Terahertz Radiation Imaging of Ferroelectric Domain Topography in Room-Temperature Hydrogen-Bonded Supramolecular Ferroelectrics

We report quasi-three-dimensional imaging of the ferroelectric domains in a hydrogen-bonded supramolecular ferroelectric 1:1 salt of 5,5'-dimethyl-2,2'-bipyridine and deuterated iodanilic acid by mapping out terahertz waves radiated from the crystal upon femtosecond laser irradiation. Polarization dependence of the effective depth radiating the terahertz waves due to the absorption anisotropy for terahertz waves allows us to distinguish domains in the inside and surface regions of the crystals. In an as-grown crystal, a large domain covering almost all the area is discerned in the inside region, while multidomains are discerned in the surface regions. We observed polarization switching via domain boundary motion in the direction perpendicular to the direction of hydrogen-bonded chains in the surface regions under an external electric field. Obtained images suggest that an uncharged 180 degrees domain wall (DW) parallel to the stacking plane of hydrogen-bonded chains covers the whole area at coercive fields. We argue that the DW dynamics stems from the anisotropic crystal structure, i.e., stacking of hydrogen-bonded chains.