Implementation and commissioning of an experimental system towards sub-eV axion-like particle searches with 0.1 PW laser at ELI-NP

This paper reports the successful development and commissioning of an integrated experimental system at ELI-NP, utilizing a 0.1 PW laser to search for axion-like particles via Four-Wave Mixing, which has been validated for background studies and is ready for a stepwise energy scale-up from 20 mJ to 2.5 J.

The Gist

Imagine the universe is a giant, dark ocean. We know there's something massive swimming in it—something that makes up most of the matter in the cosmos—but we can't see it. Scientists call this Dark Matter. One of the leading suspects for what this invisible swimmer is, is a tiny, ghostly particle called an Axion (or a cousin called an Axion-Like Particle).

The problem? These particles are so shy and interact so weakly with normal matter that finding them is like trying to hear a whisper in a hurricane.

This paper describes a massive, high-tech experiment built at ELI-NP in Romania, one of the most powerful laser facilities on Earth. The team built a "ghost hunter" to try and catch these elusive particles. Here is how they did it, explained simply.

1. The Strategy: The "Two-Laser Dance"

To find these ghosts, the scientists use a trick called Four-Wave Mixing. Think of it like a dance floor with two different types of music playing at once.

• Laser A (The Creator): A super-fast, high-energy laser (like a drumbeat).
• Laser B (The Inducer): A slightly different laser (like a bassline).

When these two laser beams cross paths in a vacuum, they create a chaotic, high-energy party. The theory is that if Axions exist, the energy from this party might briefly turn into an Axion. But Axions are invisible, so they vanish immediately.

However, if an Axion is created, the second laser (the Inducer) can "nudge" it, causing it to turn back into a photon (a particle of light) instantly. This new photon is the "smoking gun." It has a very specific color (wavelength) that the lasers don't naturally produce. If the detectors see this specific color, they might have found an Axion.

2. The Challenge: The "Noise" Problem

The biggest hurdle isn't making the lasers; it's listening for the whisper without hearing the background noise.

In a normal room, if you try to hear a pin drop, the sound of the air conditioner or people talking will drown it out. In this experiment, the "noise" comes from:

• Air molecules: Even in a vacuum, a few stray gas atoms can get hit by the lasers and glow, creating fake signals.
• Glass and Mirrors: The lasers bounce off mirrors and pass through glass. Sometimes, the glass itself gets excited and emits light that looks exactly like the Axion signal.

The scientists realized that as they make their lasers more powerful (moving from a "tabletop" experiment to a "facility-scale" one), this background noise gets louder and louder.

3. The Solution: Building a "Silent Room"

The team built a specialized experimental area (the "0.1 PW" zone) designed to be the ultimate quiet room.

• The Vacuum Chamber: They built a chamber so empty that it's a better vacuum than outer space. They used special metal seals and pumps to suck out almost every air molecule. This stops the "gas noise."
• The "Area Scan": They realized that if the lasers hit the glass mirrors, the glass might glow. To figure this out, they built a system to change the size of the laser beam.
  • Analogy: Imagine shining a flashlight on a wall. If you make the beam wider, you hit more of the wall. If the "glow" gets bigger as the beam gets wider, it's coming from the wall (the mirror). If the glow stays the same, it's coming from the air. This helps them distinguish between real Axion signals and mirror glitches.
• The "Trigger Pattern" (The Secret Code): This is their cleverest trick. They don't just fire the lasers randomly. They fire them in a specific four-step rhythm:
  1. Both Lasers On: (Looking for the Axion signal).
  2. Only Laser A On: (Checking for noise from Laser A).
  3. Only Laser B On: (Checking for noise from Laser B).
  4. Neither On: (Checking for random electronic noise).

By comparing the results of these four steps, they can mathematically subtract the noise and see if anything is left over.

4. The Results: The "Test Drive"

Before they turn on the full-power 10-PW lasers (which would be like a nuclear explosion of light), they needed to test the system with lower power (0.1 PW, or 100 terawatts).

They ran a "commissioning" test with laser pulses that were about 20 millijoules (tiny energy, but enough to test the gears).

• The Vacuum: They proved they could keep the pressure incredibly stable.
• The Timing: They synchronized the two lasers so perfectly that they hit the same spot at the exact same time, even though one laser pulse is a billion times shorter than the other.
• The Noise: They found that at these low energies, the "mirror noise" and "air noise" were successfully suppressed. The system worked exactly as designed.

The Bottom Line

This paper is essentially a technical manual and a success report for a new, ultra-sensitive detector.

They haven't found the Axion yet (that's the goal for the future when they turn the power up to the maximum). But they have proven that their "ghost hunting" machine is built correctly, the "silent room" is quiet enough, and the "secret code" works.

Now, they are ready to turn up the volume. They plan to slowly increase the laser energy from the current test levels all the way to the maximum 2.5 Joules (and eventually 10 PW). If Axions exist in the mass range they are looking for, this upgraded system will be the first to hear their whisper in the dark.

Technical Summary

1. Problem Statement

Dark Matter (DM) remains a fundamental unsolved problem in physics. Axion-like particles (ALPs) are a leading candidate for Cold Dark Matter, particularly in the sub-eV mass range. Detecting ALPs is challenging due to their extremely small coupling constants with photons.

• Current Limitations: Previous searches using table-top laser systems (10 TW class) have reached exclusion limits but cannot probe the theoretical QCD axion prediction region.
• The Challenge of Scaling: Upgrading to PW-class lasers (like the 0.1 PW system at ELI-NP) increases the probability of ALP production (which scales quadratically with creation laser intensity). However, higher intensities also drastically increase background noise from residual gas and optical elements.
• Key Issue: Moving from "table-top" to "facility-scale" experiments requires a paradigm shift in handling systematic errors, specifically distinguishing between the desired ALP signal and background photons generated by:
  1. Gas-origin: Interactions with residual atoms/molecules in the vacuum chamber.
  2. Optics-origin: Non-linear effects in mirrors, beam splitters, and glass substrates.

