Terahertz radiation imaging of ferroelectric domain topology in room-temperature organic supramolecular ferroelectrics

We demonstrate a new method to detect ferroelectric domains in inside and surface regions of organic ferroelectrics by mapping out two orthogonally polarized terahertz waves radiated from the crystal upon the irradiation of near-infrared femtosecond laser pulses. We used polarization dependence of the effective depths radiating the terahertz waves, which originate from the optical anisotropy in the terahertz frequency region. This allows us to distinguish ferroelectric domains in the inside and surface regions of the crystals. We applied this method to a room-temperature organic supramolecular ferroelectric crystal, 1:1 salt of 5,5'-dimethyl-2,2'-bipyridine and deuterated iodanilic acid. A single domain covering almost all the area of an as-grown crystal ($\sim$600 $\mu$m $\times$ 800 $\mu$m) is discerned in the inside region, while complicated multi-domain in size of $\sim$ 200 $\mu$m is observed in the surface region. By applying external electric field along the 2c-b axis (ferroelectric polarization direction), the polarization switching proceeds with successive propagations of uncharged (neutral) and quasi-one-dimensional 180$^\circ$ domain walls (DWs) along the b-axis ($\perp$ 2c-b axis). This results in the formation of another uncharged and two-dimensional 180$\circ$ DW parallel to the (100) plane, which covers all the area of the crystal. We discuss the usefulness of the present terahertz radiation imaging technique and ferroelectric DW dynamics in terms of anisotropic stacking of hydrogen-bonded chains.