🔬 mesoscale physics

Nonlinear optical charge state switching and pumping to a diamond NV center dark state

By utilizing a nanoscale cavity to generate intense infrared fields, this study identifies that the photoluminescence quenching in diamond nitrogen-vacancy centers is caused by nonlinear two-photon pumping into a previously misunderstood dark state of the neutral NV charge, thereby characterizing its energy levels and lifetime to establish performance limits for future quantum technologies.

Original authors: Prasoon K. Shandilya, Vinaya K. Kavatamane, Sigurd Flågan, David P. Lake, Denis Sukachev, Paul E. Barclay

Published 2026-07-07

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CC BY 4.0

Original authors: Prasoon K. Shandilya, Vinaya K. Kavatamane, Sigurd Flågan, David P. Lake, Denis Sukachev, Paul E. Barclay

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Diamond "Light Bulb" and Its Mystery Switch

Imagine a tiny diamond containing a special defect called a Nitrogen-Vacancy (NV) center. You can think of this defect as a microscopic light bulb. When you shine a green laser on it, it glows brightly with red light. Scientists love these "bulbs" because they are incredibly useful for quantum computers and super-sensitive sensors.

However, these light bulbs have a frustrating habit: sometimes, they suddenly go dark. This is called a "dark state." When the bulb goes dark, it stops glowing, which ruins the data scientists are trying to read. For a long time, nobody knew exactly why the bulb went dark or how to turn it back on.

The Experiment: A Tiny Whirlpool of Light

To solve this mystery, the researchers built a special playground for these light bulbs. They took a tiny disk of diamond (about the width of a human hair) and used a fiber optic cable to pump intense infrared (IR) light into it.

Think of this diamond disk as a whirlpool. When you pour water (light) into a whirlpool, it spins around the edge very fast. Because the disk is so small and the light bounces around so many times, the light becomes incredibly intense right at the edge of the disk, even if the input power is low.

They shone two types of light on the diamond:

Green light: To make the light bulbs glow.
Infrared (IR) light: The "mystery ingredient" that was supposed to be harmless but turned out to be the key.

The Discovery: The Two-Step Trap

The researchers discovered that the "dark state" isn't just a random glitch. It's a specific trap that the light bulbs fall into when hit by strong infrared light.

Here is the analogy for what happens:

The Normal Cycle: Normally, the green laser pushes the light bulb up a ladder. It climbs up, glows, and then slides back down to the bottom. This is the happy, glowing state.
The Trap: The researchers found that if you hit the bulb with enough infrared light, it acts like a two-step elevator.
- Usually, the bulb can only take one step at a time.
- But with strong infrared light, the bulb gets hit by two photons (packets of light) at the exact same time. This gives it a massive boost, launching it up to a very high, hidden floor called the $4A_2$ state.
The Dark Room: Once the bulb lands on this high floor, it gets stuck. It's like a room with no windows and no door. The bulb cannot glow from here, and it takes a long time to find its way back down to the ground floor. While it is stuck in this "dark room," the diamond stops glowing.

What They Measured

By carefully turning the infrared light up and down, the scientists were able to map out this hidden floor:

The Height: They calculated exactly how high this "dark room" is compared to the ground floor. They found it is less than 0.58 electron-volts (a unit of energy) above the ground.
The Exit Door: They figured out how much energy is needed to kick the bulb out of the dark room and back to the normal glowing state.
The Stay Duration: They measured how long the bulb stays stuck in the dark room. It stays there for about 1.78 to 6.06 microseconds. While that sounds fast, for a tiny atom, it's a long time—long enough to mess up sensitive measurements.

Why This Matters (According to the Paper)

The paper explains that this discovery is crucial for anyone trying to use these diamond light bulbs in high-tech devices.

The "Quenching" Effect: The paper shows that if you use strong infrared light (like in advanced microscopes or sensors), you might accidentally trap all your light bulbs in the dark room, making your device stop working.
New Tools: Because they now understand the rules of this "dark room," they can use it as a tool. For example, they showed that by modulating the infrared light, they could turn the diamond's glow on and off very quickly. This could be used to create new types of super-resolution microscopes (seeing things much smaller than before) or to map out invisible infrared fields.

Summary

In short, the researchers found that strong infrared light acts like a two-key switch that forces diamond light bulbs into a hidden, non-glowing "dark room." They mapped out the size of this room, how long the bulbs stay inside, and how to get them out. This knowledge helps scientists avoid accidentally turning off their quantum devices and gives them a new way to control light for advanced imaging.

Technical Summary: Nonlinear Optical Charge State Switching and Pumping to a Diamond NV Center Dark State

Problem Statement
The performance of diamond nitrogen-vacancy (NV) centers in quantum technologies is often limited by photodynamics that degrade readout fidelity. Specifically, the quenching of photoluminescence (PL) under intense excitation is frequently attributed to a "dark state," the physical nature and population mechanism of which have remained poorly understood. While the NV center exists in two primary charge states, the negatively charged $NV^-$ and the neutrally charged $NV^0$ , both possessing well-characterized optical properties, a non-fluorescent state observed in certain excitation schemes has eluded a definitive model. Previous work has explored charge state manipulation via multi-wavelength excitation (e.g., combining green and near-infrared fields) and the use of stimulated emission depletion (STED) microscopy, yet a quantitative model describing the quenching mechanism and the specific energy levels of the associated dark state has been elusive.