2. Methodology

The authors developed and commissioned an experimental system at the ELI-NP (Extreme Light Infrastructure - Nuclear Physics) facility in Romania, specifically in the 0.1 PW experimental area (E4).

Experimental Principle:
The search utilizes the Four-Wave Mixing (FWM) process in a quasi-parallel photon-photon collision geometry.

• Creation Laser: A high-power Ti:Sapphire laser ($\lambda_c = 800$ nm, ~25 fs pulse).
• Inducing Laser: A Nd:YAG laser ($\lambda_i = 1064$ nm, ~10 ns pulse).
• Mechanism: Two photons from the creation laser collide to produce an ALP (resonant state), which is then stimulated to decay into a signal photon by the inducing laser.
• Signal: The expected signal photon wavelength is $\lambda_s = 641$ nm.

System Design & Control Strategies:
To isolate the signal from backgrounds, the system integrates several advanced control subsystems:

• Vacuum Control: Two interaction chambers (VE1 for normal vacuum, VE2 for ultra-high vacuum) allow pressure scanning from $10^{-7}$ to $10^{-3}$ mbar to distinguish gas-origin backgrounds (which scale with pressure) from optics-origin backgrounds (pressure-independent).
• Spatiotemporal Overlap Control:
  • Spatial: The Nd:YAG beam diameter is designed to be 3 times larger than the Ti:Sa beam at the focus. This minimizes the impact of pointing instability on the overlap factor.
  • Temporal: A low-jitter timing system synchronizes the two lasers. Since the Nd:YAG pulse is much longer (ns) than the Ti:Sa pulse (fs), electronic synchronization is sufficient.
• Trigger Pattern Generator (TPG): A unique method to separate backgrounds by cycling through four trigger states every four laser shots:
  • S (Signal): Both lasers active (synchronized).
  • C (Creation): Only Ti:Sa active.
  • I (Inducing): Only Nd:YAG active.
  • P (Pedestal): Neither laser active (noise floor).
  • Analysis: The net signal is calculated as $S - C - I + P$.

Diagnostics:

• Knife-edge scanning to measure beam diameters and verify the 3:1 ratio.
• SPIDER (Spectral Phase Interferometry for Direct Electric-field Reconstruction) to characterize the Ti:Sa pulse duration.
• Fast Photodiodes to monitor timing jitter and pulse shapes.

3. Key Contributions

• System Integration: First full integration of a sub-eV ALP search experiment at a PW-class facility (ELI-NP), transitioning from 10 TW to 0.1 PW capabilities.
• Background Characterization Framework: Established a rigorous methodology to categorize and separate background sources (gas vs. optics) using pressure dependence and trigger-event selection.
• Stability Demonstration: Proved that the system can maintain the necessary spatiotemporal overlap over tens of thousands of laser shots despite the pointing instability inherent in high-power laser systems.
• Scalability Path: Validated a stepwise scale-up strategy, starting from 20 mJ pulses and preparing for future operation at 0.1 J, 1 J, and 2.5 J.

4. Results

The commissioning experiments were conducted with pulse energies of 18.3 mJ (Ti:Sa) and 23.9 mJ (Nd:YAG) at a vacuum pressure of $1.3 \times 10^{-7}$ mbar.

• Laser Performance:
  • Ti:Sa Pulse Duration: Measured at 26.1 ± 0.7 fs (consistent with Fourier limit).
  • Nd:YAG Pulse Duration: Measured at 10.5 ± 0.3 ns.
  • Timing Jitter: Total system jitter was 0.35 ns (standard deviation), well within the required tolerance relative to the Nd:YAG pulse width.
  • Pointing Stability: Over 49,670 shots, the Ti:Sa beam remained within $\pm 0.5\sigma$ of the Nd:YAG beam center, ensuring stable overlap.
• Beam Geometry:
  • Ti:Sa diameter: 59.7 mm; Nd:YAG diameter: 19.9 mm (Ratio $\approx$ 3:1 as designed).
  • Focal spot sizes: Ti:Sa ($\sim$8 $\mu$m) and Nd:YAG ($\sim$30 $\mu$m).
• Background Analysis:
  • Signal Detection: A clear FWM signal was observed only in the "S" (Signal) trigger pattern. The "C", "I", and "P" patterns showed negligible signal, confirming the suppression of single-laser backgrounds.
  • Pressure Dependence: In the range of $1.3 \times 10^{-7}$ to $1.0 \times 10^{-5}$ mbar, no power-law scaling (characteristic of gas-origin backgrounds) was observed.
  • Conclusion on Backgrounds: The remaining background is likely optics-origin (from mirrors/beam splitters) rather than gas-origin, as it is pressure-independent. This identifies the primary challenge for future high-energy runs.

5. Significance

• Platform Readiness: The paper confirms that the integrated experimental system at ELI-NP is fully functional and ready for high-energy ALP searches.
• New Exclusion Limits: By successfully commissioning the system at 0.1 PW, the collaboration is positioned to explore the sub-eV mass range with sensitivity far exceeding current limits, potentially reaching the theoretical QCD axion prediction.
• Methodological Benchmark: The paper provides a critical blueprint for future high-power laser experiments, demonstrating how to manage systematic errors (pointing, timing, and background separation) in facility-scale environments.
• Future Outlook: The system enables a controlled scale-up of laser energy to 2.5 J, which will allow the experiment to probe deeper into the unexplored parameter space of ALP-photon coupling.