Recently, the research team led by Professors Lu Dazhi and Yu Haohai from the State Key Laboratory of Crystal Materials has reported new advances in langasite-family oxide nonlinear optical crystals. Their study, titled “La₃ZrGa₅O₁₄: Band-Inversion Strategy in Topology-Protected Octahedron for Large Nonlinear Response and Wide Bandgap”, was published in Angewandte Chemie International Edition. Professor Yu Haohai is the corresponding author, and Professor Lu Dazhi is the first author. Shandong University is the primary institution affiliated with this work.

Mid-infrared (MIR) lasers, located in the atmospheric transmission window and corresponding to the vibrational frequencies of various molecules, hold significant applications in fields such as electro-optical countermeasures and environmental monitoring. Nonlinear optical crystals for frequency conversion and amplification are critical materials for advancing MIR laser technologies. However, oxide crystals typically exhibit strong multi-phonon absorption, limiting their transmission range to wavelengths below 5 μm, while semiconductor crystals suffer from narrow bandgaps that result in low laser damage thresholds (LDTs), hindering their ability to support high-power MIR laser systems. Additionally, research in this area is constrained by the intrinsic trade-off between bandgap width and nonlinear optical coefficients. Consequently, developing MIR nonlinear crystals that simultaneously achieve high LDTs and large nonlinear coefficients remains a major research focus and challenge in this field.

The band-inversion strategy enables synergistic improvement of the laser damage threshold and nonlinear optical response

This research team has long been dedicated to the study of lasers and nonlinear optical crystals. In recent years, by focusing on phonon-related mechanisms, they established design principles for MIR oxide crystals. They achieved the first international discovery that langasite family crystals combine high LDTs, large nonlinear optical coefficients, and broad transmission ranges. The team further developed large-size crystal growth techniques for this family of materials, successfully producing the world-record-sized optical crystal of its kind. They also designed and implemented practical nonlinear optical devices. Building on this foundation, the current work innovatively proposes a design strategy utilizing topologically symmetric-protected oxygen octahedra to induce electronic band inversion. This approach shifts the active 4delectron orbitals to the bottom of the conduction band, resulting in the development of La₃ZrGa₅O₁₄(LGZr)—a nonlinear optical crystal with exceptional comprehensive performance. LGZr synergistically enhances both its LDT and nonlinear response, making it the material within its family to exhibit the strongest nonlinear response, widest bandgap, and highest LDT discovered to date. This research provides a new paradigm for designing MIR oxide nonlinear crystals and demonstrates significant application potential in critical engineering areas such as amplification and frequency conversion for ultrafast, high-intensity MIR lasers.

The National Key Research and Development Program of China, the National Natural Science Foundation of China, the Taishan Pangdeng Scholarship, the Shandong Provincial Key Research and Development Program, the Shandong Provincial Natural Science Foundation, and the State Key Laboratory of Crystal Materials supported the related work.