Excited-State Lifetime of NV Centers for All-Optical Magnetic Field Sensing

At a Glance

Metadata	Details
Publication Date	2024-03-25
Journal	Sensors
Authors	Ludwig Horsthemke, Jens Pogorzelski, Dennis Stiegekötter, Frederik Hoffmann, Lutz Langguth
Institutions	FH Münster, Quantum Design (Germany)
Citations	9
Analysis	Full AI Review Included

Executive Summary

This research investigates the use of the magnetic field-dependent fluorescence lifetime of Nitrogen-Vacancy (NV) centers in microdiamonds to create a robust, all-optical magnetometry system.

Robust Sensing Metric: The system utilizes the fluorescence phase shift (a non-intensity quantity) upon modulated excitation, providing inherent immunity to common disturbances like laser intensity noise and optical path fluctuations.
Immunity Achievement: The phase-based approach demonstrated 100 times higher immunity to intensity fluctuations compared to traditional intensity-based magnetometry.
Optimal Performance Point: Maximum magnetic contrast in phase (5.8°) was achieved at an optimal excitation modulation frequency of 13 MHz.
MW-Free Fiber Design: The sensor is implemented in a fiber-based setup, eliminating the need for complex Microwave (MW) delivery systems, simplifying the design for industrial application.
Sensitivity Metrics: A realized noise floor of 20 µT/sqrtHz was achieved, approaching the estimated shot-noise-limited sensitivity (SNLS) of 0.95 µT/sqrtHz at 11.5 mW excitation power.
Decay Dynamics: The NV center ensemble exhibits bi-exponential fluorescence decay, with the larger decay time (14.54 ns at B=0) showing a significant magnetic contrast of 15.2%.

Technical Specifications

Parameter	Value	Unit	Context
Sample Material	NV-rich microdiamond powder	-	Ensemble fixed to fiber tip
Excitation Wavelength	520	nm	Laser Diode (PLT5 520B)
Excitation Power (LIA Test)	11.5	mW	Used for noise floor determination
Modulation Frequency Range	Up to 100	MHz	Frequency domain sweep range
Optimal Sensing Frequency	13	MHz	Frequency of maximum phase contrast
Maximum Phase Contrast	5.8	°	Achieved at 13 MHz
Noise Floor (Phase, realized)	20	µT/sqrtHz	At 13 MHz, B_bias = 20 mT
Shot-Noise-Limited Sensitivity (SNLS)	0.95	µT/sqrtHz	Theoretical limit for phase measurement
Immunity Improvement	100	times	Phase vs. magnitude approach
Magnetic Field Range (Tested)	0 to ~120	mT	Electromagnet range
Fluorescence Decay (B=0)	Bi-exponential	-	Fit components: τ_2,1 = 6.13 ns, τ_2,2 = 14.54 ns
Magnetic Contrast (Intensity)	13.9	%	Reduction in fluorescence count-rate
Magnetic Contrast (Lifetime τ_2,2)	15.2	%	Reduction in larger decay time component
Fiber Core Diameter	105	µm	Multimode fiber used for sensing head

Key Methodologies

The experiment involved two primary stages: initial characterization using Time-Correlated Single-Photon Counting (TCSPC) and subsequent frequency domain sensing using a fiber probe.

TCSPC Characterization:
- A high-NV-density microdiamond powder sample was excited using a ps-Laser (515 nm) focused to a 0.42 µm spot.
- Fluorescence histograms were recorded at varying magnetic fields (0 to 70 mT).
- Decay curves were fitted using a bi-exponential model (Equation 1) to extract magnetic field-dependent lifetimes (τ_k,i) and fractional amplitudes (a_k,i).
Fiber Sensor Fabrication:
- NV-rich diamond powder was affixed to the end facet of a 105 µm core optical fiber using glue, creating the sensor head.
Frequency Domain Setup:
- A 520 nm laser diode was modulated harmonically by an RF amplifier across a frequency range up to 100 MHz.
- Fluorescence was collected via the same fiber and detected by a Si-photodiode coupled to a Trans-Impedance Amplifier (TIA).
Transfer Function Measurement:
- A Vector Network Analyzer (VNA) or Lock-In Amplifier (LIA) was used to measure the magnitude (|H_r|) and phase (∠H_r) of the fluorescence signal relative to the excitation signal.
- Measurements were normalized to the B = 0 mT response to isolate the magnetic field effect on the fluorescence decay dynamics.
Optimal Frequency Selection:
- The modulation frequency was optimized to 13 MHz, where the phase response exhibited the maximum magnetic contrast (5.8°).
Noise and Immunity Testing:
- An artificial intensity disturbance (fluctuation of ±0.17% at 1 Hz) was introduced using a Liquid Crystal Light Valve (LCLV).
- The phase measurement showed a disturbance of only 0.02% relative to its magnetic contrast, confirming 100x superior immunity compared to the magnitude measurement (2.3% disturbance).
- Noise spectral densities were measured using the LIA at 13 MHz with a 20 mT bias field to determine the realized noise floor (20 µT/sqrtHz).

Commercial Applications

This technology enables the creation of robust, all-optical quantum sensors suitable for environments where traditional MW-based or intensity-sensitive sensors fail.

Industrial Process Monitoring: Sensing magnetic fields in high-voltage environments or areas requiring high electrical insulation resistance, as the probe is entirely non-metallic and MW-free.
Remote Sensing Probes: Deployment of compact, fiber-coupled probes for magnetic field mapping in confined or inaccessible spaces (e.g., downhole drilling, internal machinery inspection).
Biomagnetism and Medical Devices: Non-invasive magnetic sensing in biological systems, avoiding the local heating and eddy currents associated with MW delivery systems.
Quantum Sensor Development: Establishing a foundation for next-generation robust quantum magnetometers that rely on non-intensity quantities (lifetime/phase) to mitigate common noise sources (laser power drift, fiber movement).
High-Field Magnetometry: Applicable in a consistent magnetic field range (10-40 mT), making it suitable for applications outside the narrow zero-field or level-anticrossing regimes used by other high-sensitivity NV sensors.

View Original Abstract

We investigate the magnetic field-dependent fluorescence lifetime of microdiamond powder containing a high density of nitrogen-vacancy centers. This constitutes a non-intensity quantity for robust, all-optical magnetic field sensing. We propose a fiber-based setup in which the excitation intensity is modulated in a frequency range up to 100MHz. The change in magnitude and phase of the fluorescence relative to B=0 is recorded where the phase shows a maximum in magnetic contrast of 5.8∘ at 13MHz. A lock-in amplifier-based setup utilizing the change in phase at this frequency shows a 100 times higher immunity to fluctuations in the optical path compared to the intensity-based approach. A noise floor of 20μT/Hz and a shot-noise-limited sensitivity of 0.95μT/Hz were determined.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s24072093

References

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