Optically detected magnetic resonance of nitrogen-vacancy centers in diamond using two-photon excitation

This paper demonstrates the first observation of ground-state optically detected magnetic resonance (ODMR) in nitrogen-vacancy centers in diamond at room temperature using two-photon excitation with a 1040 nm femtosecond laser, establishing a promising method for fast 3D quantum sensing and imaging.

The Gist

Imagine you have a giant, clear diamond. Inside this diamond, there are tiny, invisible "defects" called Nitrogen-Vacancy (NV) centers. Think of these defects not as flaws, but as tiny, magical quantum compasses or microscopic sensors. They are so sensitive that they can feel magnetic fields, temperature changes, and even rotation, all while glowing with a faint red light.

Scientists want to use these diamond sensors to build super-precise medical devices or quantum computers. But to use them, they need to "talk" to them. This is where the story gets interesting.

The Old Way: Shining a Flashlight Everywhere

Traditionally, scientists used a green laser (like a bright flashlight) to wake up these diamond sensors.

• The Problem: When you shine a green flashlight through a thick diamond, the light hits everywhere at once. It's like trying to listen to a single person whispering in a crowded stadium while everyone else is shouting. The signal gets messy, and you can't tell exactly where in the diamond the sensor is located, especially deep inside.
• The Result: To get a clear signal, scientists usually had to grow diamonds with a very thin layer of sensors on the surface, limiting what they could study.

The New Way: The "Two-Photon" Laser Trick

In this paper, the researchers (Nguyen and Kieu) tried a clever new trick using a femtosecond laser (a laser that fires incredibly fast, tiny pulses of light). Instead of green light, they used infrared light (which is invisible to our eyes, like the heat from a remote control).

Here is the magic analogy:
Imagine the diamond sensors are like solar panels that only turn on when they get hit by two specific raindrops at the exact same time.

1. The Old Way: You throw one big, heavy raindrop (green light) at the whole field. It hits everything, but it's messy.
2. The New Way: You use a machine that fires two tiny, invisible raindrops (infrared photons) simultaneously.
   • If you are far away from the target, the two drops miss each other, and nothing happens.
   • But right at the focal point (the exact spot you are aiming at), the two drops collide perfectly. Their combined energy is enough to "wake up" the sensor.

This means the sensors only light up in that tiny, precise 3D spot where the two laser beams cross. It's like using a laser pointer to highlight a single word in a book without lighting up the whole page.

What They Did

The team set up a microscope with this special laser and a microwave generator (like a tiny radio station).

1. Mapping the Diamond: They scanned a large, rough diamond and a pile of tiny, sand-grain-sized diamonds. Because their laser only lights up the specific spot it hits, they could create a 3D map showing exactly where the "good" sensors were hiding and where the diamond was just empty space. They found that the sensors weren't spread out evenly; they were clumped in some areas and missing in others.
2. The "Radio" Test (ODMR): To prove these sensors were working, they played a specific radio frequency (microwaves) at the diamond.
   • When the radio frequency matched the "natural tune" of the sensor, the sensor stopped glowing as brightly.
   • They saw this "dimming" effect clearly, even with their new laser method. This proved they could read the sensor's data without the messy background noise of the old method.
3. The Magnetic Field Test: They brought a magnet close to the diamond. Just like a compass needle moves when you bring a magnet near it, the "tune" of the diamond sensor changed. They watched the signal shift in real-time, proving their system could detect magnetic fields with high precision.

Why This Matters

This discovery is like upgrading from a floodlight to a high-powered spotlight.

• Speed: They can scan and map diamonds much faster.
• Depth: They can see deep inside thick diamonds or even under layers of material, which was impossible before.
• Precision: They can target single sensors or tiny clusters without interference from neighbors.

In a nutshell: The researchers found a way to use a special laser to "poke" diamond sensors in 3D space with pinpoint accuracy. This opens the door to using diamonds as ultra-sensitive 3D scanners for everything from detecting tiny magnetic fields in the brain to monitoring the health of electric car batteries.

Technical Summary

1. Problem Statement

Nitrogen-Vacancy (NV) centers in diamond are powerful tools for quantum sensing, magnetic imaging, and quantum information processing. However, traditional one-photon excitation (1PE) using visible light (515–637 nm) faces significant limitations:

• Lack of Localization: 1PE excites NV centers throughout the entire sample volume, making it difficult to isolate specific defects in bulk materials.
• Depth Limitations: Visible light suffers from scattering and optical aberrations in bulk or highly scattering samples, limiting penetration depth and degrading spatial resolution.
• Background Noise: Excitation of the entire sample generates background fluorescence from other defects, reducing the signal-to-noise ratio (SNR).
• Sample Constraints: These limitations often necessitate the use of specially grown diamonds with thin, near-surface NV layers, restricting applications in bulk or micro-sized diamonds.

