A BBO-like trithiocyanate with significantly enhanced birefringence and second-harmonic generation

There is presently a great demand for nonlinear optical (NLO) crystals that can demonstrate both strong second-harmonic generation (SHG) and high birefringence in emerging spin-orbit angular momentum photonic technologies. Here we report a new chiral trithiocyanate compound, K 4 (HC 3 N 3 S 3 ) 2 ·H 2 O, which is composed of highly π-conjugated [HC 3 N 3 S 3 ] 2− rings and is structurally analogous to commercial NLO crystal β-BaB 2 O 4 (BBO). Compared with BBO, which has optimally arranged π-conjugated [B 3 O 6 ] 3− rings, K 4 (HC 3 N 3 S 3 ) 2 ·H 2 O does not have an optimal arrangement of [HC 3 N 3 S 3 ] 2− rings. However, this trithiocyanate exhibits a significantly enhanced SHG response of up to 1.8 times that of BBO and an enhanced birefringence of up to 0.402 at the wavelength of 550 nm (about 3.3 times that of BBO). The first-principles calculations show that the π-conjugated [HC 3 N 3 S 3 ] 2− is the cause of the superior optical properties of K 4 (HC 3 N 3 S 3 ) 2 ·H 2 O. These findings demonstrate that the π-conjugated [HC 3 N 3 S 3 ] 2− is an excellent bi-functional “material gene” and merits more attention in the study of optical integrated devices for emerging photonic technologies.