UV Solar‐Blind‐Region Phase‐Matchable Optical Nonlinearity and Anisotropy in a π‐Conjugated Cation‐Containing Phosphate

Abstract Wide ultraviolet (UV) transparency, strong second‐harmonic generation (SHG) response, and sufficient optical birefringence for phase‐matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non‐traditional π‐conjugated cation‐based UV NLO phosphate [C(NH 2 ) 3 ] 6 (PO 4 ) 2 ⋅3 H 2 O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH 2 PO 4 @ 1064 nm) and 532 nm (0.3 × β‐BaB 2 O 4 @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar‐blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π‐conjugated cations [C(NH 2 ) 3 ] + and their favorable arrangement with [PO 4 ] 3− anions.