Methodology
The authors investigate these dynamics using a diamond microdisk cavity supporting high-quality factor ( $Q$ ) whispering gallery modes. The experimental setup involves:

Sample: A diamond microdisk (diameter $\sim 5.3$ $\mu$ m) fabricated from optical-grade material with an NV concentration of $\sim 10^{13}–10^{14}$ cm $^{-3}$ .
Excitation: A confocal microscope provides green laser excitation (532 nm) to drive NV centers. Simultaneously, a fiber taper waveguide evanescently couples infrared (IR) light into the microdisk modes at two specific wavelengths: 966 nm and 1524 nm.
Field Intensities: The high- $Q$ and small mode volume of the cavity allow for intracavity IR field intensities exceeding 1 W/ $\mu$ m $^2$ for milliwatt-level input powers, enabling the study of nonlinear optical effects over six orders of magnitude in field strength.
Detection: PL is collected via both the microscope (free space) and the fiber taper (cavity modes). The fiber taper collection is crucial for isolating emission from NVs within the whispering gallery modes, effectively filtering out background PL from the substrate.
Modeling: The experimental data is analyzed using a seven-level rate equation model adapted from Razinkovas et al. This model accounts for photoionization and recombination processes driven by single green photons, single IR photons, and two-IR-photon processes. It specifically considers transitions between the $NV^-$ ground ( $^3A_2$ ) and excited ( $^3E$ ) states, and the $NV^0$ ground ( $^2E$ ), excited ( $^2A_2$ ), and quartet ( $^4A_2$ ) states.

Key Contributions and Results

Identification of the Dark State Mechanism: The study demonstrates that the quenching of NV emission is caused by nonlinear optical pumping into the $^4A_2$ quartet state of the neutral $NV^0$ center. This occurs via a two-photon absorption process where two IR photons drive population from the $NV^-$ excited state ( $^3E$ ) to the $NV^0$ quartet state ( $^4A_2$ ).
Energy Constraints: By analyzing the ionization thresholds required to reproduce the experimental data, the authors constrain the energy difference between the $NV^0$ quartet state ( $^4A_2$ ) and its ground state ( $^2E$ ) to $\Delta_0 \le 0.58$ eV. Consequently, the ionization energy from the $NV^-$ excited state ( $^3E$ ) to the $NV^0$ quartet state is constrained to $\le 1.28$ eV.
Recombination Thresholds: The study establishes that the recombination energy threshold from the $^4A_2$ state to the $NV^-$ ground state ( $^3A_2$ ) is $\le 2.33$ eV. Furthermore, the recombination threshold from the $NV^0$ excited state ( $^2A_2$ ) to the $NV^-$ ground state is constrained to the range $0.65$ eV $< R < 0.75$ eV.
Intrinsic Lifetime Estimation: The effective decay rate of the $^4A_2$ state is estimated, yielding an intrinsic lifetime range of $1.78 $–$ 6.06$ $\mu$ s. This value, derived from fitting the PL quenching dynamics and time-resolved modulation response, has not been previously reported in the literature.
Charge State Dynamics: The research maps out three distinct regimes of charge state behavior as a function of intracavity IR photon number ( $N_{IR}$ $N_{I R}$ ):
- Low $N_{IR}$ : Recombination dominates, increasing $NV^-$ PL.
- Intermediate $N_{IR}$ : Single-photon ionization and recombination balance, leading to complex dynamics.
- High $N_{IR}$ : Two-photon pumping to the $^4A_2$ state saturates the population. Since the decay from $^4A_2$ is limited by a constant non-radiative rate, population becomes trapped in this dark state, resulting in the quenching of PL from both $NV^0$ and $NV^-$ .
Field Sensing: The nonlinear dependence of the quenching on IR intensity allows for the imaging of local infrared fields, as demonstrated by spatially resolved PL maps showing suppression at the microdisk perimeter where whispering gallery modes are localized.

Significance and Claims
The paper claims that this new understanding of the $^4A_2$ state and its role in dark state behavior is critical for predicting the operational limits of NV-based technologies that rely on intense fields. Specifically, the authors note that these findings impact:

Levitated Systems: Experiments involving optically levitated nanodiamonds often utilize intense IR fields for trapping and cooling; understanding charge state switching is vital for maintaining spin coherence.
Spin-Optomechanics: Devices integrating NV centers with nanomechanical resonators and superconducting circuits may be affected by unintended charge state transitions under high-power operation.
Absorption-Based Magnetometers: Schemes utilizing IR absorption (e.g., at 1042 nm) for sensing may suffer performance degradation due to charge state switching at high powers.
Super-Resolution Microscopy: The results confirm that 1550 nm light can implement STED microscopy, and the nonlinear quenching mechanism offers an alternative to electron beam or near-field techniques for field imaging, potentially allowing for spatial resolution enhancement via post-processing.

The authors conclude that the ability to manipulate the $^4A_2$ state, potentially leveraging its long lifetime and spin-lattice relaxation time, could enable new applications in quantum sensing and memory, while the characterization of these limits is essential for the robust design of future quantum devices.