🔬 optics

Generation of continuous-wave laser light at 148.4 nm using cavity-enhanced second harmonic generation in $BaMgF_4$

This paper reports the first experimental demonstration of continuous-wave vacuum ultraviolet laser light generation at 148.4 nm using a periodically poled $BaMgF_4$ crystal within a power-enhancement cavity, achieving a proof-of-concept output power of approximately 16 pW for potential applications in nuclear optical clocks.

Original authors: Keerthan Subramanian, Hiroki Tanaka, Simon J. Herr, Nutan Kumari Sah, Gaurav Jha, Florian Zacherl, Srinivasa Arasada Pradeep, Valerii Andriushkov, Ke Zhang, Darius Fenner, Yumiao Wang, Milena Hugensch

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Keerthan Subramanian, Hiroki Tanaka, Simon J. Herr, Nutan Kumari Sah, Gaurav Jha, Florian Zacherl, Srinivasa Arasada Pradeep, Valerii Andriushkov, Ke Zhang, Darius Fenner, Yumiao Wang, Milena Hugenschmidt, Frank Kühnemann, Gaetano G. M. Bonetti, Shoichi Ui, Matthias Bickermann, Chenxi Ma, Xian Zheng, Michael Zopf, Bettina Lommel, Jan C. Müller, Stephan Hannig, Dmitry Budker, Ferdinand Schmidt-Kaler, Lars von der Wense

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

Imagine you are trying to build a very specific kind of clock. Most clocks tick using the vibration of atoms, but scientists are now trying to build a "nuclear clock" that ticks using the vibration of an atomic nucleus. This is like trying to hear a whisper in a hurricane; the nucleus is incredibly small and quiet, so you need a very precise, pure beam of light to make it "sing" (resonate).

The problem is that the light needed to wake up this specific nucleus (Thorium-229) is a very strange color: it's in the "vacuum ultraviolet" (VUV) range. This is a color so energetic that it doesn't exist naturally in our atmosphere and is very hard to make in a steady, continuous stream.

Here is how the researchers in this paper tackled the challenge of making this light, explained simply:

The Goal: A Steady Beam of "Invisible" Light

The team wanted to create a continuous-wave (CW) laser at a wavelength of 148.4 nanometers. Think of "continuous-wave" as a steady, unbroken stream of water from a hose, rather than a sprinkler that sprays in bursts. This steady stream is essential for the nuclear clock to work.

The Tool: The Crystal "Translator"

To make this light, they used a special crystal made of Barium Magnesium Fluoride (BaMgF4).

The Analogy: Imagine you have a radio playing a low, bass-heavy song (the laser light they started with at 296.8 nm). You want to hear a high-pitched version of that song (the 148.4 nm light). The crystal acts like a translator that takes the low notes and instantly converts them into high notes.
The Trick: This crystal isn't just a solid block; the scientists "periodically poled" it. Imagine the crystal is a long hallway with a floor pattern. They painted the floor in a specific repeating pattern (like stripes) so that every time the light wave hits a stripe, it gets a little push in the right direction. This pattern is crucial for the translation to work efficiently.

The Challenge: The "Weak" Translator

There was a catch. This specific crystal (BaMgF4) is a bit of a "shy" translator. It has a very weak ability to convert the light compared to other crystals.

The Analogy: It's like trying to fill a bucket with a leaky, tiny cup. Most other crystals are like a firehose; this one is a tiny dropper. Because the crystal is so weak at this job, the scientists had to be very clever to get enough light out.

The Solution: The "Echo Chamber"

To overcome the crystal's weakness, they built a special cavity (an optical resonator).

The Analogy: Imagine you are in a hallway with mirrors on both ends. If you clap once, the sound bounces back and forth, getting louder and louder with every echo. The scientists put their crystal inside this "sound hallway" (which is actually a vacuum chamber to keep air from blocking the light).
They shot their "low note" laser into this chamber. The light bounced back and forth thousands of times, building up a massive amount of energy inside the chamber. Every time the light passed through the "shy" crystal, it made a tiny bit of the high-pitched light. Because the light passed through the crystal so many times, those tiny bits added up to a detectable beam.

The Result: A First-Time Success

The experiment was a success, but it was a "proof of concept."

The Output: They managed to generate a steady beam of the special 148.4 nm light. However, the amount of light was very small—about 16 picowatts.
The Scale: To put that in perspective, a standard laser pointer is billions of times brighter. But in the world of this specific, hard-to-make light, getting any steady beam is a huge milestone. It's the first time this specific type of crystal has ever been used to make this kind of light.

What's Next?

The paper explains that while they succeeded, the current setup is like a prototype car. It runs, but it's not fast yet. The scientists identified why the output was low:

The Crystal Quality: The "striped" pattern on the crystal wasn't perfect, and the crystal itself absorbed some of the light.
The Order: They had to use a "9th order" method (a complex way of arranging the stripes) because they couldn't make the stripes small enough for the most efficient method.

The researchers believe that if they can make the crystal clearer, make the stripes smaller, and improve the "echo chamber," they could increase the power of the light by thousands of times. This would bring the "nuclear clock" one step closer to reality, offering a timekeeper so precise it could detect changes in gravity or the fabric of space-time itself.

In summary: The team built a specialized "echo chamber" to amplify a weak crystal's ability to turn one color of light into a very rare, high-energy color. They successfully made the first steady beam of this light, proving the method works, even though the beam is currently very dim.