While Multi-Photon Microscopy (MPM) offers superior depth penetration and intrinsic optical sectioning, no prior study had successfully demonstrated Optically Detected Magnetic Resonance (ODMR) using Near-Infrared (NIR) two-photon excitation (2PE) for NV centers.

2. Methodology

The authors developed an experimental setup combining a custom-built multiphoton microscope with microwave (MW) delivery and lock-in detection.

• Excitation Source: A home-built Ytterbium (Yb) femtosecond fiber laser operating at 1040 nm with ~50 fs pulse duration and a 7.01 MHz repetition rate. Two photons of 1040 nm provide energy equivalent to a single 520 nm photon, matching the absorption band of NV centers.
• Optical System:
  • A 20X, 0.75 NA objective lens.
  • Epi-detection scheme to collect back-propagated nonlinear signals.
  • Spectral Separation: Filters separated various nonlinear channels:
    • 2PEF (630–950 nm): For NV center fluorescence.
    • 3PEF, SHG, THG: For other defect centers and harmonic generation.
  • Detection: Signals were read out via Photomultiplier Tubes (PMTs) coupled to a lock-in amplifier (UHFLI-600) to demodulate the fluorescence intensity changes.
• Microwave Delivery:
  • A custom PCB with a copper MW loop (hole diameter ~1.77 mm) was designed to hold the diamond sample.
  • A signal generator (2.75–2.96 GHz) amplified by a 40 dB gain amplifier delivered RF power (30 dBm) to the loop.
  • Imaging was performed through the hole in the PCB to allow the objective to focus on the sample surface.
• Samples:
  1. Bulk Diamond: A ~4x4 mm red-plated HPHT (High-Pressure High-Temperature) single crystal diamond.
  2. Micro-diamonds: 15 µm powdery diamonds with varying NV concentrations.

3. Key Contributions

• First Demonstration of 2PE-ODMR: This is the first report of observing ODMR traces of NV centers using two-photon excitation with a 1040 nm femtosecond laser.
• 3D Mapping Capability: By leveraging the intrinsic optical sectioning of 2PE, the authors demonstrated the ability to map NV center distributions in 3D within bulk and micro-sized diamonds without the background noise associated with 1PE.
• Defect Screening Tool: The system successfully utilized multi-channel nonlinear imaging (2PEF, 3PEF, SHG, THG) to distinguish between different defect types (e.g., $NV^0$ vs. $NV^-$) and identify regions of high NV concentration.
• Validation of Stability: The study confirmed that NV centers remain stable under femtosecond laser excitation, with repeatable ODMR traces over multiple cycles.

4. Results

• Bulk Diamond Imaging:
  • Multi-color mosaics revealed non-uniform zoning of NV centers, likely due to the HPHT growth process and electron bombardment.
  • Spectral Analysis: Confirmed the presence of both $NV^0$ (575 nm ZPL) and $NV^-$ (637 nm ZPL). The emission signal showed a quadratic power dependence (slope $\approx$ 2.25), confirming the two-photon nature of the excitation.
  • ODMR Traces: Clear ODMR dips were observed at zero magnetic field.
    • Zero-Field Splitting: Two Lorentzian dips centered at ~2.87 GHz.
    • Hyperfine Splitting: A separation of ~5.64 MHz between the $m_s = \pm 1$ states, attributed to crystal lattice strain.
    • Contrast: A fluorescence reduction of ~7% was measured with a linewidth (FWHM) of ~26.87 MHz.
• Magnetic Field Response (Zeeman Splitting):
  • Upon applying an external static magnetic field, the ODMR spectrum split into a four-dip pattern, indicating the projection of the magnetic field onto the crystal axes.
  • The resonant frequencies shifted predictably with increasing field strength, validating the system's capability for vector magnetometry.
• Micro-diamond Analysis:
  • Imaging revealed significant heterogeneity in NV concentration among individual 15 µm diamonds.
  • Spectral Differentiation: Some micro-diamonds were dominated by $NV^0$, while others were rich in $NV^-$.
  • Selective ODMR: ODMR signals were only detected in micro-diamonds with a high concentration of $NV^-$ centers (showing a ~2.5% intensity change), while $NV^0$-dominated diamonds showed no measurable ODMR signal.

5. Significance

This work establishes Two-Photon Excitation Fluorescence (2PEF) as a superior tool for NV center research, particularly for applications requiring:

• Deep Imaging: The ability to probe NV centers deep within bulk diamonds or beneath interfaces where visible light fails.
• High Spatial Resolution: Intrinsic optical sectioning allows for the isolation of single NV centers or specific ensembles without background interference.
• Rapid 3D Sensing: The combination of fast scanning and localized excitation enables rapid 3D mapping of magnetic fields and temperature.
• Material Characterization: The technique serves as a powerful screening tool to identify optimal regions in bulk or micro-diamonds for quantum sensing applications, overcoming the limitations of uniformity in commercially available samples.

The authors conclude that 2PE-ODMR is a promising pathway for advancing high-resolution, 3D quantum sensing and imaging technologies.