Monitoring Dark-State Dynamics of a Single Nitrogen-Vacancy Center in Nanodiamond by Auto-Correlation Spectroscopy - Photonionization and Recharging

At a Glance

Metadata	Details
Publication Date	2021-04-10
Journal	Nanomaterials
Authors	Mengdi Zhang, Bai‐Yan Li, Jing Liu
Institutions	University of Indianapolis, Harbin Medical University
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research utilizes Auto-Correlation Spectroscopy (ACS) to precisely quantify the photon-induced charge conversion kinetics of single Nitrogen-Vacancy (NV) centers in nanodiamonds, providing critical data for quantum applications.

Core Achievement: Developed and validated an analytical model based on ACS to extract the transition rates (k₁₂ and k₂₀) governing the charge conversion between the fluorescent negative state (NV^-, “bright”) and the neutral dark state (NV⁰, “dark”).
Ionization Mechanism: The ionization process (NV^- → NV⁰) is confirmed to be a fast, one-photon process, exhibiting a linear dependence on excitation laser power (P^1.0). The characteristic transition time is approximately 0.1 µs.
Recharging Mechanism: The recharging process (NV⁰ → NV^-) is confirmed as a sequential two-photon process, showing a quadratic dependence on laser power (P^1.85). The characteristic transition time is significantly slower, around 20 ms.
Wavelength Control: Excitation using 532 nm (green) laser induces strong fluorescence intermittency (blinking), while 633 nm (red) excitation suppresses the ionization effect, leading to stable, mono-state emission.
Methodological Advantage: ACS allows for accurate measurement of transition dynamics across a broad temporal range (sub-microseconds to milliseconds), overcoming limitations of traditional ON/OFF histograms which struggle with fast transitions and require arbitrary thresholding.
Engineering Relevance: The derived kinetic parameters enable precise control of the NV center charge state, which is vital for improving the accuracy of quantum bit encoding and enhancing signal stability in quantum sensing and bioimaging.

Technical Specifications

Parameter	Value	Unit	Context
Nanodiamond Nominal Size	50	nm	Host material for NV centers.
Excitation Wavelength (Blinking)	532	nm	Used to induce NV^- ↔ NV⁰ charge conversion.
Excitation Wavelength (Stable)	633	nm	Used to suppress ionization (mono-state emission).
NV^- (“ON”) State Lifetime	11.88	ns	Measured via Time-Correlated Single Photon Counting (TCSPC).
NV⁰ (“OFF”) State Lifetime	6.14	ns	Measured via TCSPC.
NV^- → NV⁰ Ionization Time	~0.1	µs	Characteristic transition time (fast component).
NV⁰ → NV^- Recharging Time	~20	ms	Characteristic transition time (slow component).
Ionization Rate (k₁₂) Power Dependence	Linear (P^1.0)	N/A	Confirms one-photon absorption process.
Recharging Rate (k₂₀) Power Dependence	Quadratic (P^1.85)	N/A	Confirms sequential two-photon absorption process.
Ionization Rate Range (k₁₂)	5.33 to 160.26	ms^-1	Measured rate range across 0.02 mW to 1.5 mW excitation power.
Objective Numerical Aperture (NA)	1.2	N/A	Water-immersion objective used for confocal microscopy.
Immobilization Layer Thickness	60	nm	Polyvinyl Alcohol (PVA) coating thickness.

Key Methodologies

The study employed time-resolved single molecule spectroscopy combined with a customized scanning confocal microscope to characterize the NV center dynamics.

Nanodiamond Purification: Commercial 50 nm nanodiamonds (0.5% w/v) were cleaned by repeated centrifugation (5000 rpm for 5 min, 80% supernatant replacement) five times to remove smaller particles and impurities.
Sample Preparation: A 20 µL drop of the cleaned suspension was spin-coated onto a MgO substrate at 2000 rpm for 2 min.
Immobilization: A 60-nm-thick layer of 1.5% w/v Polyvinyl Alcohol (PVA) was applied over the sample to immobilize the NV centers and protect them from surface oxidation.
Excitation: A picosecond-pulsed laser (532 nm or 633 nm) was used for excitation, delivered via a high NA (1.2) water-immersion objective. Laser power was systematically varied (0.02 mW to 1.58 mW) for power-dependent measurements.
Detection: Fluorescence emission was collected by the same objective, filtered (685-70 band pass filter), and split onto two Single Photon Avalanche Photodiodes (SPADs) in a Hanbury Brown-Twiss (HBT) geometry.
Lifetime Measurement: Time-Correlated Single Photon Counting (TCSPC) was used to measure the fluorescence lifetime (τ) of the excited states for both “ON” and “OFF” states.
Kinetics Analysis (ACS): The time-course fluorescence intensity trajectory was analyzed using the derived analytical auto-correlation function (Equation 5) based on a three-level system model. This yielded the dark-state probability (D^eq₂) and the dark-state lifetime (τ_D).
Rate Calculation: Transition rates k₁₂ (ionization) and k₂₀ (recharging) were calculated from the fitted ACS parameters using Equation 6, confirming the power dependence and mechanism of each transition.

Commercial Applications

The ability to monitor and control the charge state dynamics of NV centers is critical for advancing several high-technology fields:

Quantum Sensing: NV centers are used as highly sensitive nanoscale sensors for magnetic and electric fields. Controlling the NV^- state stability (suppressing NV⁰ dark state formation) improves the signal-to-noise ratio and coherence time, enhancing sensor performance in complex environments (e.g., biological systems).
Quantum Computing and Communication: Stable NV^- emission is essential for reliable quantum bit (qubit) encoding. The kinetic data allows engineers to select optimal excitation parameters (wavelength and power) to minimize fluorescence intermittency and maximize the fidelity of quantum operations.
Super-Resolution Bioimaging: The controlled blinking behavior (intermittency) of NV centers is utilized in stochastic super-resolution microscopy (similar to STORM/PALM) to localize individual nanodiamonds with high precision. Precise knowledge of the fast ionization (0.1 µs) and slow recharging (20 ms) times allows for optimization of illumination cycles for localization algorithms.
Solid-State Single Photon Sources: By utilizing excitation wavelengths that prevent ionization (e.g., 633 nm), NV centers can be engineered into highly stable, room-temperature single-photon sources required for quantum cryptography and integrated photonics.

View Original Abstract

In this letter, the photon-induced charge conversion dynamics of a single Nitrogen-Vacancy (NV) center in nanodiamond between two charge states, negative (NV−) and neutral (NV0), is studied by the auto-correlation function. It is observed that the ionization of NV− converts to NV0, which is regarded as the dark state of the NV−, leading to fluorescence intermittency in single NV centers. A new method, based on the auto-correlation calculation of the time-course fluorescence intensity from NV centers, was developed to quantify the transition kinetics and yielded the calculation of transition rates from NV− to NV0 (ionization) and from NV0 to NV− (recharging). Based on our experimental investigation, we found that the NV−-NV0 transition is wavelength-dependent, and more frequent transitions were observed when short-wavelength illumination was used. From the analysis of the auto-correlation curve, it is found that the transition time of NV− to NV0 (ionization) is around 0.1 μs, but the transition time of NV0 to NV− (recharging) is around 20 ms. Power-dependent measurements reveal that the ionization rate increases linearly with the laser power, while the recharging rate has a quadratic increase with the laser power. This difference suggests that the ionization in the NV center is a one-photon process, while the recharging of NV0 to NV− is a two-photon process. This work, which offers theoretical and experimental explanations of the emission property of a single NV center, is expected to help the utilization of the NV center for quantum information science, quantum communication, and quantum bioimaging.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano11040979

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