Exceptional bi-step switching of quadratic nonlinear optical properties in a one-dimensional channel compound.

We report a new solid-state nonlinear optical switch based on a one-dimensional channel compound NH4[(CH3)(4)N] SO4 center dot H2O. It shows an exceptional bi-step switching of second harmonic generation (SHG) effects, corresponding to its SHG-Off, SHG-Low and SHG-High states, which originate from two-step structural transitions with unique centrosymmetric-polar-chiral symmetry transformations.