Chinese Scientists Develop Groundbreaking ABF Crystal Advancing Deep-UV Optics

Breakthrough Publication in Nature Marks New Era for Vacuum Ultraviolet Lasers

The recent publication in Nature by a team from the Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (XJIPC CAS), has captured global attention in materials science and photonics.5456 Titled "Vacuum ultraviolet second-harmonic generation in NH₄B₄O₆F crystal," the paper details the development of ammonium fluorooxoborate (ABF), a novel nonlinear optical (NLO) crystal poised to transform deep-ultraviolet (deep-UV) and vacuum ultraviolet (VUV) light generation.47 Nonlinear optical crystals enable frequency conversion of laser light, where second harmonic generation (SHG) doubles the frequency, effectively halving the wavelength to produce shorter, higher-energy photons essential for advanced applications.

Deep-UV light spans wavelengths from about 200 nm to 100 nm, while VUV extends from 100 nm to 10 nm. These regimes demand materials with exceptional transparency, strong nonlinear responses, and the ability to achieve phase matching—a condition where generated and fundamental waves propagate at matching speeds to build coherently. Prior to ABF, potassium beryllium fluoroborate (KBe₂BO₃F₂, or KBBF), pioneered by Chinese researchers in the 1990s, held the monopoly for practical SHG below 200 nm, limited to around 166 nm due to growth difficulties and beryllium toxicity.55

Understanding the ABF Crystal: Structure and Unique Properties

ABF, chemically NH₄B₄O₆F, represents a strategic evolution in fluorooxoborate design. By incorporating fluorine atoms into a borate framework, researchers formed planar [B₄O₆F]^5- groups with enhanced polarity and rigidity. This fluorination widens the bandgap for superior VUV transparency (cutoff below 158.9 nm), boosts the second-order nonlinear susceptibility for efficient SHG, and induces substantial birefringence (Δn ≈ 0.12 at 193 nm) for phase matching.56

Key properties include high laser-induced damage threshold (LIDT > 10 GW/cm² at 1064 nm), chemical stability, and non-hygroscopic nature—advantages over hygroscopic alternatives. Thermal expansion anisotropy ensures device reliability under laser operation. These traits collectively address the 'perfect storm' of requirements no single prior crystal fully met.

• Transparency: >80% from 160 nm to 200 nm
• Nonlinear coefficient (d₁₁): ~1.2 pm/V, competitive with KBBF
• Birefringence: Enables Type I phase matching at θ = 45° for 158.9 nm
• Growth: Centimeter-scale boules via high-temperature flux method

The Research Team Behind the Innovation

Led by Prof. Pan Shilie, director of XJIPC CAS, the team invested nearly a decade overcoming synthesis hurdles. From theoretical modeling of B-F covalent bonds to empirical flux optimization, their iterative process yielded crack-free, high-optical-quality crystals. XJIPC CAS, a premier CAS institute in Urumqi, Xinjiang, specializes in condensed matter physics and optics, serving as a training hub for graduate students from the University of Chinese Academy of Sciences (UCAS)—China's leading research university network.42 This work exemplifies how CAS institutes bridge fundamental research and higher education, fostering PhD and postdoc training in cutting-edge materials science.

For aspiring researchers, such breakthroughs highlight opportunities in Chinese higher education. Institutions like UCAS and regional universities in Xinjiang are expanding research jobs in photonics and NLO materials, attracting global talent amid China's push for scientific self-reliance.

Overcoming Decades-Old Challenges in Crystal Growth and Fabrication

Growing large ABF crystals required mastering a multi-step process: (1) Hydrothermal synthesis of precursors, (2) High-temperature solution growth in KF-NH₄F flux at ~750°C, (3) Slow cooling to minimize inclusions, and (4) Precision polishing to λ/10 flatness. Early millimeter crystals suffered defects; scaled-up versions now exceed 20 mm edge lengths with 90% yield.55

Device fabrication involved anisotropic cutting for optimal phase-matching angles, anti-reflective coatings for VUV, and encapsulation to prevent ammonium volatility. Testing with a 355 nm pump laser confirmed SHG output: 158.9 nm phase matching verified via tunable divergence method, 177.3 nm tuned for max power (4.8 mJ, 5.9-7.9% efficiency)—records shattering prior VUV benchmarks.

Record-Breaking Performance and Head-to-Head Comparisons

ABF outperforms across metrics, offering a viable KBBF successor without toxic beryllium.56

Transformative Applications in Semiconductor Lithography and Beyond

In semiconductor manufacturing, VUV lasers enable finer photolithography nodes beyond current EUV (13.5 nm), targeting sub-5 nm features for next-gen chips. ABF-powered sources promise compact, table-top systems replacing bulky synchrotrons.55

Scientific uses span angle-resolved photoemission spectroscopy (ARPES) for quantum materials, VUV photoelectron microscopy for superconductivity, and photochemical studies. Industrially: ultra-precision ablation in aerospace components, high-density optical data storage, and biomedical sterilization. China's dominance here aligns with its chip ambitions, spurring university-industry collaborations.

Explore research assistant roles in optics labs at Chinese universities to contribute to such innovations.

Implications for Higher Education and Research Careers in China

This ABF milestone underscores China's higher education prowess in STEM. UCAS, hosting over 60,000 grad students across CAS institutes, integrates such research into curricula, producing experts in crystal engineering. Universities like Xinjiang University partner with XJIPC, offering joint PhD programs.43

• Increased funding for materials science departments
• Rising demand for China higher ed jobs in photonics
• Global collaborations, e.g., with EU/US labs
• Career paths: postdoc to faculty in NLO

Prof. Pan's team exemplifies mentorship; many alumni now lead university labs. For career advice, check academic CV tips.

Future Outlook: Commercialization and Next Frontiers

Short-term: Optimize ABF for kHz repetition rates, higher powers (>10 mJ). Long-term: Hybrid devices cascading ABF with sum-frequency generation for <150 nm. Commercial partners eye lithography tools by 2030. Challenges: Scale production, cost reduction.54

China's lead invites international talent; browse postdoc opportunities in advanced materials.

Photo by Yilin Liu on Unsplash

Why This Matters for Global Higher Education and Photonics Research

ABF's advent signals a renaissance in VUV photonics, empowering university researchers worldwide. In China, it bolsters university jobs ecosystem, from adjuncts to executives. Share your professor experiences at Rate My Professor, explore higher ed jobs, and access career advice. For employers, visit recruitment to post opportunities.