The Groundbreaking Development of the ABF Crystal

Chinese scientists have unveiled a transformative advancement in nonlinear optics with the creation of the ammonium fluorooxoborate (ABF) crystal, chemically known as NH₄B₄O₆F. This innovation, led by Prof. Shilie Pan at the Xinjiang Technical Institute of Physics and Chemistry (XTIPC) under the Chinese Academy of Sciences (CAS), promises to redefine vacuum ultraviolet (VUV) laser technology. VUV light, spanning wavelengths from 100 to 200 nanometers, possesses extraordinarily high photon energy, making it indispensable for probing atomic and molecular structures in ways unattainable by longer wavelengths.

The ABF crystal addresses a critical bottleneck in generating coherent VUV light efficiently through second harmonic generation (SHG), a nonlinear optical process where two input photons at frequency ω combine to produce one photon at 2ω. Traditional methods like synchrotron radiation or excimer lasers are bulky, costly, and inefficient, whereas SHG via crystals offers compactness and high beam quality. Yet, until now, no material fully satisfied the demanding requirements: deep VUV transparency, robust nonlinear optical (NLO) response measured by the second-order susceptibility tensor d, sufficient birefringence for phase matching, large growable crystals, chemical stability, high laser-induced damage threshold (LIDT), and ease of processing.

Unpacking the Unique Structure of ABF

At the heart of ABF's superiority lies its fluorooxoborate architecture. By strategically incorporating fluorine atoms into a borate framework, the researchers formed isolated [B₄O₆F] anionic groups. These units feature B-F covalent bonds that widen the bandgap, ensuring transparency down to VUV wavelengths, while inducing structural asymmetry crucial for non-centrosymmetric crystals needed for SHG. The arrangement creates sublattices with optimized birefringence Δn, enabling phase matching—the momentum conservation condition k_2ω = 2k_ω—at unprecedented short wavelengths.

The crystal's orthorhombic symmetry and effective NLO coefficient d₃₂ = 1.09 pm/V outperform predecessors. Thermal expansion data confirms stability, and centimeter-scale boules—up to several cm long—were grown via high-temperature solution methods refined over a decade, transitioning from millimeter prototypes.

The Decade-Long Quest Led by Prof. Pan Shilie

Prof. Pan Shilie, director of XTIPC and a pioneer in functional crystals, spearheaded this effort building on China's legacy. In the 1990s, Academician Chen Chuangtian invented KBe₂BO₃F₂ (KBBF), the gold standard for VUV SHG below 200 nm. However, KBBF's beryllium toxicity, layered growth limiting thickness to ~3 mm, and hygroscopicity posed barriers. Pan's team proposed a fluorination strategy: replacing Be with safer B-F units to boost bandgap (via electronegative F), enhance SHG (asymmetry), and birefringence (planar groups).

• Innovative synthesis of precursors under controlled atmospheres.
• Optimization of flux systems for large single crystals without defects.
• Precision polishing for anisotropic devices with exact phase-matching angles (θ, φ).
• Rigorous testing with ns-pulse lasers (e.g., 355 nm pumped OPO).

This culminated in Nature publication on January 28, 2026 (DOI: 10.1038/s41586-025-10007-z), affirming China's dominance.Read the Nature paper For academics interested in such cutting-edge work, opportunities abound in research jobs at CAS institutes.

Record-Shattering Performance in VUV Generation

ABF devices achieved birefringent phase-matched SHG at 158.9 nm—the shortest ever—using fundamental wavelengths around 317.8 nm. At 177.3 nm, it delivered 4.8 mJ nanosecond pulse energy with 5.9% efficiency, shattering prior records for solid-state VUV sources. Tunable output spans 158.9–340 nm, breaching the '200 nm wall' effortlessly.

Sellmeier equations validate phase-matching angles, e.g., (90°, 70°) for 158.9–188 nm.

ABF vs. KBBF: A Superior Successor

While KBBF excels in d₁₁=0.47 pm/V, its toxicity and growth limits hinder scalability. ABF's d₃₂ yields higher d_eff across 310–400 nm fundamentals, safer composition, and 10x thicker crystals. No ionic F-K bonds like KBBF's weak layers; ABF's rigid framework ensures durability.

• Safety: Beryllium-free, non-toxic.
• Growth: Cm-scale vs. mm.
• LIDT: High, suitable for mJ pulses.
• Processability: Easily cut/polished for devices.

Electron localization function analyses confirm F's role in asymmetry.Phys.org coverage

Transformative Applications in Science and Industry

ABF enables compact all-solid-state VUV lasers for:

• Spectroscopy: High-res angle-resolved photoemission for materials science.
• Quantum Tech: VUV for Rydberg atoms, entanglement.
• Semiconductors: Lithography precursor, metrology for sub-2nm nodes.
• Superconductivity: Probe high-Tc mechanisms.
• Precision Manufacturing: Aerospace parts, biomed devices via microprocessing.

Prof. Pan notes: "ABF holds great promise for advanced equipment and unknown fields." Imagine crafting an academic CV for roles in this booming sector.

China's Strategic Edge in Crystal Research

CAS's XTIPC exemplifies China's investment in strategic materials. From KBBF to ABF, decades of theory—primitives, strategies—culminate here. Pan's awards underscore this. Ties to University of Chinese Academy of Sciences foster talent pipeline. For higher ed in China, this boosts postdoc and professor jobs in optics.

Future Horizons: Shorter Wavelengths, Higher Powers

Optimizing quality promises >10% efficiency, mJ at <150 nm. Hybrids with other NLOs, integration in quantum devices loom. Global collaborations could accelerate, but China's lead persists. Researchers, check higher-ed-jobs for openings.

Photo by Thorium on Unsplash

Stakeholder Views and Broader Impacts

Industry eyes ABF for chip fabs; academia hails design paradigm. Balanced views note scaling challenges, but consensus: game-changer. Ethical manufacturing, IP via CAS patents secure supply.