into the\ninterplay between the spin contrast and fluorescence lifetime. We show that for\nimproving the spin readout sensitivity in NV ensembles, one should aim at\nmodifying the far field radiation pattern rather than enhancing the emission\nrate.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.96.035146