π-Conjugated Cations in Phosphates: A Pathway to Solar-Blind UV Nonlinear Optical Crystals with Phase-Matching

Abstract

While birefringence phase-matching (PM) remains the most practical approach for nonlinear optical (NLO) frequency conversion, conventional phosphate crystals suffer from intrinsically low birefringence (Δn < 0.05) that hinders PM behavior in the solar-blind ultraviolet (UV) region (λ < 280 nm). Herein, we demonstrate a π-conjugated cation engineering strategy to break this limitation, reporting two phosphite-based NLO crystals: (C₂N₄OH₇)H₂PO₃ (GUPO) and C(NH₂)₃H₂PO₃ (GPO). The synergistic alignment of π-conjugated guanidinium cations and [H₂PO₃]⁻ anions enables record-breaking optical anisotropy (Δn = 0.19 @ 589.3 nm for GUPO), surpassing all known inorganic phosphates. Crucially, GUPO is the first phosphate to realize full-wavelength PM, in which the PM wavelength fully covers its optical transparency range down to 215 nm. Concurrently, GUPO exhibits exceptional second-harmonic generation (SHG) response (2.2 × KDP at 1064 nm and 1.0 × β-BBO at 532 nm), which may enable direct 266 nm laser generation. Mechanistic studies reveal that the giant birefringence originates from oriented π–π interactions between cations, while the SHG response stems from the cooperative polarization of cations. This work establishes π-conjugated cation engineering as a paradigm for designing UV NLO materials, with GUPO crystal emerging as a cheap, efficient alternative to conventional UV NLO crystals.