Targeted hydroxyl fluorination for first-principles design of deep-ultraviolet nonlinear optical crystals: Multi-property optimization & chromatic dispersion modulation in Li2CsB7O10F4

Abstract

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals serve as core components for next-generation optoelectronic applications, ranging from high-resolution lithography to precision laser spectroscopy. However, the simultaneous achievement of three critical performance metrics—wide bandgap, suitable birefringence, and high second-harmonic generation (SHG) efficiency—remains a formidable challenge in this field. To address this long-standing issue, this work proposes a targeted hydroxyl fluorination and synergistic cation regulation strategy, designed specifically to tune and optimize the key optical properties of DUV NLO crystals. Using first-principles calculations, we adopted the parent compound Li2CsB7O10(OH)4 as a structural scaffold to design the fluorinated compound series Li2A’B7O10F4 (A’= Li, Na, K, Rb, Cs). The core of this design lies in replacing the hydroxyl-containing anionic unit [BO2(OH)2]⁴⁻ in the parent structure with the functional fluorine-containing unit [BO2F2]⁴⁻—a modification tailored to enhance DUV transparency and modulate optical anisotropy. Comprehensive property simulations confirm that the Li2A’B7O10F4 series exhibit significantly enhanced optical performance relative to the parent Li2CsB7O10(OH)₄. Among all derivatives, Li2CsB7O10F4 stands out with the most optimized properties. Its bandgap increases by nearly 1 eV (from ~6.35 eV in the parent to ~7.33 eV), directly boosting DUV transparency, while its SHG coefficient remains at a comparable high level—avoiding the common trade-off between bandgap and nonlinearity. Further analysis reveals that the [BO2F2]⁴⁻ units play a pivotal role in simultaneously regulating large birefringence and low chromatic dispersion, two key factors enabling effective DUV phase-matching (PM). Notably, compared to its hydroxyl-based counterpart, Li2CsB7O10F4 achieves a striking ~69.8 nm blue-shift in the shortest PM wavelength, with the value down to 198.5 nm—breaking through the critical 200 nm "wall" for practical DUV applications. These findings demonstrate the superiority of the targeted hydroxyl fluorination and synergistic cation regulation strategy in fine-tuning the optical properties of DUV NLO materials, offering a viable path toward high-performance DUV optoelectronic devices.