Acentric nonlinear optical N-benzyl stilbazolium crystals with high environmental stability and enhanced molecular nonlinearity in solid state

We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N′-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P21 phase with a large macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of urea at the non-resonant wavelength of 1.9 µm. Compared to conventional rod-shaped N-alkyl stilbazolium crystals, an enhanced hyperpolarizability of the chromophoresβ0 in the solid state can be utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-chromophore interactions due to the larger chromophore–chromophore separation induced by the bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have grown good optical quality crystals large enough for optical characterization. With as-grown BP3 crystals without additional polishing and cutting procedures we have demonstrated THz generation by optical rectification using 180 fs pulses at the pump wavelength of 836 nm.