Extremely large nonresonant second-order nonlinear optical response in crystals of the stilbazolium salt DAPSH

We report on the extremely large nonresonant quadratic optical nonlinearity of the stilbazolium salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The phenyl-pyridinium chromophores in DAPSH crystals grown from acetone solution pack with a highly aligned polar order, resulting in a very large birefringence, Δn=1.17±0.06 at λ=0.83 μm and Δn=0.83±0.04 at λ=1.55 μm. More importantly, this leads to an extremely large diagonal quadratic susceptibility with the nonlinear optical coefficient for second-harmonic generation reaching up to d111=290±40 pm/V at 1.907 μm fundamental wavelength, which presents a considerable improvement with respect to the presently best material trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosyate (DAST) with d111=210±55 pm/V at λ=1.907 μm. The result is in agreement with the preferential packing of the chromophores and the previous studies demonstrating higher microscopic nonlinearity of the chromophores in DAPSH compared to that of DAST.