Technical Summary: Generation of Continuous-Wave Laser Light at 148.4 nm Using Cavity-Enhanced Second Harmonic Generation in BaMgF4

Problem and Motivation
The development of a nuclear optical clock based on the transition between the ground and isomeric state of the $^{229}\text{Th}$ nucleus requires a continuous-wave (CW) laser source at 148.4 nm. While previous experiments utilized pulsed lasers or frequency combs, these approaches suffer from broad spectral bandwidths or low efficiency, respectively. Although recent breakthroughs have demonstrated CW 148.4 nm sources using random quasi-phase-matched SrB $_4$ O $_7$ (SBO) crystals or four-wave mixing in cadmium vapor, the quest for a high-power, all-solid-state CW laser system remains open. The primary challenge lies in finding a nonlinear crystal that is transparent in the vacuum ultraviolet (VUV), possesses a $\chi^{(2)}$ nonlinearity, and is amenable to quasi-phase-matching (QPM) via electric-field periodic poling. Among candidates like BPO $_4$ , SrB $_4$ O $_7$ , and SrMgF $_4$ , only BaMgF $_4$ (BMF) has been confirmed as ferroelectric, making it the sole candidate for deterministic QPM via established electric-field poling techniques. However, BMF has a relatively small nonlinear coefficient ( $d_{32} \approx 0.039$ pm/V) compared to other materials, and its application to 148.4 nm generation had not been previously demonstrated.

Methodology
The authors experimentally investigated the potential of periodically poled BaMgF $_4$ (ppBMF) to generate 148.4 nm light via cavity-enhanced second harmonic generation (SHG). The methodology involved three primary stages:

Crystal Growth and Preparation: BMF crystals were grown using the Czochralski technique at the Leibniz-Institut für Kristallzüchtung (IKZ). The study compared samples grown in aggressive fluorinating atmospheres (high VUV transparency but low crystallinity) versus non-fluorinating atmospheres (higher crystallinity but lower transparency). The chosen sample was a high-crystallinity crystal that underwent post-growth vacuum annealing to reduce scattering centers, despite a trade-off of reduced VUV transmission (approx. 3.6% at 148.4 nm). The crystals were cut, polished, and prepared for periodic poling.
Periodic Poling: Using a calligraphic poling technology at the Fraunhofer-Institut für Physikalische Messtechnik (IPM), inverted domains were written into the BMF crystal. Due to the inability to achieve sub-micron periods required for first-order QPM, the team utilized 9th-order QPM with a poling period of 8.86 µm. The poling was performed using the $d_{31}$ and $d_{33}$ nonlinear coefficients, with domain lines parallel to the crystallographic $a$ -axis.
Experimental Setup: The ppBMF crystal was inserted into a bowtie-shaped enhancement cavity operating at the fundamental wavelength of 296.8 nm. The cavity was housed in a vacuum chamber ( $\sim 10^{-6}$ mbar) purged with an ozone-oxygen mixture to prevent hydrocarbon contamination from the intense UV/VUV radiation. The setup utilized a commercial TOPTICA TA-FHG pro laser (600–800 mW output) and a Hänsch-Couillaud lock to stabilize the cavity length. VUV output was detected using a micro-channel plate (MCP) for alignment and a photomultiplier tube (PMT) for power quantification.

Key Results
The experiment successfully demonstrated the generation of CW VUV light at 148.4 nm using ppBMF for the first time.

Output Power: A typical VUV output power of $(16 \pm 1)$ pW was obtained. A position-dependent maximum of $(48 \pm 3)$ pW was observed at an intra-cavity fundamental power of 1.4 W.
Scaling: The VUV power exhibited the expected quadratic scaling with the fundamental power, confirming the $\chi^{(2)}$ nonlinear process.
Stability: The system demonstrated stability for over 200 seconds when the vacuum chamber was continuously purged with ozone. Without purging, power levels dropped rapidly due to carbon layer buildup.
Comparison to Theory: The maximum observed power of 48 pW compares favorably to a theoretical estimate of $(63 \pm 18)$ pW based on the Boyd-Kleinman equation, using parameters of 3.6% crystal transmission, a 9th-order QPM ( $m=9$ ), and a nonlinear coefficient $d = 0.015$ pm/V.

Significance and Claims
The paper claims this work as the first instance of using BaMgF $_4$ crystals to generate VUV laser light. It serves as a proof-of-concept investigation establishing the viability of ppBMF for nuclear optical clock applications. The authors note that while the current output power is low, the experimental findings align closely with theoretical expectations, providing a clear path for future improvements.

The authors outline specific avenues for increasing power, including:

Improving cavity finesse by replacing MgF $_2$ prisms with dichroic elements and using anti-reflection coated crystals.
Utilizing crystals with higher VUV transparency, potentially at the cost of poling quality.
Reducing the QPM order (e.g., from 9th to 3rd) by achieving smaller poling periods, potentially using thinner crystals or ridge waveguides.
Exploring alternative phase-matching methods like order-disorder phase matching via direct laser lithography, which could be applied to non-ferroelectric crystals with larger nonlinear coefficients (e.g., SBO).

The study concludes that while current BMF-based systems are limited by low nonlinear coefficients and poling constraints, the demonstrated technique offers a solid foundation for developing high-power, all-solid-state CW lasers at 148.4 nm.